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CRITERIA FOR BUILDING DESIGN
5.8 CHAPTER FIVE
Alternatively, and for all types of lateral load resisting systems, the following equation may be used:
∑ P = R ∑ HL (5.9b)
e2
M
∆ H
where R M = 1.0 for braced-frame systems and R L = 0.85 for moment-frame and combined sys-
tems, unless a larger value is justified by analysis.
E = modulus of elasticity, E = 29,000 ksi (200,000 MPa)
4
4
I = moment of inertia in the plane of bending, in (mm )
L = story height, in (mm)
K 1 = effective length factor in the plane of bending, calculated on the basis of no sidesway, set
equal to 1.0 unless analysis indicates a smaller value may be used
K 2 = effective length factor in the plane of bending, calculated for a sidesway buckling analysis
∆ H = first-order interstory drift due to lateral forces, in (mm)
ΣH = story shear produced by lateral forces used to compute ∆ H , kips (N)
5.2.2 Design Requirements
Where required strengths are obtained by second-order analyses, the analyses are subject to the fol-
lowing. For LRFD design, analyses should be made using the LRFD load combinations. For ASD
design, make the analyses using 1.6 times the ASD load combinations, and then divide the results by
1.6 to obtain the required strength. Note, however, that the B 1 and B 2 amplifiers used in the ampli-
fied first-order analysis (Art. 5.2.1) already include the 1.6 multiplier.
Where B 2 [Eq. (5.6)] > 1.5, required strengths must be determined from a direct analysis method
given in App. 7 of the AISC Specification. This method is optional where B 2 ≤ 1.5. Where B 2 ≤ 1.5,
required strengths can be determined from either a second-order analysis, such as is outlined in
Art. 5.2.1, or if conditions 1–3 that follow are satisfied, by a direct first-order analysis.
Where a second-order analysis is used, all gravity load combinations must include a minimum
lateral load at each level of 0.002 times the design gravity load at that level. The lateral load must be
considered independently in two orthogonal directions. Where the ratio of second-order drift to first-
order drift does not exceed 1.1, compression members may be designed using a K factor of 1.0.
The conditions for a direct first-order analysis are as follows.
1. Select a member with yield strength sufficient to satisfy
(5.10)
αP r < 0.5P y
where α= 1.0 (LRFD) = 1.60 (ASD)
P r = required axial compressive strength, kips (N)
P y = member yield strength AF y , kips (N)
2. Include in all load combinations, applied in combination with other loads at each level of the
structure, an additional load N i , given by
(5.11)
N i = 2.1(∆/L)Y i > 0.0042Y i
where Y i = gravity load (from LRFD load combination or 1.6 times ASD load combination) on
level i, kips (N)
L = story height, in (mm)
∆= first-order interstory drift due to design loads, in (mm). Calculate ∆ at strength loads.
Where ∆ varies over the plan of the structure, take ∆ as the average drift weighed in pro-
portion to vertical load or, alternatively, as the maximum drift.
∆/L = maximum ratio of ∆ to L for all stories
The additional load N i must be considered independently in two orthogonal directions.
3. Apply the nonsway amplification factor B 1 [Eq. (5.5)] to the total member moments.
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