Civil Engineering Reference
In-Depth Information
Figure 14.26
Computing Stiffness Factors
1
12
I
of columns
(16)(16)
3
5461 in.
4
5461
12
k
of columns
I
455
1
12
b
w
h
3
1
12
(12)(24)
3
27,648 in.
4
Equivalent
I
of T beam
(2)
(2)
k
of T beam
27,648
24
1152
Record stiffness factors on the frame and compute distribution factors, as shown in
Figure 14.27.
Balance fixed-end moments and draw shear and moment diagrams, as shown in Fig-
ure 14.28.
Assumed Points of Inflection
Another approximate method of analyzing statically indeterminate building frames is to
assume the locations of the points of inflection in the members. Such assumptions have
the effect of creating simple beams between the points of inflection in each span, and the
positive moments in each span can be determined by statics. Negative moments occur in
the girders between their ends and the points of inflection. They may be computed by
considering the portion of each beam out to the point of inflection (P.I.) to be a can-
tilever. The shear at the end of each of the girders contributes to the axial forces in the
columns. Similarly, the negative moments at the ends of the girders are transferred to the
columns.
In Figure 14.29, beam
AB
of the building frame shown is analyzed by assuming
points of inflection at the one-fifth points and assuming fixed supports at the beam ends.
