Civil Engineering Reference
In-Depth Information
4
Design of Rectangular Beams
and One-Way Slabs
4.1
LOAD FACTORS
Load factors are numbers, almost always larger than 1.0, which are used to increase the
estimated loads applied to structures. They are used for loads applied to all types of mem-
bers, not just beams and slabs. The loads are increased to attempt to account for the uncer-
tainties involved in estimating their magnitudes. How close can you estimate the largest
wind or seismic loads that will ever be applied to the building which you are now occupy-
ing? How much uncertainty is present in your answer?
You should note that the load factors for dead loads are much smaller than the ones
used for live and environmental loads. Obviously, the reason is that we can estimate the
magnitudes of dead loads so much more accurately than we can the magnitudes of those
other loads. In this regard, you will notice that the magnitudes of loads that remain in
place for long periods of time are much less variable than are those loads applied for brief
periods such as wind and snow.
Section 9.2 of the Code presents the load factors and combinations that are to be used
for reinforced concrete design. The required strength
U
, or the load-carrying ability of a
particular reinforced concrete member, must at least equal the largest value obtained by
substituting into ACI equations 9-1 to 9-7. The following equations conform to the re-
quirements of the International Building Code (IBC)
1
as well as to the values required by
ASCE Standard 7.
2
U
1.4(
D
F
)
(ACI Equation 9-1)
(ACI Equation 9-2)
(ACI Equation 9-3)
(ACI Equation 9-4)
(ACI Equation 9-5)
(ACI Equation 9-6)
(ACI Equation 9-7)
U
1.2(
D
F
T
)
1.6(
L
H
)
0.5(
L
r
or
S
or
R
)
U
1.2
D
1.6(
L
r
or
S
or
R
)
(1.0
L
or 0.8
W
)
U
1.2
D
1.6
W
1.0
L
0.5(
L
r
or
S
or
R
)
U
1.2
D
1.0
E
1.0
L
0.2
S
U
0.9
D
1.6
W
1.6
H
U
0.9
D
1.0
E
1.6
H
1
International Code Council,
International Building Code 2003
(IBC).
2
American Society of Civil Engineers,
Minimum Design Loads for Buildings and Other Structures
. ASCE 7-02
(Reston, VA: American Society of Civil Engineers), p. 6.
81
