Design Considerations for Earthquake Forces - Simplified Design of Reinforced Concrete Buildings - page 290

11.6.3 Load Combinations for Seismic Design

The seismic forces effect E in the load combinations introduced in Chapter 2 (Section 2.2.3) is the combined
effect of horizontal and vertical earthquake induced forces and is calculated as follows:

For load combination ACI Equation 9-5:

E = ρ Q E + 0.2 S DS D

For load combination ACI Equation 9-7:

E = ρ Q E - 0.2 S DS D

where ρ is a redundancy factor based on the structural redundancy present in the building. For SDC A, B, or

C, ρ = 1. For structures assigned SDC D, E or F, ρ = 1.3, unless the conditions in ASCE 12.2.4.2 are met, in

such case it is permitted to assumed ρ = 1. Also, for drift calculation and P-delta effects ρ = 1 for all seismic

design categories (ASCE 12.3.4.1).

11.7 OVERTURNING

A building must be designed to resist the overturning effects caused by the seismic forces (ASCE 12.8.5).
The overturning moment (M x ) at any level x is determined from the following equation:

n
∑

( )

M x = F i h i − h x

i = x

where

F i = the portion of the base shear V, induced at level i.

h i and h x = the height in feet from the base to level i or x.

11.8 STORY DRIFT

ASCE 7-05 specifies maximum allowable limits for story drift resulting from the design earthquake (ASCE

Table 12.12-1). Drift control is important to limit damage to partitions, shafts and stair enclosures, glass and
other fragile nonstructural elements. The design story drift δ x

(resulting from the design earthquake) at the floor level x at the top and bottom of story under consideration
(Figure 11-11). The design story drift

is the difference of the lateral deflection

x is calculated as follows:

x = δ x - δ x −1

where δ x-1 are the magnified lateral displacement at the top and bottom of the story considered

(Figure 11-11). The magnified lateral displacement is calculated from the following equation:

δ x and

δ x = C d δ xe /I