Page 478 - Design of Reinforced Masonry Structures

7.40 CHAPTER SEVEN

lateral forces to supporting vertical seismic force–resisting elements such as shear walls
or braced frames. In all cases of force transfer, a complete, continuous load path must be
followed in order to preserve structural safety.
Minimum design seismic forces acting on a structure can be determined by methods
commonly referred to as equivalent lateral force procedure and dynamic analysis. The lat-
ter method can be further classified into response spectrum analysis and the time-history
analysis. The equivalent lateral force procedure is covered by ASCE 7-05 Section 12.8,
which is briefly reviewed in this section. ASCE 7-05 Section 12.14.8 also permits a sim-
plified lateral force analysis procedure, which is much simpler than the equivalent lateral
force procedure. Readers should refer to Refs. 7.13 and 7.14 for a comprehensive discus-
sion on this topic; the following is a summary from these references and Ref. 7.11.

7.6.3 Seismic Load Effects and Load Combinations
In accordance with MSJC-08 Section 3.1.2, design loads for strength design of masonry
are to be determined based on load combinations given by ASCE 7-05 Section 2.3 (unless
provided for differently by the local building codes). Of the seven load combinations speci-
fied in Section 2.3, the two that include seismic load are load combinations 5 and 7, which
can be expressed as follows:
U = 1.2D + 1.0E + f L + 0.2S (7.39)
1
U = 0.9D + 1.0E + 1.6H (7.40)

where D = dead load
E = earthquake load
H = load due to lateral earth pressure, ground water pressure, or pressure of bulk
materials
L = live load
S = snow load
f = load factor, which is equal to 1.0 for live loads for garages and areas occupied
1
as places of public assembly, and equal to 0.5 for all other live loads
The earthquake load E in Eqs. (7.39) and (7.40) is a function of both horizontal and ver-
tical earthquake-induced forces as specified in ASCE 7-05 Section 12.4, and is expressed
as Eq. 7.41:
E = E ± E (7.41)
h
v
where E and E , respectively, are effects of horizontal and vertical earthquake-induced
h
v
forces defined by Eqs. (7.42) and (7.43):

E = rQ E (7.42)
h
E = 0.2 S D (7.43)
DS
v
where Q = effects from horizontal earthquake forces from V or F p
E
V = total design seismic lateral force or base shear
F = seismic force acting on a component of the structure
p
r = redundancy factor
D = effects of dead load
S = 5 percent damped, design spectral acceleration parameter for short periods
DS
(0.2-s period)