Page 44 - Design of Reinforced Masonry Structures

INTRODUCTION 1.13

Attention should be paid to the factor 0.7 associated with the earthquake load (E) in the
above load combinations and a factor 1.0 to the wind load (W) even though both are lateral
loads acting on the structure. The reason for this is the fact that W represents the allowable
stress-level wind load, whereas E represents strength-level load (hence load factor = 0.7 to
convert it to allowable stress-level load).
The earthquake load included in both the strength and allowable stress design combina-
tions is defined in ASCE 7-05 Section 12.4 as explained in the next section.

1.8.4 Earthquake Load Effects and Combinations
Earthquake load effects, denoted as E, include axial, shear, and flexural member forces
resulting from horizontal and vertical earthquake forces, both of which are caused by
ground motion. These effects are determined somewhat differently for strength design and
allowable stress design as follows.
1. For use in load combination (5) for strength design, and load combinations (5) and (6)
for allowable stress design:

E = E + E [ASCE 7-05 Eq. (12.4-1)]
h
v
2. For use in load combination (7) in strength design and load combination (8) for allow-
able stress design:
E = E – E [ASCE 7-05 Eq. (12.4-2)]
v
h
where E = seismic load effect
E = effect of horizontal seismic forces
h
E = effect of vertical seismic forces
v
The horizontal load effect is determined from the following equation:
E = rQ [ASCE 7-05 Eq. (12.4-3)]
h
E
where r = redundancy factor, as defined in ASCE-05 Sec. 12.3.4
Q = effects of horizontal forces resulting from the base shear, V, or seismic force
E
acting on a component in a structure, F p
The vertical seismic load effect is determined from the following equation:
E = 0.2S D [ASCE 7-05 Eq. (12.4-4)]
v
DS

where S DS = design spectral response acceleration parameter at short periods (ASC 7-05
Sec. 11.4.4)
D = effects of dead load
The vertical seismic load effect, E , can be taken to be equal to zero when using ASCE
v
7-05 Eqs. (12.4-1), (12.4-2), (12.4-5), and (12.4-6) (excerpted later in this section) where
S DS ≤ 0.125.

1.8.5 Seismic Load Combinations
When a structure is subjected to earthquake forces, the seismic load effect, E, in both the
strength and allowable stress design load combination equations presented earlier can be