Structural Concrete - Hassoun

20.3 Analysis Procedures 811
Table 20.9 Allowable Story Drift, Δ a
(in.) a Risk Category
Structure I or II III IV
Structures, other than masonry shear wall structures, four
c
0.025 h sx 0.020 h sx 0.015 h sx
stories or less above the base in height with interior walls,
partitions, ceilings, and exterior wall systems that have been
designed to accommodate the story drifts
Masonry cantilever shear wall structures d 0.010 h sx 0.010 h sx 0.010 h sx
Other masonry shear wall structures 0.007 h sx 0.007 h sx 0.007 h sx
All other structures 0.020 h sx 0.015 h sx 0.010 h sx
a h sx
is the story height below level x.
b For seismic-force-resisting systems comprised solely of moment frames is seismic design categories D, E, and F, the allowable
story drift shall comply with the requirements of ASCE7-10, Section 12.12.1.1.
c There shall be no drift limit for single-story structures with interior walls, partitions, ceilings, and exterior wall systems that
have been designed to accommodate the story drift.
d Structures in which the basic structural system consists of masonry shear walls designed as vertical elements cantilevered
from their base or foundation support that are so constructed that moment transfer between shear walls (coupling) is negligible.
Source: ASCE 7-10 Minimum Design Loads for Buildings and Other Structures, 2010. With permission from ASCE.
where
δ x = maximum inelastic response displacement
δ xe = design-level elastic lateral displacement at floor level x under seismic lateral forces
C d = deflection amplification factor from Table 20.7
I e = occupancy importance factor from Table 20.1
2. The design story drift can then be calculated as the difference between the deflections of the
centers of masses of any two adjacent stories. Definition of story drift is shown in Fig. 20.4.
Δ = δ x − δ x−1 (20.26)
3. Check for the P-delta effect and adjust for magnification factor if needed.
P-Delta Effect. An accurate estimate of story drift can be obtained by the P-delta analysis. In
first-order structural analysis, the equilibrium equations are formulated for the un-deformed shape
of a structure. When deformations are significant, the second-order analysis must be applied, and
the P-delta effect must be considered in determining the overall stability of the structure. The
P-delta effect does not need to be applied when the ratio of secondary to primary moment, θ,
does not exceed 0.1. This ratio is given by the following equation:
θ =
p xΔI e
(20.27)
V x h sx C d
where
θ = stability coefficient
P x = total unfactored vertical design load at and above level x (dead, floor live, and snow load)
Δ = design story drift (in.)
V x = seismic shear force between level x and x–1
h sx = story height below level x (in.)
C d = deflection amplification factor from Table 20.7 (Table 12.2-1 of ASCE7–10)