Design Considerations for Earthquake Forces - Simplified Design of Reinforced Concrete Buildings - page 284

For structures assigned to seismic design categories B, C, D, E or F, the ASCE introduces three analytical
procedures to account for earthquake effects; equivalent lateral force analysis (ASCE 12.8), modal response
spectrum analysis (ASCE 12.9) and seismic response history procedure (ASCE Chapter 16). For buildings
within the scope of this publication, the equivalent lateral forces procedure provides the most suitable
approach. The following sections present the equivalent lateral force method. For other analytical procedures
References 11.1 and 11.2 should be consulted.

11.6 EQUIVALENT LATERAL FORCE PROCEDURE

For reinforced concrete structures with or without irregularities and assigned to SDC B or C, the equivalent lat-
eral force procedure can be used. Also the equivalent lateral force method can be used for regular structures
assigned to SDC D, E, or F provided that the fundamental period of the structure T < 3.5 T s where T s = S D1 /S DS
(see below for calculation of T). Limitations on the applicability of this method to irregular structures assigned
to SDC D, E, or F are given in ASCE (Table 12.6-1).

11.6.1 Design Base Shear

The seismic base shear, V, in a given direction is a fraction of the dead weight of the structure. V is calculated
from:

V = C s W

where:

C s = seismic response coefficient
W = the effective seismic weight of the structure which includes the total dead load and the loads listed
below (ASCE 12.7.2):

(1) In areas used for storage, a minimum of 25% of the floor live load (floor live load in public garages and
open parking structures need not be included)

(2) Where an allowance for partition load is included in the floor load design, the actual partition weight or a
minimum weight of 10 psf of floor area, whichever is greater

(3) Total operating weight of permanent equipment

(4) 20% of flat roof snow load where flat roof snow load exceeds 30 psf

The seismic response coefficient is calculated as follows:

S D1 S DS

R
I

C s = ≤

For T ≤ T L

R
I

⎛ ⎞ ⎛ ⎞

T

⎜ ⎟ ⎜ ⎟

S D1 T L
T 2 R
I

C s =

For T > T L

⎛ ⎞

⎜ ⎟