Civil Engineering Reference
In-Depth Information
In the preceding expressions the following values are used:
U
the design or ultimate load the structure needs to be able to resist
D
dead load
F
loads due to the weight and pressure of fluids
T
total effects of temperature, creep, shrinkage, differential settlement and
shrinkage-compensating concrete
L
live load
H
loads due to weight and lateral earth pressure of soils, groundwater pressure or
pressure of bulk materials
L
r
roof live load
S
snow load
R
rain load
W
wind load
E
seismic or earthquake load effects
When impact effects need to be considered, they should be included with the live
loads as per ACI Section 9.2.2.
Such situations occur when those loads are quickly ap-
plied, as they are for parking garages, elevators, loading docks, and others.
The load combinations presented in ACI Equations 9-6 and 9-7 contain a 0.9
D
value.
This 0.9 factor accounts for cases where larger dead loads tend to reduce the effects of
other loads. One obvious example of such a situation may occur in tall buildings that are
subject to lateral wind and seismic forces where overturning may be a possibility. As a re-
sult, the dead loads are reduced by 10% to take into account situations where they may
have been overestimated.
The reader must realize that the sizes of the load factors do not vary in relation to the
seriousness of failure. You may think that larger load factors should be used for hospitals
or high-rise buildings than for cattle barns, but such is not the case. The load factors
were developed on the assumption that designers would consider the seriousness of possi-
ble failure in specifying the magnitude of their service loads. Furthermore, the ACI load
factors are minimum values, and designers are perfectly free to use larger factors as they
desire.
For some special situations, ACI Section 9.2 permits reductions in the specified load
factors. These situations are as follows:
(a) In ACI Equations 9-3 to 9-5 the factor used for live loads may be reduced to 0.5
except for garages, for areas used for public assembly, and all areas where the live loads
exceed 100 psf.
(b) If the design wind load has been obtained from sources that do not include a wind
directionality factor, the designer is permitted to use 1.3
W
instead of 1.6
W
in Equations
9-4 and 9-6.
(c) Frequently, building codes and design load references convert seismic loads to
strength-level values (that is, in effect they have already been multiplied by a load factor).
This is the situation assumed in ACI Equations 9-5 and 9-7. If, however, service load seis-
mic forces are specified, it will be necessary to use 1.4
E
in these two equations.
