Civil Engineering Reference
In-Depth Information
2.4.2
Arrangement of Live Load
According to ACI 8.11.1, it is permissible to assume that for gravity load analysis, the live load is applied only to
the floor or roof under consideration, with the far ends of the columns assumed fixed. In the usual case where the
exact loading pattern is not known, the most demanding sets of design forces must be investigated. Figure 2-9
illustrates the loading patterns that should be considered for a three-span frame.
w
d
+ w
˜
w
d
A
D
B
C
(1) Loading pattern for negative moment at support A
and positive moment in span AB
w
d
+ w
˜
w
d
A
C
D
B
(2) Loading pattern for negative moment at support B
w
d
+ w
˜
w
d
w
d
A
B
C
D
(3) Loading pattern for positive moment in span BC
Figure 2-9 Partial Frame Analysis for Gravity Loading
2.4.3
Design Moments
When determining moments in frames or continuous construction, the span length shall be taken as the
distance center-to-center of supports (ACI 8.9.2). Moments at faces of supports may be used for member design
purposes (ACI 8.9.3). Reference 2.2 provides a simple procedure for reducing the centerline moments to face
of support moments, which includes a correction for the increased end moments in the beam due to the
restraining effect of the column between face and centerline of support. Figure 2-10 illustrates this correction.
For beams and slabs subjected to uniform loads, negative moments from the frame analysis can be reduced by
w
u
˜
2
a
/6. A companion reduction in the positive moment of w
u
˜
2
a
/12 can also be made.
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