7/30/2019 IBC Design Requirements http://slidepdf.com/reader/full/ibc-design-requirements 1/22 SE ReferenceManual 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), Occupancy Category Description of Hazard Represented by Building Collapse I Low II All buildings except those in I, III and IV III Substantial • Public Assembly >300 people •Schools, daycares >250 people •College, adult education >500 people •Healthcare (no emergency or surgery) >50 people • J ails, 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 for earthquake backup •Aviation towers, control centers •Critical nation defense related building •Buildings containing hazardous materials quantities greater than 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(L r or S or R) +( f 1 L +0.8W) 4. 1.2D +1.6W +f 1 L +0.5(L r or S or R)
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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:
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
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.
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
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)
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
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.
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
Designed using Chapters 1 through 18 of ACI 318.
• Ordinary Moment Frames
Designed using Chapters 1 through 18 of ACI 318.
§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
(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.
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.
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.