Civil Engineering Reference
In-Depth Information
Figure 16.2
Equivalent frame method.
Direct Design Method
The Code (13.6) provides a procedure with which a set of moment coefficients can be de-
termined. The method, in effect, involves a single-cycle moment distribution analysis of
the structure based on (a) the estimated flexural stiffnesses of the slabs, beams (if any),
and columns and (b) the torsional stiffnesses of the slabs and beams (if any) transverse to
the direction in which flexural moments are being determined. Some types of moment co-
efficients have been used satisfactorily for many years for slab design. They do not, how-
ever, give very satisfactory results for slabs with unsymmetrical dimensions and loading
patterns.
Equivalent Frame Method
In this method a portion of a structure is taken out by itself, as shown in Figure 16.2,
and analyzed much as a portion of a building frame was handled in Example 14.2. The
same stiffness values used for the direct design method are used for the equivalent
frame method. This latter method, which is very satisfactory for symmetrical frames as
well as for those with unusual dimensions or loadings, is presented in Chapter 17 of
this text.
16.4
COLUMN AND MIDDLE STRIPS
After the design moments have been determined by either the direct design method or the
equivalent frame method, they are distributed across each panel. The panels are divided
into column and middle strips, as shown in Figure 16.3, and positive and negative mo-
ments are estimated in each strip. The
column strip
is a slab with a width on each side of
the column centerline equal to one-fourth the smaller of the panel dimensions
2
. It
includes beams if they are present. The middle strip is the part of the slab between the two
column strips.
1
or
