Seismic Detailing Presentation

Seismic Connections Seminar

American Institute of Steel Construction I.1

Detailing High Seismic Projects

BackgroundDuctilitySeismic Design

AISC Seismic ProvisionsSeismic Load Resisting System (SLRS)Structural RequirementsDrawing Requirements

Introduction



Ductility

Ductility



Ductility

Ductility

The ability of a material to deform without fracture

Dependent on:Material PropertiesGeometryTemperatureConstraintEtc



Ductility

Toughness: resistance to unstable crack propagation in the presence of a notch Charpy V-Notch (CVN) test measures toughness

Ductility

Charpy V-Notch (CVN) testImpact test performed on a notched specimen according to ASTM A370Specimen is machined from member to be tested



Ductility

Charpy V-Notch (CVN) testSpecimen is struck and broken in a single blow in a specially designed testing machine

Ductility

Charpy V-Notch (CVN) testEnergy absorbed in breaking the specimen is measuredResults given in ft-lbs at a certain temperatureExample: Welds shall use filler metal with a minimum CVN toughness of 20 ft-lbs at 0o F



Ductility

Ductility



Ductility

Unrestrained necking down of the material

Ductility



Ductility

Potential for cracking can increase if the material is constrained in a way that keeps the material from necking down

Free to yield Restrained

Ductility



Ductility

Ductility



Ductile:YieldingBearing deformation at bolt holes

Ductility

Ductility



Ductility

Less desirableTension or shear ruptureBolt shearBlock shear

Ductility



Ductility

Ductility



Ductility

Seismic Design



Why are there special seismic requirements?

System Ductility

Seismic Design

“System Ductility” is the Ability of a System to Maintain Stability After Yielding/Overload of Some Elements

Ability of Yielding/Overloaded Elements to DeformAbility of Non-yielding Elements to Withstand Forces Redistributed by YieldingAbility of Non-yielding Elements to Withstand Deformations Caused by Yielding

Seismic Design



Conventional Building Code Philosophy Objective: Prevent collapse in extreme earthquakesObjectives are not necessarily to:

• limit damage• maintain function• provide for easy repair

Seismic Design

V

Earth

quak

e Loa

d, V

Ductility = Inelastic Deformation

Deformation, Δ

Δ

Failure (rupture or instability)

Seismic Design



Completely elastic response

As required elastic strength reduces (i.e. larger “R”-factor) required inelastic deformation increases

Earth

quak

e Loa

d, V

Deformation, ΔΔyield Δmax

Velastic

0.75Velastic

0. 5Velastic

0.25Velastic

As elastic design load decreases, required inelastic deformation increases

Seismic Design

Table 12.2-1 ASCE 7-05

Seismic Design



δxe δx δe

Elastic Response of Structure

VElastic

VYield

VDesign

Fully Yielded Strength

Ωo

R

Cd

Yielding

Lateral Deflection, Δ

Late

ral S

eism

ic F

orce

, V

Design Force Level

Response Modification Factor, R

How does seismic design provide ductility?

Seismic Design



Choose frame elements ("fuses") that will yield in an earthquake

Fuses must be ductile

Seismic Design

Seismic Design



Designed for the expected flexural yield strength of the beam, qualify through testing

Ensure beams can rotate inelastically to expected strength

Fuse: flexural yielding of beam ends

Seismic Design

Seismic Design



Designed for the expected tensile and compressive strengths of the brace

Ensure braces can deform inelastically to expected strength of brace

Fuse: tension yielding of braces

Seismic Design

Seismic Design



Link

Ensure links can deform inelastically to expected shear strength

Fuse: Shear yielding of links

Seismic Design

Seismic Design



Fuse: Web Element

Seismic Design

Seismic Design



AISC Seismic Provisions

1: Scope2: Referenced Standards3: General Seismic Design4: Loads, Load Combinations, Strengths5: Contract Documents

General

Each system type

All Projects

Special information and procedures

Seismic Provisions for Structural Steel Buildings, ANSI/AISC 341, Part I:

9-17: Structural Systems

18: Quality Assurance Plan (App. Q)

Appendices P, R, S, T, W, X

6: Materials7: Connections8: Members

All Frame Members




ScopeDesignFabricationErection

Structural Steel Members in the SLRSConnections in the SLRSAll Column Splices

R>3




Seismic Load Resisting Systems (SLRS)


Assembly of structural elements in the building that resists seismic loads.




Moment FramesSpecial Moment Frames (SMF)Intermediate Moment Frames (IMF)Ordinary Moment Frames (OMF)Special Truss Moment Frames (STMF)

Braced FramesSpecial Concentrically Braced Frames (SCBF)Ordinary Concentrically Braced Frames (OCBF)Eccentrically Braced Frames (EBF)Buckling-Restrained Braced Frames (BRBF)

Special Plate Shear Walls (SPSW)




Part 4 – Moment FramesSpecial Moment Frames (SMF)Intermediate Moment Frames (IMF)Ordinary Moment Frames (OMF)

Special Moment Frames (SMF)

Expected to withstand significant inelastic deformations (R = 8)



Expected to withstand limited inelastic deformations (R = 4.5)

Requirements are less stringent than SMF

Intermediate Moment Frames (IMF)

Expected to withstand minimal inelastic deformations (R = 3.5)

Ordinary Moment Frames (OMF)



Part 3 – Braced FramesOrdinary Concentrically Braced Frame Systems

(OCBF)Special Concentrically Braced Frame Systems

(SCBF)Eccentrically Braced Frame Systems (EBF)

Concentric Braced Frames



Expected to withstand limited inelastic deformation (R = 3.25)

Ordinary Concentrically Braced Frames (OCBF)

Special Concentrically Braced Frames (SCBF)





>2t




Eccentrically Braced Frames (EBF)





Part 5 – Other SystemsBuckling Restrained Braced Frames (BRBF)Special Plate Shear Walls (SPSW)Special Truss Moment Frames (STMF)



Buckling Restrained Braced Frames (BRBF)

Expected to withstand significant inelastic deformations (R = 7 to 8)

Unbonded Brace TypeUnbonded Brace Type

DecouplingDecoupling BucklingRestraintBucklingRestraint

Encasing mortarEncasing mortar

Yielding steel coreYielding steel core

Steel tubeSteel tube

Debonding material between steel core and mortar

Debonding material between steel core and mortar




Courtesy ofK.C. Tsai

Courtesy ofSTAR Seismic






Fuse: Web Element

Connections designed for the expected shear strength of the web

Horizontal and Vertical Boundary Elements:

Ensure web can deform inelastically to expected shear strength of webs





Steel plate

Horizontal boundary element (HBE)

Vertical boundary element (HBE)



Special Truss Moment Frames (STMF)



Structural Requirements



Demand Critical WeldsProtected ZonesGusset Plate Details (SCBF only)Weld Access Holes (OMF only)Prequalified Connections (SMF and IMF only)k-areaContinuity Plates




Backing BarsWeld TabsColumn SplicesBolted Joints

Demand Critical Welds




Demand Critical Weld: Weld so designated by the Seismic Provisions

Special CVN requirements for enhanced ductility

All welds in members and connections within SLRS shall use filler metal with minimum CVN of 20 ft-lbs at 0o F




CVN requirements for filler metal-Demand Critical Welds

20 ft-lbs at -20o FWhere frame is normally at 50o F or higher, 40 ft-lbs at 70o FWhere frame is normally less than 50o F, qualification temperature shall be 20o F above lowest anticipated service temperature (LAST)


Although demand critical welds are identified in the Seismic Provisions, there may be other welds that warrant this designation by the designer.

Consider inelastic demandConsequence of failureCJP groove welds between columns and base plates




Examples of demand critical welds in SMF and IMF include the following CJP groove welds:

Welds of beam flanges to columnsWelds of single plate shear connections to columnsWelds of beam webs to columnsColumns splice welds, including column bases and tapered transitions

Example “demand critical”welds


Examples of demand critical welds in OMF include the following CJP groove welds:

Welds of beam flanges to columnsWelds of single plate shear connections to columnsWelds of beam webs to columns

Example of “demand critical” welds




Examples of demand critical welds in EBF include the following welds:

CJP groove of between link beams and columnsWelds joining web plate and flange plates in built-up EBF link beamsColumn splice welds if made with CJP groove welds

Example of “demand critical”welds

(Designed as a fixed connection when link is between brace and column)


Protected Zone



Protected Zone

Protected Zone: Area of members in which limitations apply to fabrication and attachments.

Areas of Expected YieldingFabrication Discontinuities RepairedDetrimental Attachments Not Permitted -No welding or other attachments

Fracture

Shear Stud weld

Protected Zone



db/2

db

Protected Zone

L

L/4

d

Gussets

Braces at expected hinge locations

Protected Zone



Link Length e

Protected Zone

In protected zone, tack welds for attaching backing and weld tabs shall be placed where they will be incorporated into final weld

Protected Zone



Gusset Plate Details

Gusset Plate Details

Special details required for SCBF

Option 1: Connection is strong enough to restrain buckling

Option 2: Connection is ductile enough to allow the brace to buckle. Gusset plates are detailed to accommodate inelastic rotation



Gusset Plate Details-Option 1

(Courtesy of Fred Niemeier and Gary Broccard of J. S. AlbericiConstruction Co., Inc.)




>2t






Weld Access Holes



Allows for access for welding or backingOMF onlySee contract documents for IMF and SMF

Weld Access Holes

Special weld access hole geometry for OMF

Weld Access Holes



Surface roughnessNot to exceed 500 micro in.

AWS C41-77 Comparator: Sample 4

Weld Access Holes

Prequalified Connections




Prequalified Connection: Connection that complies with the requirements of Appendix P or ANSI/AISC 358




k-area

k-area

k-area: The region of the web that extends from the k dimension a distance of 1½ in. into the web beyond the k dimension.

Toe of fillet

(1.5”)



Rotary straightening of W-shapes creates zone of higher yield and tensile strength but lowers notch toughness and ductilityWelding or thermal cutting in k-area can lead to cracking

k-area

Try to avoid welding or cutting in this areaIf welding or cutting is performed in k-area, NDT should be performed to confirm that cracking has not occurred

Toe of fillet

(1” to 1.5”)

k-area



Welding doubler plates to flanges with fillet welds and use of generous continuity plate corner clips may reduce cracking potential

Doubler plate

Generous continuity plate corner clips

k-area

Continuity Plates



Corners shall be clippedCurved clips shall have a minimum radius of 0.5 in.

Continuity Plates

Along web, clip extends a distance of at least 1.5 in. beyond published “k” detail dimensionAlong flange, clip shall not exceed 0.5 in. beyond published “k1” detail dimension

Not m

ore t

han

(k1

+ 0.5”

)

Not less than (k + 1.5”) to avoid welding in k-region

“Clip” in continuity plate to avoid column fillet

Continuity Plates



SMF and IMF

Continuity plates shall be consistent with Prequalified Connection Standard (ANSI/AISC 358) or testing per Seismic ProvisionsAppendix P or S

Continuity plate

Continuity Plates

Backing Bars



Steel backing can create stress concentrations

Unfused Backing

Potential point of brittle fracture initiation

Tension force in flange

Backing Bars

At bottom flange, backing shall be removedFollowing removal, reinforce with a 5/16 in. fillet

weld

Backing removed and reinforced with fillet weld

Backing Bars



At top flange only: Backing may remain in place if it is attached with

5/16 in. fillet weld

Backing need not be removed, but if it is not, attach backing to column flange with reinforcing fillet

Backing may remain

Backing Bars

Bottom flange weld preparation – note bevel of bottom flange, backing (back-up bar), land and root opening for first weld pass

Bevel of bottom flange

Backing

Root opening and land

Backing Bars



Backing (back-up bar)Weld tab

Weld tab and backing at top flange Weld tab and backing at bottom flange

First pass (root pass) of weld Weld completed but prior to removal of weld tab and backing

Backing Bars

Bottom flange of beam showing removal of backing and weld tab. A reinforcing fillet has been added where the backing has been removed.

Reinforcing fillet weld

Backing fillet weld to column flange at beam top flange

Backing Bars



Weld Tabs

Weld tabsWeld tabs (“runoff tabs”) are extensions of the parts being welded that allow the weld to be started and stopped outside of the jointProvide for similar geometry as the preparationGenerally required to be removed after welding

Weld tab and backing at frame girder bottom flange

Weld tab and backing

Weld Tabs



Not the same as “end dams” (which should not be used)

Weld dam improperly substituted for weld tab

Weld Tabs

Weld tab

The function of the weld tab can be seen clearly in the photograph. The weld can stop and start outside of the joint.

Weld Tabs



Bottom flange backing and weld tabs to be removed

Top flange backing left in place with reinforcing filletWeld tabs removed and weld ground to smooth transition

Weld Tabs

Column Splices



Splices in SLRS made with fillet or PJP welds

4 ft or more from connection or at column midheight

Column splice

Column Splices

Beveled transitions at CJP groove welds

Structural design drawings must show when they are required

2.51

Column Splices



Beveled transitions at PJP groove welds

Seismic Provisions specifically indicate that column splices made with PJP groove welds do not require beveled transitions

2.51

Column Splices

Column Splices

Non-SLRS Splices



Column splices in columns not in SLRS:

4 ft or more from connection or at column midheight

Column splice

Column Splices

Bolted Joints



Bolts in SLRS shall be pretensioned high-strength ASTM A325 or A490 bolts

Twist-off type tension control bolt assemblies of equivalent mechanical properties may be substituted for A325 or A490 fastener assemblies

Bolted Joints

Faying surfaces shall be prepared as slip-critical with a Class A surface

Bearing strength shall be provided using either standard holes or short-slotted holes with slot perpendicular to line of force

Bolted Joints



For brace diagonals, oversized holes are permitted if connection is designed as slip-critical and oversized hole is in one ply

Alternative hole type is permitted if designated in Connection Prequalification Standard or justified by testing (Appendix P or T)

Bolted Joints

Bolts and welds shall not be designed to share force in a joint or same force component in a connection

Bolts

WeldsVertical force from brace and beam shear (and possibly the horizontal force) is resisted by bolts and welds, but designed so that either welds or bolts take total load

Line of action of vertical force

Bolted Joints



Drawing Requirements


AISC Seismic ProvisionsStructural Design Drawings

Designation of the SLRSDesignation of the members and connections that are part of the SLRSConnection configurationsConnection material and sizesLowest Anticipated Service Temperature (LAST)

if < 50o F




Location of demand critical weldsLocations and dimensions of protected zonesLocations where gusset plates are to be detailed to accommodate inelastic rotation


Welding requirements as specified in Appendix W, Section W2.1

Locations where backup bars are removedLocations where supplemental fillet welds are required when backing is permitted to remainLocations where fillet welds are used to reinforce groove welds





Locations where weld tabs are removedSplice locations where tapered transitions are requiredShape of the weld access hole if a special shape is required

AISC Seismic ProvisionsShop Drawings

Designation of the members and connections that are part of the SLRSConnection material specificationsLocation of demand critical shop weldsLocations and dimensions of protected zonesGusset plates drawn to scale when they are detailed to accommodate inelastic rotation



AISC Seismic ProvisionsShop Drawings


Access hole dimensions, surface profile and finish requirementsLocations where backup bars are removedLocations where weld tabs are removedNDT to be performed by the fabricator, if any(See Appendix Q)

AISC Seismic ProvisionsErection Drawings

Designation of the members and connections that are part of the SLRSField connection material specifications and sizesLocation of demand critical field weldsLocations and dimensions of protected zonesLocations of pretensioned bolts



AISC Seismic ProvisionsErection Drawings


Locations where backup bars are removedLocations where supplemental fillet welds are required when backing is permitted to remainLocations where weld tabs are removed

Questions?

Seismic Detailing Presentation

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