Page 42 - Design of Reinforced Masonry Structures

INTRODUCTION 1.11

To account for these uncertainties, the nominal strength of a member is reduced by mul-
tiplying it with the strength reduction factor f; its value depends on member loads, such as
moment, shear, compression, etc.

1.8 LOAD COMBINATIONS

1.8.1 General

All structures and their load-carrying elements/components should be analyzed and/or
designed for one or more loads acting simultaneously. A structure at some point in time may
be subjected to only one type of load, for example, a dead load, whereas at other times it might
be subjected to several different loads acting simultaneously, for example, dead and live loads,
or dead, live, and lateral loads such as wind- and earthquake-induced loads. In order to main-
tain uniformity in application of load combinations by design professionals, the many loads
which are assumed to act singly or simultaneously are specified in the ASCE 7-05 Standard
[1.30] for both strength design (SD) and allowable stress design (ASD) philosophies.
Reinforced masonry structures can be designed based on either strength design phi-
losophy or allowable stress design philosophy. Allowable stress design has been in use for
many years for designing masonry structures, but the trend is shifting toward using strength
design. Design loads and their combinations are used differently when using these two
different design philosophies. Although discussion in this book is based on only strength
design load combinations, both strength and allowable stress design load combinations are
presented in this section for completeness. A discussion of load combinations has been
provided in Commentary to ASCE 7-05 Standard.

1.8.2 Load Combinations for Strength Design
The following are the basic load combinations for which structures, components, and their
foundations are to be designed so that their strength equals or exceeds the effects of factored
loads in the following combinations:
1. 1.4(D + F)
2. 1.2(D + F + T) + 1.6(L + H) + 0.5(L or S or R)
r
3. 1.2D + 1.6(L or S or R) + 0.5(L or 0.8W)
r
4. 1.2D + 1.6W + L + 0.5(L or S or R)
r
5. 1.2D + 1.0E + L + 0.2S
6. 0.9D + 1.6W + 1.6H

7. 0.9D + 1.0E + 1.6H
where D = dead load
E = earthquake load
F = loads due to fluid with well-defined pressures and maximum heights
H = loads due to lateral earth pressure, ground water pressure, or pressure of bulk
materials
L = live load
L = roof live load
r
R = rain load
S = snow load
T = self-straining force
W = wind load