IBC Design Requirements

7/30/2019 IBC Design Requirements

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SE Reference Manual Chapter 16

Chapter 16 CH16-1

CHAPTER 16

Design Requirements

Chapter 16 of the IBC/CBC prescribes general design requirements for structures

regulated by the code. Relevant information from Chapter 16 is presented below:

§1604.5 Occupancy Category

Per Table 1604.5 (IBC/CBC) or Table 1-1 (ASCE 7),

OccupancyCategory

Description of Hazard Represented by Building Collapse

I LowII All buildings except those in I, III and IVIII Substantial

• Public Assembly >300 people• Schools, daycares >250 people

• College, adult education >500 people

• Healthcare (no emergency or surgery) >50 people

• Jails, detention centers

• Any building with more than 5000 people

• Public utility buildings (not in IV)

• Buildings containing hazardous materials (not in IV)IV Essential facilities

• Hospitals with emergency and surgery

• Fire, rescue, police

• Emergency shelters for earthquakes

• Power stations, public utility buildings designated forearthquake backup

• Aviation towers, control centers

• Critical nation defense related building

• Buildings containing hazardous materials quantities greaterthan in Table 307.1.(2).

§1605 – L oad Combinations

Strength Design

IBC/CBC 1605.2.1

1. 1.4(D +F)2. 1.2(D +F +T) +1.6(L +H) +0.5(L r or S or R)3. 1.2D +1.6(Lr or S or R) +(f 1L +0.8W)4. 1.2D +1.6W +f 1L +0.5(Lr or S or R)




Chapter 16 CH16-5

Roof Loads (§1607.11)

Roof live loads may be reduced per 1607.11.2.1 based on the roof slope andtributary area, except for landscaped roofs (20psf minimum). Special purposeroofs (§1607.11.2.2) shall be treated similar to floors.

§1609 – Wind Loads

• Wind loads are per Section 6 of ASCE 7. See section ‘Wind Loads’.

§1617 – Earthquake Loads

The IBC references ASCE 7 for the majority of the earthquake load provisions. IBC (andASCE 7) assigns a ‘Seismic Design Category’ to each structure. Seismic designcategories are used to determine permissible lateral systems, height limitations, type of lateral analysis and seismic detailing requirements. Earthquake loads are described in the

section titledEarthquake Loads. Other relevant items are discussed here. The codereferences are to ASCE 7.

The general layout of the seismic provisions of ASCE 7 is as follows:

Chapter Description11 Seismic design criteria

- Importance Factor & Seismic Design Category (SDC)- Mapped accelerations etc.

12 Seismic design requirements for buildings- Design basis

-

Provisions for structural system selection, horizontal and verticalcombinations of lateral systems etc.- Seismic load combinations- Equivalent lateral force calculations- Response spectrum analysis- Drift limits- Detailing requirements for different SDC etc.

13 Seismic design requirements for non-structural components, includingarchitectural and MEP components.

14 Seismic design and detailing for different materials – Not used byIBC/CBC

15 Seismic design requirements for non-building structures, including thosesimilar to buildings (pipe racks, towers etc.) and those not similar tobuildings (tanks, stacks, chimneys etc.).

16 Seismic response history procedures (time history analysis procedures)17 Design requirements for base isolated structures18 Design requirements for structures with damping systems.19 Soil-structure interaction for seismic design20 Site classification for seismic design




Chapter 16 CH16-6

21 Site-specific ground motion procedures for seismic design22 Seismic ground motions and long-period transition maps

Relevant Provisions of ASCE 7 Earthquake Design:

• §12.2.5.1 Dual Systems

Dual systems are defined as a combination, in any given direction of loading, of moment frames (special or intermediate) and shear walls, braced frames (SCBF,EBF, BRBF etc.)—see Table 12.2-1 for a complete listing of allowed dualsystems.

The moment frames shall be designed to resist a minimum of 25% of the designbase shear. The actual seismic force distribution shall be based on the appropriaterigidities of the systems.

• §12.2.2 Combinations in Different Directions

Different seismic systems can be used in each of the orthogonal directions of the

structure. The appropriate values of R, Cd, andΩo should be used for each system.

• §12.2.3 Combinations in the Same Direction

For non-dual systems used in combination in the same direction, use the least

value of R for any of the systems. The Cd andΩovalues shall correspond to the Rfactor being used in a given direction and shall not be less than the largestrespective values for that R factor.

Exception:

Different systems in each independent line of lateral system are permitted to bedesigned for the least value of the R factor in that line if the following conditionsare met:

1. Occupancy Category I or II.2. Height is two stories or less.3. Light frame construction or flexible diaphragms.

• §12.2.3.1 Vertical Combinations of Lateral Systems

R used in any story shall not exceed the lowest R value used in any story above.

Cd andΩo shall not be less than the largest value of each factor used in any storyabove.

Exceptions:




Chapter 16 CH16-8

Table 1

Structural Design Requirements

Description SDC ‘A’ SDC B SDC C SDC D SDC E & F

Building height limits Table 12.2-1 Table 12.2-1 Table 12.2-1 Table 12.2-1 &§12.2.5.4

Table 12.2-1 &§12.2.5.4

Redundancy/Reliabilityfactor

(§12.3.4)

ρ = 1.0 ρ = 1.0 ρ = 1.0 ρ = 1.3

ρ=1.00 permitted if conditions in §12.3.4.2& Table 12.3-3 are met.

Same as SDC D

Analysis Procedures1,2 (§12.6 & Table 12.6-1)

OC – Occupany Category

(See Note 2 for adescription of analyticalprocedures 1 through 5)

1(§11.7.2)

OC I, II & ≤ 3 storieswith building frame orbearing wall system –

2,3,4,5

All other structures –3,4,5

Any structure with site

class E or F – 3,4,5

Same as SDC B Same as SDC B

&

All other light framedstructures – 3,4,5

Regular with T<3.5Ts –3,4,5,

Irregular with T<3.5Ts and irregularities listed

in Note 3 - 3,4,5

Any structure withtorsional irregularity

(Table 12.3-1, Type 1aor 1b) - 4,5

Same as SDC D

For SDC F, thesimplified design

procedure (type 2) isnot permitted.




Chapter 16 CH16-10

Description SDC ‘A’ SDC B SDC C SDC D SDC E & F

Design & DetailingLoad Path Connections

(§12.1.3 & 11.7.3)Provide continuouspath to the lateralsystem within the

structure.

Connection betweena smaller portion &main structure shallbe for Fp=0.05wp

Connections between asmaller portion & themain structure shall becapable of carrying the

greater of:

Fp=0.133SDSwp or

Fp=0.05wp

Same as SDC B Same as SDC B Same as SDC B

Anchorage of Concreteor Masonry Walls(§12.11 & 11.7.5)

Horizontal force shallbe greater of:Fp=0.05Wp

&Fp≥ 280plf (§11.7.5)

Horizontal force to begreater of:Fp=0.10Wp

&Fp=0.40SDSIWp

&Fp=400SDSI

&Fp≥ 280plf

Wall Design to be based

on greater of:Fp=0.10Wp &

Fp=0.40SDSIWp

If anchor spacing >4ft,design wall to spanbetween anchors

Minimumrequirements as per

SDC B

For flexiblediaphragms,

Fp=0.8SDSIWp (§12.11.2.1)

Maximumlength/width ratio for

sub-diaphragms shallbe 2.5 to 1.0(§12.11.2.2.1)

Same as SDC C Same as SDC C




Chapter 16 CH16-14

collectors andconnections between

diaphragms andcollectors to vertical

elements.

Force increase notrequired if load

combinations withover-strength are used.

See also ‘Collectors …’above in this table.

Notes: 1. OC – Occupancy Group, ASCE 7 Table 1-1, CBC Table 1604.5 and 1604A.5.2. Analysis Procedures: 1 – Minimum Lateral Force (§11.7), 2 – Simplified Design Procedure, 3 – Equivalent Lateral Force, 4 –

Response Spectrum, 5 – Time History. See Section ‘Earthquake Loads’.3. Irregular structures with T <3.5Ts and having only Horizontal Irregularities (Table 12.3-1) type 2, 3, 4, or 5 or Vertical

Irregularities (Table 12.3-2) type 4, 5a or 5b.



SE Reference Manual Loads & Analysis

IBC Earthquake Loads IBCSL1

LOADS & ANALYSIS

EARTHQUAKE LOADS

Per IBC §1613, the earthquake loads shall be per ASCE 7. The relevant provisions are

presented below. All references are to ASCE 7, unless otherwise noted.

Per IBC and ASCE 7, the earthquake forces and the associated detailing is based on the‘Seismic Design Category (SDC)’ assigned to the building.

Seismic Design Category (SDC), IBC §1613.5.6, ASCE 7 §11.6:

The SDC is a function of the ‘Occupancy Category’ (IBC/CBC Table 1604.5 and ASCE7 Table 1-1) and the mapped accelerations at the site. See IBC/CBC Tables 1613.5.6(1)and 1613.5.6(2) (ASCE 7 Tables 11.6-1 and 11.6-2, respectively) for SDC classification.

SDC A and B indicate low seismic risk; SDC C indicates moderate seismic risk; whileSDC D, E and F apply to high seismic risk. The detailing requirements as well asconstruction quality assurance requirements for SDC ‘D’, ‘E’, and ‘F’ are much morestringent than for the lower categories.

Earthquake Loads

The code permits a variety of analytical procedures – see Table 1 in Chapter ‘DesignRequirements’. The Equivalent Lateral Force Procedure per ASCE 7 §12.8 is presentedbelow.

Equivalent Lateral (Static) Force Procedure (ASCE 7 §12.8 & 11.4)

• Step 1 – Obtain Mapped Spectral Accelerations: ASCE 7 §11.4.1

From the maps (IBC Figure 1613.5(1) through 1613.5(14) or ASCE 7 Chapter 22,obtain the following:

Ss =Short period earthquake spectral response acceleration, &S1 =1-second period earthquake spectral response acceleration

• Step 2 - Determine Site Coefficients Fa and Fv: ASCE 7 §11.4.3

Determine Site Coefficients Faand Fv from Tables 11.4-1 & 11.4-2.

If site class is not known, assume ‘D’.





Note: OSHPD requires that if a structure is not assigned to SDC E of F, it shall

be assigned to SDC D (§1613A.5.6)

• Step 6 – Compute Base Shear: ASCE 7 §12.8

V =CsW (Eqn 12.8-1)

W = Dead load +25% live load for storage areas +Actual partition load(or 10psf minimum) +weight of permanent equipment +snow load(§12.7.2)

• Step 6a – Approximate Period Calculation: ASCE 7 §12.8.2.1

T = x

nT ahC T = (Eqn 12.8-7)

§12.8.2 If the period is computed from analysis, T ≤ C uT a.

where, C T & x are given below (Table 12.8-2)hn =Height of building in feetCu is given below (Table 12.8-1)

Alternative methods for periods for moment frames and shear wallbuildings are presented in §12.8.2.1.

• Step 6b – Cs calculation: ASCE 7 §12.8.1.1

( ) I R

S

C

DS

s=

(Eqn 12.8-2)

where, R =Response reduction factor, Table 12.2-1I =Importance factor, IBC Table 1604.5A, ASCE & Table 1-1.

I =1, 1.25 & 1.5 for Occupancy Category I/II, III & IV,respectively.

Structure Type Cu x SD1 Cu

Steel MRF 0.028 0.8 ≥ 0.4 1.4

Concrete MRF 0.016 0.9 0.3 1.4

Steel EBF 0.03 0.75 0.2 1.5

All others 0.02 0.75 0.15 1.6

≤ 0.1 1.7





The value of Cs need not exceed:

( )T I

R

S C

D

s

1= for T ≤ T L (Eqn 12.8-3)

( ) 2

1

T I

R

T S C

L D

s= for T > T L (Eqn 12.8-4)

Cs shall not be less than:

01.0= s

C (OSHPD/DSA, 03.0= s

C ) (Eqn 12.8-5)

Where S1 ≥ 0.6g, Cs shall not be less than:

I RS C

s

15.0= (Eqn 12.8-6)

• Step 7 – Vertical Distribution of Base Shear: ASCE 7 §12.8.3

At each level the seismic force is given as:

V C F vx x

= (Eqn 12.8-11)

∑=

=n

i

k

ii

k

x x

vx

hw

hwC

1

(Eqn 12.8-12)

If T≤ 0.5 k =1.0If T≥ 2.5 k =2.0Use k =2 or linear interpolation between the period limits.

• Seismic Load Effect: ASCE 7 §12.4.2

E = E h ± E v (Eqn 12.4-1)

E = ρ Q E + 0.2S DS D

E = ρ Q E - 0.2S DS D

Where, QE =Effect of horizontal seismic forces0.2 SDSD =Vertical acceleration effect (Eqn 12.4-4)





Where seismic over strength factor needs to be included in the design, §12.4.3,

E m = E mh ± E v

Em=ΩοQE ±0.2SDSD

The load combinations with over strength are given in §12.4.3.2.

• Redundancy Factor ‘ρ’: ASCE 7 §12.3.4.2

For SDC A, B, or C, 0.1= ρ

For SDC D, E, or F 3.1= ρ For all structures, or

0.1= ρ if one of the following two

conditions are met.

a. Each story resisting more than 35% of the base shear (typically the lower stories in a building ) shall comply with the following:

1. Loss of one of the following does not result in more than 33%reduction in story strength:

i. An individual brace or connection theretoii. Moment connections at both ends of one beamiii. A shear wall or wall pier with height-to-length ratio >1.0iv. Moment resistance at the base of a single cantilever column.

2. Loss of one of the above does not result in an extreme torsional

irregularity (Type 1b, Table 12.3-1).

b. For structures regular in plan at all levels with at least two perimeter bays of the seismic force-resisting in each direction at each level resisting more than35% of the base shear.

For shear walls: Number of bays =(n*Wall length)/story height

Where, n =2, For shear walls in light framing.n =1, for all other shear wall building.

In addition to the above,ρ =1.0 is permitted for the following:

1. Drift & P-∆ calculations2. Design of non-structural components & non-building structures that are not

similar to buildings.3. Design of collectors, splices, their connections etc., for which the load

combinations with over-strength as used.





4. For design of any member or connection for which the load combinations withover-strength are used.

• Displacement amplification: ASCE 7 §9.5.5.7

For allowable stress design, displacement is computed for earthquake loadswithout dividing by 1.4 and usingρ =1.0.

The design deflection at the center of mass at any level is calculated as,

I

C xed

x

δ δ = (Eqn 12.8-15)

where, δx =Maximum inelastic displacement at level x.Cd =Displacement amplification factor, Table 12.2-1

δxe =deflection from an elastic lateral analysis of the building.

The deflections/drifts can be determined for the seismic forces at the actual periodcalculated for the building, without applying theC uT a limit in Step 6a.

Exceptions to Static Force Procedure

:

Where applicable, the equivalent lateral force procedure may be substituted by one of theprocedures below. See Table-1 of ‘Chapter 16 Design Requirements’ for moreinformation.

• Minimum Lateral Force: IBC §11.7.1

Applies to SDC A only. At each floor the minimum base shear shall be:

Fx =0.01Wx where, Fx =Design seismic force @ story x

Wx =Seismic weight @ story x

• Simplified Procedure: §12.14

Note: Not permitted by OSHPD & DSA (§1613A.5.6.2).

This procedure can be used in lieu of the other analytical procedures for theanalysis/design of simple buildings with bearing walls or building frame systems,if the building meets certain limitations. See §12.14.1.1 for a complete list andbelow for the major limitations:

1. The building shall be in Occupancy Category I or II and shall notexceed 3 stories in height.




IBC Lateral System Selection LSS1

SELECTION OF LATERAL SYSTEMS FOR SEISMIC DESIGN

CHAPTERS 19 & 22

As with seismic loads and detailing requirements, the IBC/CBC places limits on the type

of structural systems that can be used for lateral design based on Seismic DesignCategory (SDC)--see section ‘Chapter 16 ’ and ‘ Earthquake Loads’ for more information.

The brief list below specifies the minimum concrete and steel system requirements for agiven SDC. It is always permitted to provide a better lateral system and take advantage of

the lower seismic design forces (ACI 318, R21.2.1)

For a detailed listing of lateral systems and associated limitations, see ASCE 7 Table12.2-1. All reference in the following are to IBC/CBC, unless noted otherwise.

Concrete (Chapter 19 & ACI 318)

Seismic Design Categories A & B (Low Seismic Risk) §1910.2 & 1910.3

• Ordinary Shear Walls

Designed using Chapters 1 through 18 of ACI 318.

Note: For SDC A, shear walls can be ordinary plan concrete walls per Chapter

22 of ACI 318 or detailed plain concrete walls per IBC §1908.1.14.

• Ordinary Precast Concrete Shear Walls


• Ordinary Moment Frames


§108.1.1 – Provide at least two reinforcing bars continuously at top and bottom in

beams and develop at (or continuous through) the columns.

§1908.1.2 – Columns with clear height to maximum dimension ratio of five or

less shall also be designed for shear.

Seismic Design Category C (Intermediate or Moderate Seismic Risk) §1908.1.4

• Ordinary Shear Walls

:






Note: Plain concrete shear walls not permitted except as basement (retaining)

walls for one or two family dwellings with stud framing above.

• Intermediate Precast Concrete Shear Walls

:

Designed using Section 21.13 in addition to Chapters 1 through 18 of ACI 318.

• Intermediate Moment Frames

:

Designed using Section 21.12 in addition to Chapters 1 through 18 of ACI 318.

• Discontinuous Members (§1908.1.12)

: Columns supporting discontinuous lateralsystems above (such as shear walls) shall be designed for the special seismic load

combinations (i.e. using Ωo) with appropriate transverse reinforcement, per

21.12.5.2 of ACI 318, over the full height as well as above and below as required.

Seismic Design Categories D, E, F (High Seismic Risk) §1908.1.4

• Special Shear Walls

:

Cast-in-place walls designed using Sections 21.2 and 21.7 in addition to Chapters

1 through 18 of ACI 318.

Precast walls shall also satisfy §21.8 of ACI 318 in addition to the above.

• Special Moment Frames

:

Cast-in-place frames designed using Sections 21.2, 21.3, 21.4 and 21.5 in additionto Chapters 1 through 18 of ACI 318.

Precast frames designed per Sections 21.2, 21.3, 21.4 and 21.6 of ACI 318,

including all requirements for ordinary moment frames.

• Diaphragms and Foundations

:

Designed using Sections 21.2 and 21.9 for diaphragms and 21.2 and 21.10 for

foundations, in addition to Chapters 1 through 18 of ACI 318.

• Frame members not part of the lateral system

:

Designed/checked per section 21.11 to ensure that they can continue to carry the

gravity loads at the maximum lateral displacements corresponding to the design

level seismic forces.





Steel (Chapter 22, AISC Steel Specifications (ASD/LRFD AISC 360) & AISC

Seismic Provisions, AISC 341)

Seismic Design Categories A, B & C (Low or Moderate Seismic Risk) §2205.2.1

Steel structures may be designed using the following two options:

1. Use R = 3 per ASCE 7, Table 12.2-1, Item H for ‘Structural Steel Systems Not

Specifically Detailed for Seismic Resistance’ in conjunction with the typical AISCLRFD or ASD Specifications.

2. Use an R factor per ASCE 7, Table 12.2-1 and design per AISC Seismic

Provisions (ASIC 341), Part I.

Seismic Design Categories D, E & F (High Seismic Risk) §2205.2.2

3. Steel structures shall be designed using AISC Seismic Provisions (ASIC 341-02),

Part I.



SLRS-SCBF

(1) §13.2 Bracing Members

§13.2a Braces: y F

E r

KL 4≤

See §13.2a for KL/r greater than above limit.

§13.2d Compactness (§8.2b, Table I-8-1):

WF & Channels: y F

E t

b 30.0≤

For h/tw under flexure and axial load, see Table I-8-1.

Rectangular HSS: yw F

E t

hor t

b 64.0≤

Circular HSS: y F

E t

OD 044.0≤

Angles: y F

E t

b 30.0≤

OSHPD/DSA do not permit the use of rectangular HSS sections unless filled with concrete.

§13.2c Lateral Force Distribution

For a line of bracing:

total tensionhTotal V V V 7.03.0 ≤Σ≤

Vh tension = Horizontal component of axial force for bracesin tension.

VTotal = Total horizontal force in line of bracing.

Note: Exception if all braces are designed to resist the

load combinations including Ω o in compression.

§13.2e Built-up Members:

For each individual element between stitches, l/r ≤ 0.4lt/r,where lt/r is for whole member.

Shear strength of stitches≥ tensile strength of each

element.

Stitches to be placed uniformly along length. No less than2 stitches. No bolted stitches within the middle ¼ of the brace clear length.

STEEL SPECIAL CONCENTRIC BRACED FRAMES (AISC 341, §13, ASCE 7, Table 12.2-1)

R = 6, Ω = 2, Cd=5

(3) §13.4.a Beam Design for V-Type & Inverted V-Type Bracing

1. Beam shall be continuous between columns and designed to carry all applicable gravity loadcombinations without braces.

2. For load combinations that include earthquake effects, use t he following:a. (1.2 + 0.2S DS )D + P b + f 1L + f 2S b. (0.9 - 0.2S DS )D ± P b where, P b = unbalanced post-buckling force based on Pst = R yFyAg & Psc = 0.3Pn, where Pn

is the nominal compressive capacity of the brace.3. Both flanges of the beam shall be braced as follows:

a. At the point of brace intersection.

b.

At a maximum spacing of y y pd b r F E

M

M

L L

+==2

1076.012.0(A-1-7)

where, M1 & M2 (k-in) are the smaller and larger moments at t he ends of the unbracedlength. The ratio is positive for reverse curvature and negative for single curvature. Note: For TS beams, see Appendix 1 §A.1.7. of AISC Specification.

Lateral braces shall be per Eqns. A-6-7 & A-6-8 of Appendix 6 of the AISC Spec. with Mr being either R yZFy (LRFD) or R yZFy/1.5(ASD) and Cd = 1.0.

(2) §13.3 Bracing Connections

§13.3a Tensile strength of connections (including beamconnection) shall be the lesser of:1. Pst = R yFyAg (LRFD) or Pst = R yFyAg/1.5 (AS2. Maximum load effect that can be transferred

by the system

§13.3b Flexural strength of the connection shall be base1.1R yM p (LRFD) or (1.1/1.5)R yM p (ASD) of theabout the critical buckling axis (typically out of

This strength requirement does not apply if theconnection can accommodate the inelastic rotatto the brace post-buckling deformations. This caccomplished by using single plate gussets withsetback from the yield line for out-of-plane rota

the brace end. The gusset plate shall be designeresist the compressive strength of the brace with buckling.

Net Area: Typical connections use slotted HSS membewelded to the gusset. The net area in tension calculated as the gross area minus the slot w

times the thickness of the HSS. This area neereplaced via a plate welded to the two non-slfaces of the HSS (curved plates for round HSside plates need to be adequately extended eiof the slot via a shear lag analysis (see §D3 oAISC Specification).

(4) §13.2d Columns

WF: y F

E t

b 30.0≤

Rectangular HSS: yw F

E t

hor t

b 64.0≤

Column strength and splice design shall be per §13.5 => See sheet SLRS-Col1&2 for details.

Same limits as braces, Table I-8-1.



SE Reference Manual Concrete Design

Shear Wall Design CSWD1

REINFORCED CONCRETE

SHEAR WALL DESIGN

Referencec: ACI-318 & IBC 2006/CBC 2007.

Wall Type

SDC

Reinforcement

Limits

Shear

Design

Axial & Flexural

Design

Other

Ordinary

Shear Wall

A, B 14.3

11.10.8, 11.10.9 11.10

14.2, 14.3

(10.2, 10.3. 10.10-10.14, 10.17)

-

SpecialShear Wall

A, B, C,D, E, F

21.7.2 21.7.3,21.7.4

21.7.5(10.2, 10.3. 10.10-

10.14, 10.17)

BoundaryElements

21.7.6

Notes: 1. Provisions 10.10-10.14, 10.17 address slenderness, moment magnification,

bearing strength etc. and typically do not govern the design.

2. Precast walls follow similarly to above, except ‘Intermediate Precast Walls’ (permitted in SDC A, B, C) shall also comply with 21.13.

Reinforcement Limits (§11.10.9, 14.3, 21.7.2)

Note: 1. OSHPD/DSA – Minimum reinforcement parallel to all edges of the wall and boundaries of all openings shall be twice the shear reinforcement required per lineal

foot of wall (§1908A.1.37).

2. For seismic design reinforcement development lengths (& splices) shall be per 21.5.4 – See ‘Reinforcement Development & Lap Splices’, pp. RDL3-RDL5.




Shear Wall Design CSWD6

Boundary element requirements for Special Walls (§21.7.6)

:

Boundary element requirements can be evaluated by either one of the two methodsdescribed below:

a)

For walls that are effectively continuous from the base to the top with a singlecritical section for axial and flexural loads (§21.7.6.2):

Provide boundary elements where:( )

wu

w

h

l c

/600 δ ≥ (21-8)

where, c = neutral axis depth for (1.2 + 0.2S DS )D + ρQE + f 1L

δu = design displacement at top of wall (i.e. Cd∆x/I)

δu/hw ≥ 0.007 (§21.7.6.2(a))

For δu/hw = 0.007, c = 0.24w.

At some height along the wall, the above requirement will not be applicable

Extend the boundary element reinforcing beyond this elevation by a distance not

less than the larger of: w or Mu/4Vu.

b) For walls not designed per above, provide boundary elements at wall boundaries,

and edges of openings where maximum compressive stress exceeds 0.2f’c.

Discontinue boundary detailing where the stress is less than 0.15f’c (§21.7.6.3).

Shear wall

Axial Stress

Flexural Stress

Resultant Stress

Pu

Mu

If σres > 0.2f’c, provide boundary

reinforcing.

Factored Loads

Use gross section properties

with elastic force distribution§21.7.6.3

0.15f’c

0.2f’c Boundary detailing

required.

Extend up to and

discontinue above




Concrete Beam Design CBD1

REINFORCED CONCRETE

BEAM DESIGN

Reference: ACI 318

General Design Provisions for Beams:

Analysis

Reinforcement Limits

:

• Check ρ d b f

f w

y

c'3min = ρ

Α s min ≥ §10.5.1

d b f

w

y

200min

= ρ

( d b f

f A w

y

c

s

'6

min= for T-beams with flanges in tension)

The above limits need not apply if As provided, at each section, exceeds by 1/3rd

the steel area required by analysis (§10.5.3).

In ACI 318-05, section strength is governed by available ductility (i.e. amount of

reinforcement at a given section and tensile stress in the reinforcement) and thestrength-reduction factor, φ, e.g. the higher the ductility, the smaller the strength

reduction and vice-versa.

Each section is classified as either compression-controlled, tension-controlled or

in transition. These categories are based on the net tensile strain (εt) in the

extreme tension steel and are defined at a concrete ultimate strain (εcu) of 0.003.

• Balanced strain conditions: @ εcu = 0.003, εsb = f y/Es §10.3.2

εsb = 0.002 for f y = 60ksi §10.3.3

• Compression-controlled: @ εcu = 0.003, εt ≤ εsb §10.3.3

• Tension-controlled: @ εcu = 0.003 , εt ≥ 0.005 §10.3.4

• Transition-range: @ εcu = 0.003 , 0.002 < εt ≥ 0.005 §10.3.4





• Beam Deflection (§9.5.2)

Per Table 9.5(a), deflections need not be computed if the following minimum

depths are provided (for normal weight concrete & 60ksi reinforcement):Beams

-

Simple span: L/16 L/20

Slabs

- Cantilever: L/8 L/10- One end continuous: L/18.5 L/24

- Both ends continuous: L/21 L/28

Deflection Calculation (§9.5.2.3):

Deflection is to be based on beam formulas and Ie shown below,

cr

a

cr

g

a

cr

e I M

M I

M

M I

−+

=

33

1 (Eqn 9-8)

t

g r

cr y

I f M = where, cr f f '5.7= &

2

D yt = (Eqn 9-9)

Ma is the service level moment (if Ma < Mcr => No cracking =>Ie = Ig)

23

)(3

)(kd d nA

kd b I scr −+=

−+

=

s

s

nA

b

d nA

b

kd

112

Long-term deflection factors (§9.5.2.5)

:

'501 ρ

ξ λ

+

= (Eqn 9-11)

where, ρ’ = compressive steel ratioξ = 2.0 for 5 years or more, 1.4 for 12 months or more etc.

Notes: 1. See ACI 318, Table 9.5(a) for quick estimates of slab/beam thickness.2.See ACI 318, Table 9.5(b) or UBC/CBC Table 19-C-2 for permissible

deflections (typically use ∆ D+L ≤ L/240 & ∆ L ≤ L/360).

Using transformed section from

working stress design.

PCA Notes on ACI 318-95, pg. 8-3





Non-Seismic Beam Reinforcement Detailing (§12.10, 12.11, & 12.12)

:

Positive Moment Reinforcement:

• Distance to extend reinforcement past where it is no longer required =

MAX(d,12d b) – except at supports of simple spans and ends of cantilevers.

• Extend bars at least d past the critical section.

• Minimum reinforcement to be extended at least 6” into support (Bars ‘B’):

Simple span beams: + s A3

1

Continuous beams:+ s A

4

1

• At simple supports & points of inflection, bar size shall be limited such that d satisfies the following:

a

u

n

d l V

M L +≤ §12.11.3

Mn = Nominal flexural capacity of beam

Vu = Factored shear at the section

la = Embedment length beyond center of support or

maximum of beameffective depth & 12d b at an inflection point.

Notes: 1. This provision limits the bar size to ensure adequate Ld is available.

2. Use 1.3(M n /V n ) in above equation if a compressive reaction confinesthe end of the bars.

§12.11.1





Seismic Provisions for Special Moment Frame (SMRF) Beams:

• P u ≤ 0.10A g f’ c §21.3.1.1

• 4≥

d

ClearSpan§21.3.1.2

• 3.0≥ Depth

Width§21.3.1.3

• Width ≥ 10” §21.3.1.4

• d b f

f A A w

y

c

bot stop s

'3& ≥ &

y

w

f

d b200≥ §21.3.2.1

025.0max = ρ , where d b

A

w

s

= ρ

• At face of joint, A s min bot = ½ A s top §1921.3.2.2

• Anywhere along the beam length, Mn min at top & bottom ≥ ¼ Mn max.

• Lap splices permitted only if confined over full length by hoop or spiralreinforcement (see figure below). §21.3.2.3

• Beam shear demand,2

clr g

clr

prB prA

e

Lw

L

M M V +

+= §21.3.4.1

Where, M prA & M prB = Moment capacities @ beam ends using 1.25f y & φ=1.0Wg = Factored gravity load

Lclr = Clear span

• If 2

u

e

V V ≥ & c g u f A P '05.0< §21.3.4.2

Ve = seismic shear demand from analysis.

Assume Vc = 0 & design stirrups to carry entire shear demand, Ve (shear

from analysis)—see CBD4 for Vs.

Provide 2 continuous bars top

and bottom, typical.




Since Ve approximates the maximum shear that can develop in a member,

use φ = 0.75 (§21.3.4.1 & 9.3.4).

Detailing Requirements for SMRF Beams

Note: For development lengths, splices etc. see ‘Concrete Column Design’

section.

IBC Design Requirements

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