Civil Engineering Reference
In-Depth Information
For interior columns, the critical section (for both column and middle strips) is to be
taken at the face of the supports, but not at a distance greater than 0.175
1
from the center
of the column. At exterior supports with brackets or capitals, the moment used in the span
perpendicular to the edge shall be computed at a distance from the face of the support ele-
ment not greater than one-half of the projection of the bracket or capital beyond the face
of the supporting element.
Sometimes the total of the design moments (that is, the positive moment plus the average
of the negative end moments) obtained by the equivalent frame method for a particular span
may be greater than
M
o
2
n
/8,
as used in Chapter 16. Should this happen, the Code
(13.7.7.4) permits a reduction in those moments proportionately, so their sum does equal
M
o
.
w
u
2
17.6
COMPUTER ANALYSIS
The
equivalent frame method
was developed with the intention that the moment distribution
method was to be used for the structural analysis. Truthfully, the method is so involved that it
is not satisfactory for hand calculations. It is possible, however, to use computers and plane
frame analysis programs if the structure is especially modeled. (In other words, we must es-
tablish various nodal points in the structure so as to account for the changing moments of in-
ertia along the member axes.) There are also some computer programs on the market
especially written for these frames. One of the best known is called ADOSS (analysis and de-
sign of concrete floor systems) and was prepared by the Portland Cement Association.
PROBLEMS
17.1
Determine the end moments for the beams and columns of the frame shown for which the fixed-end moments, car-
ryover factors, and distribution factors (circled) have been computed. Use the moment distribution method. (
Ans.
44.9
ft-k and 21.0 ft-k at column bases)
17.2
Using the equivalent frame method, repeat Example
17.1 if the left-hand beam span is 40 ft and the right-hand
one is 30 ft. Assume no changes in distribution and carry-
over factors. The service live load is 90 psf, and the ser-
vice dead load (including slab weight) is 130 psf.
