Civil Engineering Reference
In-Depth Information
live loads are combined with the other loads on the structure to determine the total design
forces.
The American Concrete Institute in ACI 318-02 requires that the load combinations
used for design are those specified in ASCE 7-02. In accordance with ASCE 7-98, the fol-
lowing load combinations are used.
1.
1.4Dead
2.
1.2Dead
1.6LL1
1.6LL2
0.5LR1
0.5LR2
3.
1.2Dead
1.6LL1
0.5LR1
4.
1.2Dead
1.6LL2
0.5LR2
5.
1.2Dead
1.6LR1
1.6LR2
0.5LL1
0.5LL2
6.
1.2Dead
1.6LR1
0.5LL1
7.
1.2Dead
1.6LR2
0.5LL2
8.
1.2Dead
1.6LR1
1.6LR2
0.8Wind
9.
1.2Dead
1.6LR1
0.8Wind
10.
1.2Dead
1.6LR2
0.8Wind
11.
1.2Dead
1.6Wind
0.5LL1
0.5LL2
0.5LR1
0.5LR2
12.
1.2Dead
1.6Wind
0.5LL1
0.5LR1
13.
1.2Dead
1.6Wind
0.5LL2
0.5LR2
14.
1.2Dead
0.5LL1
0.5LL2
15.
1.2Dead
0.5LL1
16.
1.2Dead
0.5LL2
17.
0.9Dead
1.6Wind
Because the frame is symmetric and the gravity loads acting on it are symmetric, the wind
load only needs to be considered in one direction. Frame symmetry will need to be main-
tained when we select members so that this limitation in the load combinations remains
valid. If symmetry cannot be maintained, load combinations 8 through 13 and 17 will
need to be repeated for wind acting from the other direction.
21.4
PRELIMINARY DESIGN AND ANALYSIS
Prior to beginning an analysis of the structure, we must determine the size of its members.
The problem is that we cannot determine the size until we know the forces in the mem-
bers, and we cannot determine the forces until we know the sizes so that we can perform
an analysis. To break this cycle, we will estimate the size of the beams and columns, and
then we must analyze the structure. Using the results of this analysis, we will refine the
design, and then update and reanalyze the structure as necessary. For a structure of this
size, reasonable estimates of member sizes are 12
for
girders. During design, we will try to keep the width of the columns and the width of the
12
for columns and 12
18
