Design Considerations for Earthquake Forces - Simplified Design of Reinforced Concrete Buildings - page 291

where

C d = deflection amplification factor presented in ASCE Table 9.5.2.2

δ xe = elastic lateral deflection (in.) due to the code prescribed seismic forces

I = occupancy importance factor defined in 11.6.1

δ x

δ x-1

Figure 11-11 Interstory Drift,

11.9 P- EFFECT

Seismic forces cause the structure to deflect laterally. As a result, secondary moments are induced in the
structural members due to the displaced gravity load as shown in Figure 11-12. This secondary moment
effect is known as the P- effect. P- effects are not required to be considered if the stability index θ is equal

to or less than 0.10. The stability index θ is calculated as follows (ASCE Eq. 12.8-16):

P x Δ
V x h sx C d

θ=

where:

P x = total unfactored vertical design load at and above level x (kips)

= design story drift (inches) occurring simultaneously with V x .

V x = seismic shear force (kips) acting between level x and x-1

h sx = story height (feet) below level x

C d = deflection amplification factor defined in 11.8

The stability coefficient, θ , shall not exceed θ max calculated as follows:

0.5
β

θ max =

C d ≤

0.25

where:

β = the ratio of shear demand to shear capacity for the story between level x and x-1 which may

be conservatively taken equal to 1.0.