Structural Concrete - Hassoun

808 Chapter 20 Seismic Design of Reinforced Concrete Structures
• The structure shall have at least two lines of lateral resistance in each of two major axis
directions.
1. The seismic base shear, V, in a given direction shall be determined in accordance with
where
V = FS DS
R W (20.19)
S DS = 2 3 F aS s
where F a is permitted to be taken as 1.0 for rock sites, 1.4 for soil sites, or determined in
accordance with Section 20.2.2. For the purpose of this section, sites are permitted to be
considered to be rock if there is no more than 10 ft (3 m) of soil between the rock surface
and the bottom of the spread footing or mat foundation. In calculating S DS , S S shall be in
accordance with Section 20.2.2, but need not be taken larger than 1.5.
F = 1.0 for one-story buildings above grade plane
F = 1.1 for two-story buildings above grade plane
F = 1.2 for three-story buildings above grade plane
R = response modification factor from Table 20.7 (ASCE 7-10, Table 12.2-1)
W = effective seismic weight of structure that shall include total dead and other loads listed in the
following text (ASCE 7-10, Section 12.14.8.1)
(a) In areas used for storage, a minimum of 25% of the floor live load.
Exceptions:
i. Where the inclusion of storage loads adds no more than 5% to the effective seismic
weight at that level, it need not be included in the effective seismic weight.
ii. Floor live load in public garages and open parking structures need not be included.
(b) Where provision for partitions is required by ASCE 7-10, Section 4.3.2 (Provision for
Partitions) in the floor load design, the actual partition weight, or minimum weight of
10 psf (0.48 kN/m 2 ) of floor area, whichever is greater.
(c) Total operating weight of permanent equipment.
(d) Where the flat roof snow load, P f , exceeds 30 psf (1.44 kN/m 2 ), 20% of the uniform
design snow load, regardless of actual roof slope.
2. The lateral seismic forces calculation: Lateral seismic forces can be determined from
(ASCE 7-10, Section 12.14.8.2)
F x = FS DS
R w x (20.20)
where
F x = seismic force applied at level x
w x = portion of effective seismic weight of structure, W, at level x
20.3.4 Summary: Simplified Analysis Procedure
Step 1. Check whether the structure satisfies conditions described in Section 20.3.3 for qualification
for the simplified analysis procedure.
Step 2. Determine the value of S DS as described in Section 20.2.2.
Step 3. Choose appropriate R factor from Table 20.7.
Step 4. Determine the effective seismic weight, W, of the structure as described in Section 20.3.3.