4.4 Procedures for Lateral-Force- Resisting · PDF file4.4 Procedures for...

4.4 Procedures for Lateral-Force-Resisting Systems

This section provides Tier 2 evaluation proceduresthat apply to lateral force resisting systems: momentframes, shear walls and braced frames.

4.4.1 Moment Frames

Chapter 4.0 - Evaluation Phase (Tier 2)

4 - 36 Seismic Evaluation Handbook FEMA 310

Commentary:

Moment frames develop their resistance to lateralforces through the flexural strength and continuityof beam and column elements.

In an earthquake, a frame with suitable proportionsand details can develop plastic hinges that willabsorb energy and allow the frame to surviveactual displacements that are larger than calculatedin an elastic-based design.

In modern moment frames, the ends of beams andcolumns, being the locations of maximum seismicmoment, are designed to sustain inelastic behaviorassociated with plastic hinging over many cyclesand load reversals. Frames that are designed anddetailed for this ductile behavior are called"special" moment frames.

Frames without special seismic detailing depend onthe reserve strength inherent in the design of themembers. The basis of this reserve strength is theload factors in strength design or the factors ofsafety in working-stress design. Such frames arecalled "ordinary" moment frames. For ordinarymoment frames, failure usually occurs due to asudden brittle mechanism, such as shear failure inconcrete members.

For evaluations using this Handbook, it is notnecessary to determine the type of frame in thebuilding. The performance issue is addressed byappropriate acceptance criteria in the specifiedprocedures. The fundamental requirements for allductile moment frames are that:

1. They have sufficient strength to resistseismic demands,

2. They have sufficient stiffness to limitinterstory drift,

3. Beam-column joints have the ductility tosustain the rotations they are subjected to,

4. Elements can form plastic hinges, and5. Beams will develop hinges before the

columns at locations distributed throughoutthe structure (the strong column/weakbeam concept).

These items are covered in more detail in theevaluation statements that follow.

It is expected that the combined action of gravityloads and seismic forces will cause the formation ofplastic hinges in the structure. However, aconcentration of plastic hinge formation atundesirable locations can severely undermine thestability of the structure. For example, in a weakcolumn situation (see Figure 4-13 next page),hinges can form at the tops and bottoms of all thecolumns in a particular story, and a storymechanism develops. This condition results in aconcentration of ductility demand and displacementin a single story that can lead to collapse.

In a strong column situation (see Figure 4-13 nextpage) the beams hinge first, yielding is distributedthroughout the structure, and the ductility demandis more dispersed.

Figure 4-13. Plastic Hinge Formation

4.4.1.1 General

4.4.1.1.1 REDUNDANCY: The number of lines ofmoment frames in each direction shall be greaterthan equal to 2 for Life Safety and for ImmediateOccupancy. The number of bays of moment framesin each line shall be greater than or equal to 2 forLife Safety and 3 for Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with the procedures in Section 4.2 shall beperformed. The adequacy of all elements andconnections in the frames shall be evaluated.


FEMA 310 Seismic Evaluation Handbook 4 - 37

STRONG

WEAKCOLUMNS

COLUMNS

Commentary:

Redundancy is a fundamental characteristic oflateral force resisting systems with superior seismicperformance. Redundancy in the structure willensure that if an element in the lateral forceresisting system fails for any reason, there isanother element present that can provide lateralforce resistance. Redundancy also providesmultiple locations for potential yielding,distributing inelastic activity throughout thestructure and improving ductility and energydissipation. Typical characteristics of redundancyinclude multiple lines of resistance to distribute thelateral forces uniformly throughout the structure,and multiple bays in each line of resistance toreduce the shear and axial demands on any oneelement (see Figure 4-14).

A distinction should be made between redundancyand adequacy. For the purpose of this Handbook,redundancy is intended to mean simply "more thanone." That is not to say that for large buildingstwo elements is adequate, or for small buildingsone is not enough. Separate evaluation statementsare present in the Handbook to determine theadequacy of the elements provided.

When redundancy is not present in the structure, ananalysis which demonstrates the adequacy of thelateral force elements is required.

Redundant Frame Nonredundant Frame

Figure 4-14. Redundancy Along a Line of Moment Frame

4.4.1.2 Moment Frames with Infill Walls

4.4.1.2.1 INTERFERING WALLS: All infill wallsplaced in moment frames shall be isolated fromstructural elements.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thedemands imparted by the structure to the interferingwalls, and the demands induced on frame shall becalculated. The adequacy of the interfering walls andthe frame to resist the induced forces shall beevaluated.4.4.1.3 Steel Moment Frames



Commentary:

When an infill wall interferes with the momentframe, the wall becomes an unintended part of thelateral-force-resisting system. Typically thesewalls are not designed and detailed to participate inthe lateral-force-resisting system and may besubject to significant damage.

Interfering walls should be checked for forcesinduced by the frame, particularly when damage tothese walls can lead to falling hazards near meansof egress. The frames should be checked for forcesinduced by contact with the walls, particularly ifthe walls are not full height, or do not completelyinfill the bay.

Commentary:

The following are characteristics of steel momentframes that have demonstrated acceptable seismicperformance:

1. The beam end connections develop the plastic moment capacity of the beam or panel zone,

2. There is a high level of redundancy in thenumber of moment connections,

3. The column web has sufficient strength to sustain the stresses in the beam-column joint,

4. The lower flanges have lateral bracing sufficient to maintain stability of theframe, and

5. There is flange continuity through the column.

Prior to the 1994 Northridge earthquake, steelmoment-resisting frame connections generallyconsisted of complete penetration flange welds anda bolted or welded shear tab connection at the web.This type of connection, which was an industrystandard from 1970 to 1995, was thought to beductile and capable of developing the full capacityof the beam sections. However, over 200 buildingsexperienced extensive brittle damage to this type ofconnection during the Northridge earthquake. Asa result, an emergency code change was made tothe 1994 UBC (ICBO, 1994) removing theprequalification of this type of connection. Thereasons for this unexpected performance are stillunder investigation. A full discussion of thevarious fractures mechanisms and ways ofpreventing or repairing them is given in FEMA 267(SAC, 1995) and FEMA 267A (SAC, 1997).

Commentary:

Infill walls used for partitions, cladding or shaftwalls that enclose stairs and elevators should beisolated from the frames. If not isolated, they willalter the response of the frames and change thebehavior of the entire structural system. Lateraldrifts of the frame will induce forces on walls thatinterfere with this movement. Claddingconnections must allow for this relative movement.Stiff infill walls confined by the frame will developcompression struts that will impart loads to theframe and cause damage to the walls. This isparticularly important around stairs or other meansof egress from the building.

4.4.1.3.1 DRIFT CHECK: The drift ratio of thesteel moment frames, calculated using the QuickCheck Procedure of Section 3.5.3.1, shall be lessthan the 0.025 for Life Safety and 0.015 forImmediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the beams and columns, including P-∆effects, shall be evaluated using the m-factors in Table4-3.

4.4.1.3.2 AXIAL STRESS CHECK: The axialstress due to gravity loads in columns subjected tooverturning forces shall be less than 0.10Fy for LifeSafety and Immediate Occupancy. Alternatively,the axial stress due to overturning forces alone,calculated using the Quick Check Procedure ofSection 3.5.3.6, shall be less than 0.30Fy for LifeSafety and Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thegravity and overturning demands for non-compliantcolumns shall be calculated and the adequacy of thecolumns to resist overturning forces shall be evaluatedusing the m-factors in Table 4-3.

4.4.1.3.3 MOMENT-RESISTINGCONNECTIONS: All moment connections shallbe able to develop the strength of the adjoiningmembers or panel zones.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the members and connections shall beevaluated using the m-factors in Table 4-3.



Commentary:

Columns that carry a substantial amount of gravityload may have limited additional capacity to resistseismic forces. When axial forces due to seismicoverturning moments are added, the columns maybuckle in a nonductile manner due to excessive axialcompression.

The alternative calculation of overturning stressesdue to seismic forces alone is intended to provide ameans of screening out frames with high gravityloads, but are known to have small seismicoverturning forces.

When both demands are large, the combined effectof gravity and seismic forces must be calculated todemonstrate compliance.

Commentary:

Prior to the 1994 Northridge earthquake, steelmoment-resisting frame connections generallyconsisted of full penetration flange welds and abolted or welded shear tab connection at the web.This type of connection, (see Figure 4-15 on thefollowing page) which was an industry standardfrom 1970 to 1995, was thought to be ductile andcapable of developing the full capacity of the beamsections. However, over 200 buildings experiencedextensive brittle damage to this type of connectionduring the Northridge earthquake. As a result, anemergency code change was made to the 1994UBC (ICBO, 1994) removing the prequalificationof this type of connection. The reasons for this

Commentary:

Moment-resisting frames are more flexible thanshear wall or braced frame structures. Thisflexibility can lead to large interstory drifts thatmay potentially cause extensive structural andnonstructural damage to welded beam-columnconnections, partitions, and cladding. Drifts mayalso induce large P-∆ demands, and pounding whenadjacent buildings are present.

An analysis of non-compliant frames is required todemonstrate the adequacy of frame elementssubjected to excessive lateral drifts.

Figure 4-15. Northridge-Type Connection

4.4.1.3.4 PANEL ZONES: All panel zones shallhave the shear capacity to resist the shear demandrequired to develop 0.8ΣMp of the girders framingin at the face of the column.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thedemands in non-compliant joints shall be calculatedand the adequacy of the panel zones for web shearshall be evaluated using the m-factors in Table 4-3.

4.4.1.3.5 COLUMN SPLICES: All column splicedetails located in moment resisting frames shallinclude connection of both flanges and the web forLife Safety, and the splice shall develop thestrength of the column for Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thegravity and seismic demands shall be calculated andthe adequacy of the splice connection shall beevaluated.



unexpected performance are still underinvestigation. A full discussion various fracturemechanisms and ways of preventing or repairingthem is given in FEMA 267 (SAC, 1995) andFEMA 267A (SAC, 1997).

For this Handbook, the Tier 1 evaluation statementis considered non-compliant for full penetrationflange welds and a more detailed analysis isrequired to determine the adequacy of thesemoment-resisting connections.

Commentary:

Panel zones with thin webs may yield or bucklebefore developing the capacity of the adjoiningmembers, reducing the inelastic performance andductility of the moment frames.

When panel zones cannot develop the strength ofthe beams, compliance can be demonstrated bychecking the panel zones for actual shear demands.

Commentary:

The lack of a substantial connection at the splicelocation may lead to separation of the splicedsections and misalignment of the columns resultingin loss of vertical support and partial or totalcollapse of the building. Tests onpartial-penetration weld splices have shown limitedductility.

An inadequate connection also reduces the effectivecapacity of the column. Splices are checkedagainst calculated demands to demonstratecompliance.

4.4.1.3.6 STRONG COLUMN/WEAK BEAM:The percent of strong column/weak beam joints ineach story of each line of moment resisting framesshall be greater than 50% for Life Safety and 75%for Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the columns to resist calculated demandsshall be evaluated using an m-factor equal to 2.5.Alternatively, the story strength shall be calculated,and checked for the capacity to resist one half the totalpseudo lateral force.

4.4.1.3.7 COMPACT MEMBERS: All momentframe elements shall meet compact sectionrequirements set forth by the Load and ResistanceFactor Design Specification For Structural SteelBuildings (AISC, 1993). This statement shall applyto the Immediate Occupancy Performance Levelonly.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of non-compliant beams and columns shallbe evaluated using the m-factors in Table 4-3.

4.4.1.3.8 BEAM PENETRATIONS: All openingsin frame-beam webs shall be less than 1/4 of thebeam depth and shall be located in the center halfof the beams. This statement shall apply to theImmediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theshear and flexural demands on non-compliant beamsshall be calculated. The adequacy of the beamsconsidering the strength around the penetrations shallbe evaluated.



Commentary:

Noncompact frame elements may experiencepremature local buckling prior to development oftheir full moment capacities. This can lead to poorinelastic behavior and ductility.

The adequacy of the frame elements can bedemonstrated using reduced m-factors inconsideration of reduced capacities for noncompactsections.

Commentary:

Members with large beam penetrations may fail inshear prior to the development of their full momentcapacity, resulting in poor inelastic behavior andductility.

Commentary:

When columns are not strong enough to forcehinging in the beams, column hinging can lead tostory mechanisms and a concentration of inelasticactivity at a single level. Excessive story drifts mayresult in an instability of the frame due to P-∆effects. Good post-elastic behavior consists ofyielding distributed throughout the frame. A storymechanism will limit forces in the levels above,preventing the upper levels from yielding. Joints atthe roof level need not be considered.

If it can be demonstrated that non-compliantcolumns are strong enough to resist calculateddemands with sufficient overstrength, acceptablebehavior can be expected.

The alternative procedure checks for the formationof a story mechanism. The story strength is thesum of the shear capacities of all the columns aslimited by the controlling action. If the columns areshear critical, a shear mechanism forms at the shearcapacity of the columns. If the columns arecontrolled by flexure, a flexural mechanism formsat a shear corresponding to the flexural capacity.

Should additional study be required, a Tier 3evaluation would include a non-linear pushoveranalysis. The formation of a story mechanismwould be acceptable, provided the targetdisplacement is met .

4.4.1.3.9 GIRDER FLANGE CONTINUITYPLATES: There shall be girder flange continuityplates at all moment resisting frame joints. Thisstatement shall apply to the Immediate OccupancyPerformance Level only.

Tier 2 Evaluation Procedure: The adequacy of thecolumn flange to transfer girder flange forces to thepanel zone without continuity plates shall beevaluated.

4.4.1.3.10 OUT-OF-PLANE BRACING:Beam-column joints shall be braced out-of-plane.This statement shall apply to the ImmediateOccupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theaxial demands on non-compliant columns shall becalculated and the adequacy of the column to resistbuckling between points of lateral support shall beevaluated considering a horizontal out-of-plane forceequal to 6% of the critical column flange compressionforce acting concurrently at the non-compliant joint.

4.4.1.3.11 BOTTOM FLANGE BRACING: The bottom flange of beams shall be bracedout-of-plane. This statement shall apply to theImmediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the beams shall be evaluated consideringthe potential for lateral torsional buckling of thebottom flange between points of lateral support.



Commentary:

The lack of girder flange continuity plates maylead to a premature failure at the column web orflange at the joint. Beam flange forces aretransferred to the column web through the columnflange, resulting in a high stress concentration atthe base of the column web. The presence ofcontinuity plates, on the other hand, transfers thebeam flange forces along the entire length of thecolumn web.

Adequate force transfer without continuity plateswill depend on the strength and stiffness of thecolumn flange in weak-way bending.

Commentary:

Joints without proper bracing may buckleprematurely out-of-plane before the strength of thejoint can be developed. This will limit the ability ofthe frame to resist seismic forces.

The combination of axial load and moment on thecolumns will result in higher compression forces inone of the column flanges. The tendency for highlyloaded joints to twist out-of-plane is due tocompression buckling of the critical columncompression flange.

Compliance can be demonstrated if the columnsection can provide adequate lateral restraint forthe joint between points of lateral support.

The critical section is at the penetration with thehighest shear demand. Shear transfer across theweb opening will induce secondary moments in thebeam sections above and below the opening thatmust be considered in the analysis.

4.4.1.4 Concrete Moment Frames

4.4.1.4.1 SHEAR STRESS CHECK : The shearstress in the concrete columns, calculated using theQuick Check Procedure of Section 3.5.3.2, shall beless than 100 psi or 2 or Life Safety andf cImmediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the concrete frame elements shall beevaluated using the m-factors in Table 4-4.



capacity of all elements exceeds the shearassociated with flexural capacity,

2. Concrete confinement is provided by beamstirrups and column ties in the form ofclosed hoops with 135-degree hooks atlocations where plastic hinges will occur.

3. Overall performance is enhanced by longlap splices that are restricted to favorablelocations and protected with additionaltransverse reinforcement.

4. The strong column/weak beam requirementis achieved by suitable proportioning of themembers and their longitudinal reinforcing.

Older frame systems that are lightly reinforced,precast concrete frames, and flat slab framesusually do not meet the detail requirements forductile behavior.

Commentary:

The shear stress check provides a quick assessmentof the overall level of demand on the structure.The concern is the overall strength of the building.

Commentary:

Concrete moment frame buildings typically aremore flexible than shear wall buildings. Thisflexibility can result in large interstory drifts thatmay lead to extensive nonstructural damage andP-delta effects. If a concrete column has a capacityin shear that is less than the shear associated withthe flexural capacity of the column, brittle columnshear failure may occur and result in collapse. Thiscondition is common in buildings in zones ofmoderate seismicity and in older buildings in zonesof high seismicity. The columns in these buildingsoften have ties at standard spacing equal to thedepth of the column, whereas current codemaximum spacing for shear reinforcing is d/2. Thefollowing are the characteristics of concretemoment frames that have demonstrated acceptableseismic performance:

1. Brittle failure is prevented by providing asufficient number of beam stirrups, columnties, and joint ties to ensure that the shear

Commentary:

Beams flanges in compression require out-of-planebracing to prevent lateral torsional buckling.Buckling will occur before the full strength of thebeam is developed, and the ability of the frame toresist lateral forces will be limited.

Top flanges are typically braced by connection tothe diaphragm. Bottom flange bracing occurs atdiscrete locations, such as at connection points forsupported beams. The spacing of bottom flangebracing may not be close enough to preventpremature lateral torsional buckling when seismicloads induce large compression forces in thebottom flange.

Note that this condition is not considered alife-safety concern, and need only be examined forthe Immediate Occupancy Performance Level.

4.4.1.4.2 AXIAL STRESS CHECK: The axialstress due to gravity loads in columns subjected tooverturning forces shall be less than 0.10f'c for LifeSafety and Immediate Occupancy. Alternatively,the axial stress due to overturning forces alone,calculated using the Quick Check Procedure ofSection 3.5.3.6, shall be less than 0.30f'c for LifeSafety and Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thegravity and overturning demands for non-compliantcolumns shall be calculated and the adequacy of thecolumns to resist overturning forces shall be evaluatedusing the m-factors in Table 4-4.

4.4.1.4.3 FLAT SLAB FRAMES: The lateral-force-resisting system shall not be a frameconsisting of columns and a flat slab/plate withoutbeams.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the slab-column system for resistingseismic forces and punching shear shall be evaluatedusing the m-factors in Table 4-4.

4.4.1.4.4 PRESTRESSED FRAME ELEMENTS:The lateral-load-resisting frames shall not includeany prestressed or post-tensioned elements.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the concrete frame including prestressedelements shall be evaluated using the m-factors inTable 4.4.

4.4.1.4.5 SHORT CAPTIVE COLUMNS: Thereshall be no columns at a level with height/depthratios less than 50% of the nominal height/depthratio of the typical columns at that level for LifeSafety and 75% for Immediate Occupancy.

Tier 2 Evaluation Procedure: The adequacy of thecolumns for the shear force required to develop themoment capacity at the top and bottom of the clearheight of the columns shall be evaluated.Alternatively, evaluate the columns as force controlledelements in accordance with the alternative equationsin Section 4.2.4.3.2.



Commentary:

The concern is the transfer of the shear andbending forces between the slab and column, whichcould result in a punching shear failure and partialcollapse. The flexibility of thelateral-force-resisting system will increase as theslab cracks.

Continuity of some bottom reinforcement throughthe column joint will assist in the transfer of forcesand provide some resistance to collapse bycatenary action in the event of a punching shearfailure.

Commentary:

Frame elements that are prestressed orpost-tensioned may not behave in a ductile manner.The concern is the inelastic behavior of prestressedelements.

Commentary:

Columns that carry a substantial amount ofgravity load may have limited additionalcapacity to resist seismic forces. When axialforces due to seismic overturning moments areadded, the columns may crush in a nonductilemanner due to excessive axial compression.

The alternative calculation of overturningstresses due to seismic forces alone is intendedto provide a means of screening out frames withhigh gravity loads, but are known to have smallseismic overturning forces.

When both demands are large, the combined effectof gravity and seismic forces must be calculated todemonstrate compliance.

4.4.1.4.6 NO SHEAR FAILURES: The shearcapacity of frame members shall be able to developthe moment capacity at the top and bottom of thecolumns.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theshear demands shall be calculated for non-compliantcolumns and the adequacy of the columns for shearshall be evaluated.

4.4.1.4.7 STRONG COLUMN/WEAK BEAM:The sum of the moment capacity of the columnsshall be 20% greater than that of the beams atframe joints.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the columns to resist calculated demandsshall be evaluated using an m-factor equal to 2.0.Alternatively, the story strength shall be calculated,and checked for the capacity to resist one half the totalpseudo lateral force.



Commentary:

If the shear capacity of a column is reached beforethe moment capacity, there is a potential for asudden non-ductile failure of the column, leading tocollapse.

Columns that cannot develop the flexural capacityin shear should be checked for adequacy againstcalculated shear demands. Note that the shearcapacity is affected by the axial loads on thecolumn and should be based on the most criticalcombination of axial load and shear.

Commentary:

When when columns are not strong enough to forcehinging in the beams, column hinging can lead tostory mechanisms and a concentration of inelasticactivity at a single level. Excessive story driftsmay result in an instability of the frame due to P-∆effects. Good post-elastic behavior consists ofyielding distributed throughout the frame. A storymechanism will limit forces in the levels above,preventing the upper levels from yielding. Joints atthe roof level need not be considered.

If it can be demonstrated that non-compliantcolumns are strong enough to resist calculateddemands with sufficient overstrength, acceptablebehavior can be expected. Reduced m-factors areused to check the columns at near elastic levels.

The alternative procedure checks for the formationof a story mechanism. The story strength is thesum of the shear capacities of all the columns aslimited by the controlling action. If the columnsare shear critical, a shear mechanism forms at theshear capacity of the columns. If the columns arecontrolled by flexure, a flexural mechanism formsat a shear corresponding to the flexural capacity.

Commentary:

Short captive columns tend to attract seismicforces because of high stiffness relative to othercolumns in a story. Significant damage has beenobserved in parking structure columns adjacent toramping slabs, even in structures with shear walls.Captive column behavior may also occur inbuildings with clerestory windows, or in buildingswith partial height masonry infill panels.

If not adequately detailed, the columns may suffera non-ductile shear failure which may result inpartial collapse of the structure.

A captive column that can develop the shearcapacity to develop the flexural strength over theclear height will have some ductility to preventsudden non-ductile failure of the vertical supportsystem.

4.4.1.4.8 BEAM BARS: At least two longitudinaltop and two longitudinal bottom bars shall extendcontinuously throughout the length of each framebeam. At least 25% of the longitudinal barsprovided at the joints for either positive or negativemoment, shall be continuous throughout the lengthof the members for Life Safety and ImmediateOccupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theflexural demand at several sections along the length ofthe non-compliant beams shall be calculated, and theadequacy of the beams shall be evaluated using anm-factor equal to 1.0.

4.4.1.4.9 COLUMN-BAR SPLICES: All columnbar lap splice lengths shall be greater than 35 db forLife Safety and 50 db for Immediate Occupancy,and shall be enclosed by ties spaced at or less than8 db for Life Safety and Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theflexural demands at non-compliant column splicesshall be calculated and the adequacy of the columnsshall be evaluated using the m-factors in Table 4-4.

4.4.1.4.10 BEAM-BAR SPLICES: The lap splicesfor longitudinal beam reinforcing shall not belocated within lb/4 of the joints and shall not belocated in the vicinity of potential plastic hingelocations.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theflexural demands in non-compliant beams shall becalculated and the adequacy of the beams shall beevaluated using the m-factors for non-ductile beams inTable 4-4.



Commentary:

Located just above the floor level, column barsplices are typically located in regions of potentialplastic hinge formation. Short splices are subjectto sudden loss of bond. Widely spaced ties canresult in a spalling of the concrete cover and lossof bond. Splice failures are sudden andnon-ductile.

Columns with non-compliant lap splices arechecked using reduced m-factors to account forthis potential lack of ductility. If the membershave sufficient capacity, the demands on thesplices are less likely to exceed the capacity of thebond.

Commentary:

Lap splices located at the end of beams and invicinity of potential plastic hinges may not be able

Commentary:

The requirement for two continuous bars is acollapse prevention measure. In the event ofcomplete beam failure, continuous bars willprevent total collapse of the supported floor,holding the beam in place by catenary action.

Previous construction techniques used bent uplongitudinal bars as reinforcement. These barstransitioned from bottom to top reinforcement atthe gravity load inflection point. Some amount ofcontinuous top and bottom reinforcement isdesired because moments due to seismic forces canshift the location of the inflection point.

Because non-compliant beams are vulnerable tocollapse, the beams are required to resist demandsat an elastic level. Continuous slab reinforcementadjacent to the beam may be considered ascontinuous top reinforcement.

4.4.1.4.11 COLUMN-TIE SPACING: Framecolumns shall have ties spaced at or less than d/4for Life Safety and Immediate Occupancythroughout their length and at or less than 8 db forLife Safety and Immediate Occupancy at allpotential plastic hinge locations.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theflexural demand in non-compliant columns shall becalculated and the adequacy of the columns shall beevaluate using the m-factors in Table 4-4.

4.4.1.4.12 STIRRUP SPACING: All beams shallhave stirrups spaced at or less than d/2 for LifeSafety and Immediate Occupancy throughout theirlength. At potential plastic hinge locations stirrupsshall be spaced at or less than the minimum of 8 dbor d/4 for Life Safety and Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theflexural demand in non-compliant beams shall becalculated and the adequacy of the beams shall beevaluate using the m-factors in Table 4-4.

4.4.1.4.13 JOINT REINFORCING: Beam-column joints shall have ties spaced at or less than8db for Life Safety and Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thejoint shear demands shall be calculated and theadequacy of the joint to develop the adjoiningmembers forces shall be evaluated.



Commentary:

Beam-column joints without shear reinforcementmay not be able to develop the strength of theconnected members, leading to a non-ductilefailure of the joint. Perimeter columns areespecially vulnerable because the confinement ofjoint is limited to three sides (along the exterior) ortwo sides (at a corner).

Commentary:

Widely spaced ties will reduce the ductility of thecolumn, and it may not be able to maintain fullmoment capacity through several cycles. Columnswith widely spaced ties have limited shear capacityand non-ductile shear failures may result.

Elements with non-compliant confinement arechecked using reduced m-factors to account for thispotential lack of ductility.

to develop the full moment capacity of the beam asthe concrete degrades during multiple cycles.

Beams with non-compliant lap splices are checkedusing reduced m-factors to account for thispotential lack of ductility. If the members havesufficient capacity, the demands are less likely tocause degradation and loss of bond betweenconcrete and the reinforcing steel.

Commentary:

Widely spaced ties will reduce the ductility of thecolumn, and it may not be able to maintain fullmoment capacity through several cycles. Columnswith widely spaced ties have limited shear capacityand non-ductile shear failures may result.

Elements with non-compliant confinement arechecked using reduced m-factors to account for thispotential lack of ductility.

4.4.1.4.14 JOINT ECCENTRICITY: There shallbe no eccentricities larger than 20% of the smallestcolumn plan dimension between girder and columncenterlines for Immediate Occupancy. Thisstatement shall apply to the Immediate OccupancyPerformance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thejoint shear demands including additional shear stressesfrom joint torsion shall be calculated and the adequacyof the beam-column joints shall be evaluated.

4.4.1.4.15 STIRRUP AND TIE HOOKS: Thebeam stirrups and column ties shall be anchoredinto the member cores with hooks of 135° or more.This statement shall apply to the ImmediateOccupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theshear and axial demands in non-compliant membersshall be calculated and the adequacy of the beams andcolumns shall be evaluated using the m-factors inTable 4-4.

4.4.1.5 Precast Concrete Moment Frames

4.4.1.5.1 PRECAST CONNECTION CHECK:The precast connections at frame joints shall havethe capacity to resist the shear and momentdemands calculated using the Quick CheckProcedure of Section 3.5.3.5.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the precast connections shall be evaluatedas force controlled elements using the procedures inSection 4.2.4.3.2.



Commentary:

Precast frame elements may have sufficientstrength to meet lateral force requirements, butconnections often cannot develop the strength of themembers, and may be subject to prematurenon-ductile failures. Failure mechanisms mayinclude fractures in the welded connections betweeninserts, pull out of embeds, and spalling ofconcrete.

Since full member capacities cannot be realized,the behavior of this system is entirely dependent onthe performance of the connections.

Commentary:

Joint eccentricities can result in high torsionaldemands on the joint area, which will result inhigher shear stresses.

Commentary:

To be fully effective, stirrups and ties must beanchored into the confined core of the member.90o hooks that are anchored within the concretecover are unreliable if the cover spalls duringplastic hinging. The amount of shear resistanceand confinement will be reduced if the stirrups andties are not well anchored.

Elements with non-compliant confinement arechecked using reduced m-factors to account forthis potential lack of ductility.

The shear capacity of the joint my be calculatedas follows:

Qcl=λγAj(f'c)1/2 psi, where γ is:

ρ''<0.003 ρ''>0.003

Int. joints w/ transverse beams 12 20Int. joints w/o transverse beams 10 15Ext. joints w/ transverse beams 8 15Ext. joints w/o transverse beams 6 12Corner joints 4 8λ = 0.75 for lightweight concreteAj = joint cross-sectional area

4.4.1.5.2 PRECAST FRAMES: For buildingswith concrete shear walls, lateral forces shall not beresisted by precast concrete frame elements.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the precast frame elements shall beevaluated as force controlled elements using theprocedures in Section 4.2.4.3.2.

4.4.1.5.3 PRECAST CONNECTIONS: Forbuildings with concrete shear walls, the connectionbetween precast frame elements such as chords,ties, and collectors in the lateral-force-resistingsystem shall develop the capacity of the connectedmembers.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the connections for seismic forces shall beevaluated as force controlled elements using theprocedures in Section 4.2.4.3.2.

4.4.1.6 Frames Not Part of theLateral-Force-Resisting System



Commentary:

This section deals with secondary componentsconsisting of frames that were not designed to bepart of the lateral-force-resisting system. These arebasic structural frames of steel or concrete that aredesigned for gravity loads only. Shear walls orother vertical elements provide the resistance tolateral forces. In actuality, however, all frames actas part of the lateral-force-resisting system. Lateraldrifts of the building will induce forces in thebeams and columns of the secondary frames.Furthermore, in the event that the primary elementsfail, the secondary frames become the primarylateral force resisting components of the building.

If the walls are concrete (infilled in steel frames ormonolithic in concrete frames), the building shouldbe treated as a concrete shear wall building (TypesC2 or C2A) with the frame columns as boundaryelements. If the walls are masonry infills, theframes should be treated as steel or concrete frameswith infill walls of masonry (Types S5, S5A, C3 orC3A). Research is continuing on the behavior ofinfill frames. Lateral forces are resisted bycompression struts that develop in the masonryinfill and induce forces on the frame elementseccentric to the joints.

The concern for secondary frames is the potentialloss of vertical-load-carrying capacity due toexcessive deformations and p-delta effects.

Commentary:

Precast frame elements may have sufficientstrength to meet lateral force requirements, butconnections often cannot develop the strength of themembers, and may be subject to prematurenon-ductile failures. Failure mechanisms mayinclude fractures in the welded connections between

Commentary:

Precast frame elements may have sufficientstrength to meet lateral force requirements, butconnections often cannot develop the strength of themembers, and may be subject to prematurenon-ductile failures. Failure mechanisms mayinclude fractures in the welded connections betweeninserts, pull out of embeds, and spalling ofconcrete.


between inserts, pull out of embeds, and spalling ofconcrete.


4.4.1.6.1 COMPLETE FRAMES: Steel orconcrete frames classified as secondary componentsshall form a complete vertical load carrying system.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thegravity and seismic demands for the shear walls shallbe calculated and the adequacy of the shear walls shallbe evaluated.

Figure 4-16. Incomplete Frame

4.4.1.6.2 DEFLECTION COMPATIBILITY:Secondary components shall have the shearcapacity to develop the flexural strength of theelements for Life Safety and shall have ductiledetailing for Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theflexural and shear demands at maximum interstorydrifts for non-compliant elements shall be calculatedand the adequacy of the elements shall be evaluated.

4.4.1.6.3 FLAT SLABS: Flat slab/plates classifiedas secondary components shall have continuousbottom steel through the column joints for LifeSafety. Flat slabs/plates shall not be permitted forthe Immediate Occupancy Performance Level.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the joint for punching shear for all gravityand seismic demands, and shear transfer due toseismic moments, shall be evaluated.



Commentary:

Frame components, especially columns, that arenot specifically designed to participate in thelateral-force-resisting system will still undergodisplacements associated with overall seismicinterstory drifts. If the columns are located somedistance away from the lateral-force-resistingelements, the added deflections due to semi-rigidfloor diaphragms will increase the drifts. Stiffcolumns, designed for potentially high gravityloads, may develop significant bending momentsdue to the imposed drifts. The moment-axial forceinteraction may lead to a nonductile failure of thecolumns and a collapse of the building.

Commentary:

If the frame does not form a complete vertical loadcarrying system, the walls will be required toprovide vertical support as bearing walls. (seeFigure 4-16). A frame is incomplete if there are nocolumns cast into the wall, there are no columnsadjacent to the wall, and beams frame into the wall,supported solely by the wall.

During an earthquake, shear walls might becomedamaged by seismic forces, limiting their ability tosupport vertical loads. Loss of vertical supportmay lead to partial collapse.

Compliance can be demonstrated if the wall isjudged adequate for combined vertical and seismicforces.

WALL

`MISSING'MEMBERS

=

Commentary:

Flat slabs not designed to participate in thelateral-force-resisting system may still experienceseismic forces due to displacements associated with

Figure 4-17. Continuous Bottom Steel

4.4.2 Shear Walls

4.4.2.1 General



ContinuousBottomSteel

Commentary:

Shear walls, as the name implies, resist lateralforces primarily in shear. In the analysis of shearwalls, it is customary to consider the shear takenby the length of the wall and the flexure taken byvertical reinforcement added at each end, much asflexure in diaphragms is designed to be taken bychords at the edges. Squat walls that are longcompared to their height, are dominated byshearing behavior. Flexural forces require only aslight local reinforcement at each end. Slenderwalls that are tall compared to their length are

overall building drift. The concern is the transfer ofthe shear and bending forces between the slab andcolumn, which could result in a punching shearfailure.

Continuity of some bottom reinforcement throughthe column joint will assist in the transfer of forcesand provide some resistance to collapse bycatenary action in the event of a punching shearfailure (see Figure 4-17). Bars can be consideredcontinuous if they have proper lap splices,mechanical couplers, or are developed beyond thesupport.

usually dominated by flexural behavior, and mayrequire substantial boundary elements at each end.

It is a good idea to sketch a complete free-bodydiagram of the wall (as indicated in Figure 4-18) sothat no forces are inadvertently neglected. An erroroften made in the design of wood shear walls is totreat the walls one story at a time, considering onlythe shear force in the wall and overlooking theaccumulation of overturning forces from the storiesabove.

When the earthquake direction being considered isparallel to a shear wall, the wall develops in-planeshear and flexural forces as described above.When the earthquake direction is perpendicular to ashear wall, the wall contributes little to the lateralforce resistance of the building and the wall issubjected to out-of-plane forces tending to separateit from the rest of the structure. This sectionaddresses the in-plane behavior of shear walls.Out-of-plane strength and anchorage of shear wallsto the structure is addressed in Section 4.5.

Solid shear walls usually have sufficient strength,though they may be lightly reinforced. Problemswith shear wall systems arise when walls are notcontinuous to the foundation, or when numerousopenings break the walls up into small piers withlimited shear and flexural capacity.

Figure 4-18. Wall Free-Body Diagram.

4.4.2.1.1 REDUNDANCY: The number of lines ofshear walls in each direction shall be greater thanor equal to 2 for Life Safety and for ImmediateOccupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with the procedures in Section 4.2 shall beperformed. The adequacy of all walls and connectionsshall be evaluated.

Figure 4-19. Redundancy in Shear Walls

4.4.2.2 Concrete Shear Walls



Commentary:

In highly redundant buildings with many longwalls, stresses in concrete shear walls are usuallylow. In less redundant buildings with largeopenings and slender walls, the stresses can behigh. In the ultimate state, when overturning forcesare at their highest, a thin wall may fail in bucklingalong the compression edge, or it may fail intension along the tension edge. Tension failuresmay consist of slippage in bar lap splices, or baryield and fracture if adequate lap splices have beenprovided.

In the past, designs have been based on liberalassumptions about compression capacity, and havesimply packed vertical rebar into the ends of thewalls to resist the tensile forces. Recent codes,recognizing the importance of boundary members,

A distinction should be made between redundancyand adequacy. For the purpose of this Handbook,redundancy is intended to mean simply "more thanone". That is not to say that for large buildingstwo elements is adequate, or for small buildingsone is not enough. Separate evaluation statementsare present in the Handbook to determine theadequacy of the elements provided.


Commentary:

Redundancy is a fundamental characteristic oflateral force resisting systems with superior seismicperformance. Redundancy in the structure willensure that if an element in the lateral forceresisting system fails for any reason, there isanother element present that can provide lateralforce resistance. Redundancy also providesmultiple locations for potential yielding,distributing inelastic activity throughout thestructure and improving ductility and energyabsorption. Typical characteristics of redundancyinclude multiple lines of resistance to distribute thelateral forces uniformly throughout the structure,(see Figure 4-19) and multiple bays in each line ofresistance to reduce the shear and axial demands onany one element.

Fa

Fb

V

F

V+F

SHEAR FROM

ABOVE

STORYFORCE

Fb

Fa

OVER-TURNING

FORCE

P

Figure 4-20. Boundary Elements.

4.4.2.2.1 SHEAR STRESS CHECK : The shearstress in the concrete shear walls, calculated usingthe Quick Check procedure of Section 3.5.3.3, shallbe less than 100 psi or 2 for Life Safety andf cImmediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the concrete shear wall elements shall beevaluated using the m-factors in Table 4-4.

4.4.2.2.2 REINFORCING STEEL: The ratio ofreinforcing steel area to gross concrete area shallbe greater than 0.0015 in the vertical direction and0.0025 in the horizontal direction for Life Safetyand Immediate Occupancy. The spacing ofreinforcing steel shall be equal to or less than 18"for Life Safety and for Immediate Occupancy.


4.4.2.2.3 COUPLING BEAMS: The stirrups in allcoupling beams over means of egress shall bespaced at or less than d/2 and shall be anchoredinto the core with hooks of 135° or more for LifeSafety and Immediate Occupancy. In addition, thebeams shall have the capacity in shear to developthe uplift capacity of the adjacent wall forImmediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theshear and flexural demands on non-compliant couplingbeams shall be calculated and the adequacy of thecoupling beams shall be evaluated. If the couplingbeams are inadequate, the adequacy of the coupledwalls shall be evaluated as if they were independent.



Commentary:

If the walls do not have sufficient reinforcing steel,they will have limited capacity in resisting seismicforces. The wall will also behave in a nonductilemanner for inelastic forces.

Commentary:

The shear stress check provides a quick assessmentof the overall level of demand on the structure. Theconcern is the overall strength of the building.

E D G E B A R

B A R G R O U P

B A R C A G E

have special requirements for proportions, barsplices, and transverse reinforcement. Examples ofboundary members with varying amounts ofreinforcing are shown in Figure 4-20. Existingbuildings often do not have these elements, and theacceptance criteria are designed to allow for this.

Another development in recent codes is therequirement to provide shear strength compatiblewith the flexural capacity of the wall to ensureductile flexural yielding prior to brittle shearfailure. Long continuous walls and walls withembedded steel or large boundary elements canhave high flexural capacities with the potential toinduce correspondingly high shear demands thatare over and above the minimum design sheardemands.

Figure 4-21. Coupled Walls.

4.4.2.2.4 OVERTURNING: All shear walls shallhave aspect ratios less than 4 to 1. Wall piers neednot be considered. This statement shall apply to theImmediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theoverturning demands for non-compliant walls shall becalculated and the adequacy of the shear walls shall beevaluated.

4.4.2.2.5 CONFINEMENT REINFORCING: Forshear walls with aspect ratios greater than 2.0, theboundary elements shall be confined with spirals orties with spacing less than 8 db. This statementshall apply to the Immediate OccupancyPerformance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theshear and flexural demands on the non-compliantwalls shall be calculated and the adequacy of the shearwalls shall be evaluated.

4.4.2.2.6 REINFORCING AT OPENINGS:There shall be added trim reinforcement around all



Commentary:

Fully effective shear walls require boundaryelements to be properly confined with closelyspaced ties (see Figure 4-20). Degradation of theconcrete in the vicinity of the boundary elementscan result in buckling of rebar in compression andfailure of lap splices in tension. Non-ductile failureof the boundary elements will lead to reducedcapacity to resist overturning forces.

COUPLINGBEAMS

BEAM

WALL WALL

FOUNDATION

Commentary:

Tall, slender shear walls may have limitedoverturning resistance. Displacements at the top ofthe building will be greater than anticipated ifoverturning forces are not properly resisted.

Often sufficient resistance can be found inimmediately adjacent bays, if a load path is presentto activate the adjacent column dead loads.

Commentary:

Coupling beams with sufficient strength andstiffness can increase the lateral stiffness of thesystem significantly beyond the stiffnesses of theindependent walls. When the walls deflectlaterally, large moments and shears are induced inthe coupling beams as they resist the imposeddeformations. Coupling beams also link thecoupled walls for overturning resistance (seeFigure 4-21).

Coupling beam reinforcement is often inadequatefor the demands that can be induced by themovement of the coupled walls. Seismic forcesmay damage and degrade the beams so severelythat the system degenerates into a pair ofindependent walls. This changes the distribution ofoverturning forces which may result in potentialstability problems for the independent walls. Theboundary reinforcement may also be inadequate forflexural demands if the walls act independently.

If the beams are lightly reinforced, theirdegradation could result in falling debris that is apotential life-safety hazard, especially at locationsof egress.

openings. This statement shall apply to theImmediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theflexural and shear demands around the openings shallbe calculated and the adequacy of the piers andspandrels shall be evaluated.

Figure 4-22. Conventional Trim Steel

4.4.2.2.7 WALL THICKNESS: Thickness ofbearing walls shall not be less than 1/25 theminimum unsupported height or length, nor lessthan 4". This statement shall apply to theImmediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: The adequacy of thewalls to resist out-of-plane forces in combination withvertical loads shall be evaluated.

4.4.2.2.8 WALL CONNECTIONS: There shall bea positive connection between the shear walls andthe steel beams and columns for Life Safety, andthe connection shall be able to develop the strengthof the walls for Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theshear and flexural demands on the shear walls shall becalculated and the adequacy of the connection totransfer shear between the walls and the steel frameshall be evaluated.



Commentary:

Slender bearing walls may have limited capacityfor vertical loads and higher potential for damagedue to out-of-plane forces and magnified moments.Note that this condition is not considered alife-safety concern and need only be examined forthe Immediate Occupancy performance level.

Commentary:

Insufficient shear transfer between the steel andconcrete elements will limit the ability of the steelto contribute to the performance of the shear walls.The connections to the column are especiallyimportant as the columns will develop a portion ofthe shear wall overturning moment. Theconnections should include welded studs, weldedreinforcing steel, or fully encased steel elementswith longitudinal reinforcing and ties.

Commentary:

Conventional trim steel is adequate only for smallopenings (see Figure 4-22). Large openings willcause significant shear and flexural stresses in theadjacent piers and spandrels. Inadequatereinforcing steel around these openings will lead tostrength deficiencies, nonductile performance anddegradation of the wall.

4.4.2.2.9 COLUMN SPLICES: Steel columnsencased in shear wall boundary elements shall havesplices that develop the tensile strength of thecolumn. This statement shall apply to theImmediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thetension demands due to overturning forces onnon-compliant columns shall be calculated and theadequacy of the splice connections shall be evaluated.

4.4.2.3 Precast Concrete Shear Walls

4.4.2.3.1 SHEAR STRESS CHECK : The shearstress in the precast panels, calculated using theQuick Check Procedure of Section 3.5.3.3, shall beless than 100 psi or 2 for Life Safety andf cImmediate Occupancy.


4.4.2.3.2 REINFORCING STEEL: The ratio ofreinforcing steel area to gross concrete area shallbe greater than 0.0015 in the vertical direction and0.0025 in the horizontal direction for Life Safetyand Immediate Occupancy. The spacing ofreinforcing steel shall be equal to or less than 18"for Life Safety and Immediate Occupancy.




Commentary:

Columns encased in shear wall boundary elementsmay be subjected to high tensile forces due to shearwall overturning moments. If the splice cannotdevelop the strength of the column, the ability ofthe column to contribute to overturning resistancewill be limited.

The presence of axial loads may reduce the nettensile demand on the boundary element columns toa level below the capacity of the splice.

Commentary:

Precast concrete shear walls are constructed insegments that are usually interconnected byembedded steel elements. These connectionsusually possess little ductility, but are important tothe overall behavior of the wall assembly.Interconnection between panels increases theoverturning capacity by transferring overturningdemands to end panels. Panel connections at thediaphragm are often used to provide continuousdiaphragm chords. Failure of these connectionswill reduce the capacity of the system.

Commentary:


Commentary:


Shear friction between the concrete and steelshould only be used when the steel is completelyencased in the concrete.

4.4.2.3.3 WALL OPENINGS: Openings shallconstitute less than 75% of the length of anyperimeter wall for Life Safety and 50% forImmediate Occupancy with the wall piers havingaspect ratios of less than 2.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the remaining wall shall be evaluated forshear and overturning resistance, and the adequacy ofthe shear transfer connection between the diaphragmand the wall shall be evaluated. The adequacy of theconnection between any collector elements and thewall shall also be evaluated.

4.4.2.3.4 CORNER OPENINGS: Walls withopenings at a building corner larger than the widthof a typical panel shall be connected to theremainder of the wall with collector reinforcing.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the diaphragm to transfer shear andspandrel panel forces to the remainder of the wallbeyond the opening shall be evaluated.

4.4.2.3.5 PANEL-TO-PANEL CONNECTIONS:Adjacent wall panels shall be interconnected totransfer overturning forces between panels bymethods other than steel welded inserts. Thisstatement shall apply to the Immediate OccupancyPerformance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theoverturning demands shall be calculated and theadequacy of the welded inserts to transfer overturningforces shall be evaluated as force controlled elementsin accordance with Section 4.2.4.3.2.



Commentary:

Welded steel inserts can be brittle and may not beable to transfer the overturning forces betweenpanels. Latent stresses may be present due toshrinkage and temperature effects. Brittle failuremay include weld fracture, pull-out of theembedded anchors, or spalling of the concrete.

Failure of these connections will result inseparation of the wall panels, and a reduction inoverturning resistance.

Commentary:

In tilt-up construction, typical wall panels are oftenof sufficient length that special detailing forcollector elements, shear transfer, and overturningresistance is not provided. Perimeter walls that aresubstantially open, such as at loading docks, havelimited wall length to resist seismic forces, and maybe subject to overturning or shear transferproblems that were not accounted for in theoriginal design.

Walls will be compliant if an adequate load pathfor shear transfer, collector forces and overturningresistance can be demonstrated.

Commentary:

Open corners often are designed as entrances withthe typical wall panel replaced by a spandrel paneland a glass curtain wall. Seismic forces in theseelements are resisted by adjacent panels and,therefore, must be delivered through collectors.

If the spandrel and other wall elements areadequately tied to the diaphragm, panel forces canbe transferred back to adjacent wall panels throughcollector elements in the diaphragm.

4.4.2.3.6 WALL THICKNESS: Thickness ofbearing walls shall not be less than 1/25 theminimum unsupported height or length, nor lessthan 4". This statement shall apply to theImmediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: The adequacy of thewalls to resist out-of-plane forces shall be evaluated.

4.4.2.4 Reinforced Masonry Shear Walls

4.4.2.4.1 SHEAR STRESS CHECK : The shearstress in the reinforced masonry shear walls,calculated using the Quick Check Procedure ofSection 3.5.3.3, shall be less than 50 psi for LifeSafety and Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the reinforced masonry shear wallelements shall be evaluated using the m-factors inTable 4-5.

4.4.2.4.2 REINFORCING STEEL: The totalvertical and horizontal reinforcing steel ratio inreinforced masonry walls shall be greater than0.002 for Life Safety and 0.003 for ImmediateOccupancy with the minimum of 0.0007 for LifeSafety and 0.001 for Immediate Occupancy ineither of the two directions; the spacing ofreinforcing steel shall be less than 48" for LifeSafety and 24" for Immediate Occupancy; and allvertical bars shall extend to the top of the walls.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the reinforced masonry shear wallelements shall be evaluated using the m-factors inTable 4-5.

4.4.2.4.3 REINFORCING AT OPENINGS: Allwall openings that interrupt rebar shall have trimreinforcing on all sides. This statement shall applyto the Immediate Occupancy Performance Levelonly.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theflexural and shear demands around the openings shallbe calculated and the adequacy of the walls shall beevaluated using only the length of the piers betweenreinforcing steel.



Commentary:


Commentary:


Commentary:

Conventional trim steel is adequate only for smallopenings. Large openings will cause significantshearing and flexural stresses in the adjacent piersand spandrels. Inadequate reinforcing steel aroundthese openings will lead to strength deficiencies,non-ductile performance and degradation of thewall.

Commentary:

Slender bearing walls may have limited capacityfor vertical loads and higher potential for damagedue to out-of-plane forces and magnified moments.Note that this condition is not considered alife-safety concern and only needs to be examinedfor the Immediate Occupancy performance level.

4.4.2.4.4 PROPORTIONS: The height-to-thickness ratio of the shear walls at each story shallbe less than 30. This statement shall apply to theImmediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: The adequacy of thewalls to resist out-of-plane forces in combination withvertical loads shall be evaluated.

4.4.2.5 Unreinforced Masonry Shear Walls

4.4.2.5.1 SHEAR STRESS CHECK: The shearstress in the unreinforced masonry shear walls,calculated using the Quick Check Procedure ofSection 3.5.3.3, shall be less than 15 psi for clayunits and 30 psi for concrete units for Life Safetyand Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the unreinforced masonry shear wallelements shall be evaluated using the m-factors inTable 4-5.

4.4.2.5.2 PROPORTIONS: The height-to-thickness ratio of the shear walls at each story shallbe less than the following for Life Safety and forImmediate Occupancy:

Top story of multi-story building: 9First story of multi-story building: 15All other conditions: 13

Tier 2 Evaluation Procedure: No Tier 2 evaluationprocedure is available for unreinforced masonry shearwall proportions in non-compliance. A Tier 3evaluation is necessary to achieve the selectedperformance level.

4.4.2.5.3 MASONRY LAY-UP: Filled collarjoints of multiwythe masonry walls shall havenegligible voids.

Tier 2 Evaluation Procedure: No Tier 2 evaluationprocedure is available for unreinforced masonry shearwall proportions in non-compliance. A Tier 3evaluation is necessary to achieve the selectedperformance level.



Commentary:


Commentary:

Slender unreinforced masonry bearing walls withlarge height-to-thickness ratios have a potential fordamage due to out-of-plane forces which may resultin falling hazards and potential collapse of thestructure.

Commentary:

When walls have poor collar joints, the inner andouter wythes will act independently. The walls maybe inadequate to resist out-of-plane forces due to alack of composite action between the inner andouter wythes.

Mitigation to provide out-of-plane stability andanchorage of the wythes may be necessary toachieve the selected performance level.

Commentary:

Slender bearing walls may have limited capacityfor vertical loads and higher potential for damagedue to out-of-plane forces and magnified moments.Note that this condition is not considered alife-safety concern and need only be examined forthe Immediate Occupancy performance level.

4.4.2.6 Infill Walls in Frames

4.4.2.6.1 WALL CONNECTIONS: All infill wallsshall have a positive connection to the frame toresist out-of-plane forces for Life Safety, and theconnection shall be able to develop the out-of-planestrength of the wall for Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theout-of-plane demands on the wall shall be calculatedand the adequacy of the connection to the frame shallbe evaluated.

Figure 4-23. Infill Wall

4.4.2.6.2 PROPORTIONS: The height-to-thickness ratio of the infill walls at each story shallbe less than 9 for Life Safety in regions of highseismicity, 13 for Immediate Occupancy in regionsof moderate seismicity, and 8 for ImmediateOccupancy in regions of high seismicity.

Tier 2 Evaluation Procedure: No Tier 2 evaluationprocedure is available for unreinforced masonry shearwall proportions in non-compliance. A Tier 3evaluation is necessary to demonstrate compliancewith the selected performance level.



INFILL WALL

GAP ON 3SIDES

SHORT COLUMNS

SHEAR WALL

Commentary:

Slender masonry infill walls with largeheight-to-thickness ratios have a potential fordamage due to out-of-plane forces. Failure of thesewalls out-of-plane will result in falling hazards anddegradation of the strength and stiffness of thelateral force resisting system.

The out-of-plane stability of infill walls isdependent on many factors including flexuralstrength of the wall and confinement provided bythe surrounding frame. If the infill is unreinforced,the flexural strength is limited by the flexuraltension capacity of the material. The surroundingframe will provide confinement, induce infill thrustforces and develop arching action against

Commentary:

Performance of frame buildings with masonry infillwalls is dependent upon the interaction between theframe and infill panels. In-plane lateral forceresistance is provided by a compression strutdeveloping in the infill panel that extendsdiagonally between corners of the frame. If gapsexist between the frame and infill, this strut cannotbe developed (see Figure 4-23). If the infill panelsseparate from the frame due to out-of-plane forces,the strength and stiffness of the system will bedetermined by the properties of the bare frame,which may not be detailed to resist seismic forces.Severe damage or partial collapse due to excessivedrift and p-delta effects may occur.

A positive connection is needed to anchor the infillpanel for out-of-plane forces. In this case, apositive connection can consist of a fully groutedbed joint in full contact with the frame, or completeencasement of the frame by the brick masonry.The mechanism for out-of-plane resistance of infillpanels is discussed in the commentary to Section4.4.2.6.2.

If the connection is non-existent, mitigation withadequate connection to the frame is necessary toachieve the selected performance level.

4.4.2.6.3 SOLID WALLS: The infill walls shall notbe of cavity construction.

Tier 2 Evaluation Procedure: No Tier 2 evaluationprocedure is available for infill walls innon-compliance.

4.4.2.6.4 INFILL WALLS: The infill walls shallbe continuous to the soffits of the frame beams.

Tier 2 Evaluation Procedure: The adequacy of thecolumns adjacent to non-conforming infill walls shallbe evaluated for the shear force required to develop theflexural capacity of the column over the clear heightabove the infill.

4.4.2.7 Walls in Wood-Frame Buildings

4.4.2.7.1 SHEAR STRESS CHECK : The shearstress in the shear walls, calculated using the QuickCheck Procedure of 3.5.3.3, shall be less than thefollowing values for Life Safety and ImmediateOccupancy:

Structural panel sheathing: 1000 plfDiagonal sheathing: 700 plfStraight sheathing: 80 plfAll other conditions: 100 plf

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the wood shear wall elements shall beevaluated using the m-factors in Table 4-6.



Commentary:

When the infill walls are of cavity construction, theinner and outer wythes will act independently. dueto a lack of composite action, increasing thepotential for damage from out-of-plane forces.Failure of these walls out-of-plane will result infalling hazards and degradation of the strength andstiffness of the lateral force resisting system.

Mitigation to provide out-of-plane stability andanchorage of the wythes is necessary to achieve theselected performance level.

Commentary:

Discontinuous infill walls occur when full baywindows or ventilation openings are providedbetween the top of the infill and bottom soffit of theframe beams. The portion of the column above the

Commentary:


out-of-plane forces. The height-to-thickness limitsin the evaluation statement are based on archingaction models that will exceed any plausibleacceleration levels in various seismic zones.

Further investigation of non-compliant infill panelsrequires a Tier 3 level analysis.

infill is a short captive column which may attractlarge shear forces due to increased stiffness relativeto other columns (see Figure 4-24). Partial infillwalls will also develop compression struts withhorizontal components that are highly eccentric tothe beam column joints. If not adequately detailed,concrete columns may suffer a non-ductile shearfailure which may result in partial collapse of thestructure. Because steel columns are not subject tothe same kind of brittle failure, this is not generallyconsidered a concern in steel frame infill buildings.

A column that can develop the shear capacity todevelop the flexural strength over the clear heightabove the infill will have some ductility to preventsudden catastrophic failure of the vertical supportsystem.

4.4.2.7.2 STUCCO (EXTERIOR PLASTER)SHEAR WALLS: Multistory buildings shall notrely on exterior stucco walls as the primarylateral-force-resisting system.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theoverturning and shear demands for non-compliantwalls shall be calculated and the adequacy of thestucco shear walls shall be evaluated using them-factors in Table 4-6.

4.4.2.7.3 GYPSUM WALLBOARD ORPLASTER SHEAR WALLS: Interior plaster orgypsum wallboard shall not be used as shear wallson buildings over one story in height.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theoverturning and shear demands for non-compliantwalls shall be calculated and the adequacy of thegypsum wallboard or plaster shear walls shall beevaluated using the m-factors in Table 4-6.

4.4.2.7.4 NARROW WOOD SHEAR WALLS:Narrow wood shear walls with an aspect ratiogreater than 2 to 1 for Life Safety and 1.5 to 1 forImmediate Occupancy shall not be used to resistlateral forces developed in the building.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theoverturning and shear demands for non-compliantwalls shall be calculated and the adequacy of thenarrow shear walls shall be evaluated using them-factors in Table 4-6.



Commentary:

Gypsum wallboard or gypsum plaster sheathingtends to be easily damaged by differentialfoundation movement or earthquake shaking.

Commentary:

Narrow shear walls are highly stressed and subjectto severe deformations that will damage thecapacity of the walls. Most of the damage occursat the base, and consists of sliding of the sill plateand deformation of hold-down anchors whenpresent. As the deformation continues, theplywood pulls up on the sill plate causing splitting.Splitting of the end studs at the bolted attachmentof hold down anchors is also common.

Commentary:

Exterior stucco walls are often used (intentionallyand unintentionally) for resisting seismic forces.Stucco is relatively stiff, but brittle, and the shearcapacity is limited. Building movements due todifferential settlement, temperature changes andearthquake or wind forces can cause cracking inthe stucco and loss of lateral strength. Lateralforce resistance is unreliable because sometimesthe stucco will delaminate from the framing and thesystem is lost. Multistory buildings should not relyon stucco walls as the primary lateral-force-resisting system.

Though the capacity of these walls is low, mostresidential buildings have numerous wallsconstructed with plaster or gypsum wallboard. Asa result, plaster and gypsum wallboard walls mayprovide adequate resistance to moderate earthquakeshaking.

One problem that can occur is incompatibility withother lateral-forcing-resisting elements. Forexample, narrow plywood shear walls are moreflexible than long stiff plaster walls; as a result, theplaster or gypsum walls will take all the load untilthey fail and then the plywood walls will start toresist the lateral loads. In multistory buildings,plaster or gypsum wallboard walls should not beused for shear walls except in the top story.

4.4.2.7.5 WALLS CONNECTED THROUGHFLOORS: Shear walls shall have interconnectionbetween stories to transfer overturning and shearforces through the floor.

Tier 2 Evaluation Procedure: No Tier 2 evaluationprocedure is available for walls in non-compliance.

4.4.2.7.6 HILLSIDE SITE: For a sloping sitegreater than one-half story, all shear walls on thedownhill slope shall have an aspect ratio less than 1to 1 for Life Safety and 1 to 2 for ImmediateOccupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theshear and overturning demands on the downhill slopewalls shall be calculated including the torsional effectsof the hillside. The adequacy of the shear walls on thedownhill slope shall be evaluated.

4.4.2.7.7 CRIPPLE WALLS: All cripple wallsbelow first floor level shear walls shall be braced tothe foundation with shear elements.

Tier 2 Evaluation Procedure: No Tier 2 evaluationprocedure is available for cripple walls innon-compliance.

4.4.2.7.8 OPENINGS: Walls with garage doors orother large openings shall be braced with plywoodshear walls or shall be supported by adjacentconstruction through substantial positive ties. Thisstatement shall apply to the Immediate OccupancyPerformance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theoverturning and shear demands on non-compliantwalls shall be calculated and the adequacy of the shearwalls shall be evaluated using the m-factors in Table4-6



Commentary:

Buildings on a sloping site will experiencesignificant torsion during an earthquake. Tallerwalls on the downhill slope are more flexible thanthe supports on the uphill slope. Therefore,significant displacement and racking of the shearwalls on the downhill slope will occur. If the wallsare narrow, significant damage or collapse mayoccur.

Commentary:

Cripple walls are short stud walls that enclose acrawl space between the first floor and the ground.Often there are no other walls at this level, andthese walls have no stiffening elements other thanarchitectural finishes. If this sheathing fails, thebuilding will experience significant damage and, inthe extreme case, may fall off its foundation. Tobe effective, all exterior cripple walls below thefirst floor level should have adequate shearstrength, stiffness, and proper connection to thefloor and foundation. Cripple walls that changeheight along their length, such as along slopingwalls on hillside sites, will not have a uniformdistribution of shear along the length of the wall,due to the varying stiffness. These walls may besubject to additional damage on the uphill side dueto concentration of shear demand.

Mitigation with shear elements needed to completethe load path is necessary to achieve the selectedperformance level.

Commentary:

In platform construction, wall framing isdiscontinuous at floor levels. The concern is thatthis discontinuity will prevent shear andoverturning forces from being transferred betweenshear walls in adjacent stories.

Mitigation with elements or connections needed tocomplete the load path is necessary to achieve theselected performance level.

4.4.2.7.9 HOLD-DOWN ANCHORS: All wallsshall have properly constructed hold-downanchors. This statement shall apply to theImmediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theoverturning and shear demands for non-compliantwalls shall be calculated and the adequacy of the shearwalls shall be evaluated using the m-factors in Table4-6.

4.4.3 Braced Frames

Figure 4-24. Braced Frames

4.4.3.1 General



Commentary:

Walls with large openings may have little or noresistance to shear and overturning forces. Theymust be specially detailed to resist these forces, orbraced to other parts of the structure withcollectors. Special detailing and collectors are notpart of conventional construction procedures. Lackof this bracing can lead to collapse of the wall.

Commentary:

Buildings without hold-down anchors may besubject to significant damage due to uplift andracking of the shear walls. Note that this conditionis not considered a life-safety concern and onlyneeds to be examined for the Immediate Occupancyperformance level.

Commentary:

Braced frames develop their lateral force resistancethrough axial forces developed in the diagonalbracing members. The braces induce forces in theassociated beams and columns, and all aresubjected to stresses that are primarily axial. Whenthe braces are eccentric to beam/column joints,members are subjected to shear and flexure in

addition to axial forces. A portal frame with kneebraces near the frame joints is one example.

Braced frames are classified as eitherconcentrically braced frames or eccentricallybraced frames (see Figure 4-24). Concentricallybraced frames have braces that frame intobeam/column joints or concentric connections withother braces. Minor connection eccentricities maybe present and are accounted for in the design.Eccentrically braced frames have braces that arepurposely located away from joints, andconnections that are intended to induce shear andflexure demands on the members. The eccentricityis intended to force a concentration of inelasticactivity at a predetermined location that willcontrol the behavior of the system. Moderneccentrically braced frames are designed with strictcontrols on member proportions and specialout-of-plane bracing at the connections to ensurethe frame behaves as intended.

LINK BEAM

LINK BEAM

CONCENTRICALLY BRACED FRAMES

ECCENTRICALLY BRACED FRAME

4.4.3.1.1 REDUNDANCY: The number of lines ofbraced frames in each principal direction shall begreater than or equal to 2 for Life Safety andImmediate Occupancy. The number of braced baysin each line shall be greater than 2 for Life Safetyand 3 for Immediate Occupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with the procedures in Section 4.2 shall beperformed. The adequacy of all elements andconnections in the braced frames shall be evaluated.

4.4.3.1.2 AXIAL STRESS CHECK: T he axialstress in the diagonals, calculated using the Quick

Check Procedure of Section 3.5.3.4, shall be lessthan 18 ksi or 0.50Fy for Life Safety and ImmediateOccupancy.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the braced frame elements shall beevaluated using the m-factors in Table 4-3.

4.4.3.1.3 STIFFNESS OF DIAGONALS: All

diagonal elements required to carry compressionshall have Kl/r ratios less than 120. This statementshall apply to the Immediate OccupancyPerformance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thecompression demands in non-compliant braces shall becalculated and the adequacy of the braces shall beevaluated for buckling.

4.4.3.1.4 CONNECTION STRENGTH: All thebrace connections shall develop the yield capacityof the diagonals. This statement shall apply to theImmediate Occupancy Performance Level only.



Commentary:

Redundancy is a fundamental characteristic oflateral force resisting systems with superior seismicperformance. Redundancy in the structure willensure that if an element in the lateral forceresisting system fails for any reason, there isanother element present that can provide lateralforce resistance. Redundancy also providesmultiple locations for potential yielding,distributing inelastic activity throughout thestructure and improving ductility and energyabsorption. Typical characteristics of redundancyinclude multiple lines of resistance to distribute thelateral forces uniformly throughout the structure,and multiple bays in each line of resistance toreduce the shear and axial demands on any oneelement.

A distinction should be made between redundancyand adequacy. For the purpose of this Handbook,redundancy is intended to mean simply "more thanone". That is not to say that for large buildingstwo elements is adequate, or for small buildingsone is not enough. Separate evaluation statementsare present in the Handbook to determine theadequacy of the elements provided.


Commentary:

The axial stress check provides a quick assessmentof the overall level of demand on the structure.The concern is the overall strength of the building.

Commentary:

Code design requirements have allowedcompression diagonal braces to have Kl/r ratios ofup to 200. Cyclic test have demonstrated thatelements with high Kl/r ratios are subjected to largebuckling deformations resulting in brace orconnection fractures. They cannot be expected toprovide adequate performance. Limited energydissipation and premature buckling cansignificantly reduce strength, increase the buildingdisplacements and jeopardize the performance ofthe framing system.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thedemands on the non-compliant connections shall becalculated and the adequacy of the brace connectionsshall be evaluated.

4.4.3.1.5 COLUMN SPLICES: All column splicedetails located in braced frames shall develop thetensile strength of the column. This statement shall

apply to the Immediate Occupancy PerformanceLevel only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thetension demands on non-compliant columns shall becalculated and the adequacy of the splice connectionsshall be evaluated.

4.4.3.1.6 OUT-OF-PLANE BRACING: Bracedframe connections attached to beam bottom flangeslocated away from beam-column joints shall bebraced out-of-plane at the bottom flange of thebeams. This statement shall apply to the ImmediateOccupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Thedemands shall be calculated and the adequacy of thebeam shall be evaluated considering a horizontalout-of-plane force equal to 2% of the bracecompression force acting concurrently at the bottomflange of the beam.

4.4.3.2 Concentrically Braced Frames



Commentary:

Since connection failures are usually nonductile innature, it is more desirable to have inelasticbehavior in the members.

Commentary:

Columns in braced frames may be subject to largetensile forces. A connection that is unable to resistthis tension may limit the ability of the frame toresist lateral forces. Columns may uplift and slideoff bearing supports, resulting in a loss of verticalsupport and partial collapse.

Commentary:

Brace connections at beam bottom flanges that donot have proper bracing may have limited ability toresist seismic forces. Out-of-plane buckling mayoccur before the strength of the brace is developed.Connections to beam top flanges are braced by thediaphragm, so V-bracing need not be considered.

This statement is intended to target chevron typebracing, where braces intersect the beam frombelow at a location well away from a column.Here only the beam can provide out-of-planestability for the connection. At beam/columnjoints, the continuity of the column will providestability for the connection.

To demonstrate compliance, the beam is checkedfor the strength required to provide out-of-planestability using the 2% rule.

Commentary:

Common types of concentrically braced frames areshown in Figure 4-25.

Braces can consist of light tension-only rodbracing, double angles, pipes, tubes or heavywide-flange sections.

Figure 4-25. Bracing Types

4.4.3.2.1 K-BRACING: The bracing system shallnot include K-braced bays.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the columns shall be evaluated for alldemands including concurrent application of theunbalanced force that can be applied to the column bythe braces. The unbalanced force shall be taken as thehorizontal component of the tensile capacity of onebrace, assuming the other brace has buckled incompression. The m-factors in Table 4-3 shall beused.

4.4.3.2.2 TENSION-ONLY BRACES:Tension-only braces shall not comprise more than70% of the total lateral-force-resisting capacity instructures over two stories in height. Thisstatement shall apply to the Immediate OccupancyPerformance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the tension-only braces shall be evaluatedusing the m-factors in Table 4-3.

4.4.3.2.3 CHEVRON BRACING: The bracingsystem shall not include chevron-, or V-bracedbays. This statement shall apply to the ImmediateOccupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theadequacy of the beams shall be evaluated for alldemands including concurrent application of theunbalanced force that can be applied to the beams bythe braces. The unbalanced force shall be taken as thevertical component of the tensile capacity of onebrace, assuming the other brace has buckled incompression. The m-factors in Table 4-3 shall beused.



CHEVRON `V' `K'

Commentary:

In K-brace configurations, diagonal bracesintersect the column between floor levels (seeFigure 4-25). When the compression bracebuckles, the column will be loaded with thehorizontal component of the adjacent tension brace.This will induce large midheight demands that canjeopardize the stability of the column and verticalsupport of the building.

In most cases, columns have not been designed toresist this force. The risk to the vertical supportsystem makes this an undesirable bracingconfiguration.

Commentary:

Tension-only brace systems may allow the brace todeform with large velocities during cyclic responseafter tension yielding cycles have occurred.Limited energy dissipation and premature fracturecan significantly reduce the strength, increase thebuilding displacements and jeopardize theperformance of the framing system.

Concrete braced frames are rare and are notpermitted in some jurisdictions because it isdifficult to detail the joints with the kind ofreinforcing that is required for ductile behavior.

Commentary:

In chevron and V-brace configurations, diagonalbraces intersect the beam between columns (seeFigure 4-25). When the compression bracebuckles, the beam will be loaded with the verticalcomponent of the adjacent tension brace. This willinduce large midspan demands on the beam thatcan jeopardize the support of the floor.

4.4.3.2.4 CONCENTRIC JOINTS: All thediagonal braces shall frame into the beam-columnjoints concentrically. This statement shall apply tothe Immediate Occupancy Performance Level only.

Tier 2 Evaluation Procedure: An analysis inaccordance with Section 4.2 shall be performed. Theaxial, flexural, and shear demands including thedemands due to eccentricity of the braces shall becalculated. The adequacy of the joints shall beevaluated.

4.4.3.3 Eccentrically Braced Frames

No evaluation statements or Tier 2 procedures havebeen provided specifically for eccentrically bracedframes. Eccentrically braced frames shall be checkedfor the general braced frame evaluation statements andTier 2 procedures in Section 4.4.3.1.



Commentary:

Frames that have been designed as concentricallybraced frames may have local eccentricities withinthe joint. A local eccentricity is where the lines ofaction of the bracing members do not intersect thecenterline of the connecting members. Theseeccentricities induce additional flexural and shearstresses in the members that may not have beenaccounted for in the design. Excessive eccentricitycan cause premature yielding of the connectingmembers or failures in the connections, therebyreducing the strength of the frames.

Commentary:

Eccentrically braced frames have braces that arepurposely located away from joints, andconnections that are intended to induce shear and

In most cases, beams have not been designed toresist this force. The risk to the vertical supportsystem makes chevron and V-bracing undesirablebracing configurations.

flexure demands on the members. The eccentricityis intended to force a concentration of inelasticactivity at a predetermined location that willcontrol the behavior of the system. Moderneccentrically braced frames are designed with strictcontrols on member proportions and specialout-of-plane bracing at the connections to ensurethe frame behaves as intended.

The eccentrically braced frame is a relatively newtype of frame that is recognizable by a diagonalwith one end significantly offset from the joints(Figure 4-26). As with any braced frame, thefunction of the diagonal is to provide stiffness andtransmit lateral forces from the upper to the lowerlevel. The unique feature of eccentrically bracedframes an offset zone in the beam, called the "link".The link is specially detailed for controlled yielding This detailing is subject to very specificrequirements, so an ordinary braced frame thathappens to have an offset zone that looks like a linkmay not necessarily behave like an eccentricallybraced frame.

An eccentrically braced frame has the followingessential features:

1. There is a link beam at one end of eachbrace.

2. The length of the link beam is limited tocontrol shear deformations and rotationsdue to flexural yielding at the ends of thelink.

3. The brace and the connections are designedto develop forces consistent with thestrength of the link.

4. When one end of a link beam is connectedto a column, the connection is a fullmoment connection.

5. Lateral bracing is provided to preventout-of-plane beam displacements thatwould compromise the intended action.

In most cases where eccentrically braced framesare used, the frames comprise the entire lateralforce resisting system. In some tall buildings,eccentrically braced frames have been added as

Figure 4-26. Eccentrically Braced Frames



LINK

BEAMS

stiffening elements to help control drift in momentresisting steel frames.

There are no evaluation statements for eccentricallybraced frames because their history isso short, but the engineer is alerted to their possiblepresence in a building. For guidance in dealing witheccentrically braced frames, the evaluating engineeris referred to the Recommended Lateral ForceRequirements and Commentary (SEAOC, 1996). Itshould be noted that some engineers who werefamiliar with current research, designedeccentrically braced frames before the SEAOCprovisions were developed. These frames may notsatisfy all of the detailing requirements present inthe current code. Any frame that was clearlydesigned to function as proper eccentrically bracedframe should be recognized and evaluated with dueregard for any possible shortcomings that willaffect the intended behavior.

4.4 Procedures for Lateral-Force- Resisting · PDF file4.4 Procedures for...

Documents

Transcript of 4.4 Procedures for Lateral-Force- Resisting · PDF file4.4 Procedures for...