Civil Engineering Reference
In-Depth Information
Other Items
The next four chapters of this topic are devoted to several other important items relating
to beams. These include different shaped beams, compression reinforcing, cracks, bar de-
velopment lengths, and shear.
Further Notes on Beam Sizes
From the standpoints of economy and appearance, only a few different sizes of beams
should be used in a particular floor system. Such a practice will save appreciable amounts
of money by simplifying the formwork and at the same time will provide a floor system
that has a more uniform and attractive appearance.
If a group of college students studying the subject of reinforced concrete were to design
a floor system and then compare their work with a design of the same floor system made by
an experienced structural designer, the odds are that the major difference between the two de-
signs would be in the number of beam sizes. The practicing designer would probably use
only a few different sizes, whereas the average student would probably use a larger number.
The designer would probably examine the building layout to decide where to place
the beams and then would make the beam subject to the largest bending moment as small
as practically possible (that is, with a fairly high percentage of reinforcing). Then he or
she would proportion as many as possible of the other similar beams with the same out-
side dimensions. The reinforcing percentages of these latter beams might vary quite a bit
because of their different moments.
4.5
DETERMINING STEEL AREA WHEN BEAM DIMENSIONS
ARE PREDETERMINED
Sometimes the external dimensions of a beam are predetermined by factors other than
moments and shears. The depth of a member may have been selected on the basis of the
minimum thickness requirements discussed in Section 4.2 for deflections. The size of a
whole group of beams may have been selected to simplify the formwork as discussed in
Section 4.4. Finally, a specific size may have been chosen for architectural reasons. Next
we briefly mention three methods for computing the reinforcing required. Example 4.5 il-
lustrates the application of each of these methods.
Appendix Tables
The value of
M
u
/
bd
2
can be selected from the tables. For most
situations this is the quickest and most practical method.
The tables given in Appendices A
and B of this text apply only to tensilely reinforced rectangular sections. Furthermore, we
must remember to check
can be computed, and
values.
Use of
Formula
The following equation was previously develope
d in Sectio
n 3.4 for rectangular sections.
c
0.85
f
2
R
n
0.85
f
1
1
f
y
c
