Civil Engineering Reference
In-Depth Information
self as well as the external loads. The weight estimates for the beams selected in this text
are generally very close because the authors were able to perform a little preliminary pa-
perwork before making their estimates. You are not expected to be able to glance at a
problem and give an exact estimate of the weight of the beam required. Following the
same procedures as did the authors, however, you can do a little figuring on the side and
make a very reasonable estimate. For instance, you could calculate the moment due to the
external loads only, select a beam size, and calculate its weight. From this beam size, you
should be able to make a very good estimate of the weight of the final beam section.
Another practical method for estimating beam sizes is to assume a minimum overall
depth
h
equal to the minimum depth specified by the ACI if deflections are not to be cal-
culated. The ACI minimum for the beam in question may be determined by referring to
Table 4.1. Then the beam width can be roughly estimated equal to about one-half of the
assumed value of
h
and the weight of this estimated beam calculated
bh
144
times the con-
crete weight per cubic foot.
After
M
u
is determined for all of the loads, including the estimated beam weight, the
section is selected. If the dimensions of this section are significantly different from those
initially assumed, it will be necessary to recalculate the weight and
M
u
and repeat the
beam selection. At this point you may very logically ask, “What's a significant change?”
Well, you must realize that we are not interested academically in how close our estimated
weight is to the final weight, but rather we are extremely interested in how close our cal-
culated
M
u
is to the actual
M
u
. In other words, our estimated weight may be considerably
in error, but if it doesn't affect
M
u
by more than say 1% or forget it.
In Example 4.2 beam proportions are estimated as just described, and the dimensions
so selected are taken as the final ones. As a result, you can see that it is not necessary to
check the beam weight and recalculate
M
u
and repeat the design.
In Example 4.3 a beam is designed for which the total value of
M
u
(including the
beam weight) has been provided, as well as a suggested steel percentage.
Finally, with Example 4.4, the authors have selected a beam whose weight is un-
known. Without doubt many students initially have a little difficulty understanding how to
make reasonable member weight estimates for cases such as this one. To show how eas-
ily, quickly, and accurately this may be done for beams, this example is included.
We dreamed up a beam weight estimated out of the blue equal to 400 lb/ft. (We could
just as easily and successfully have made it 10 lb/ft or 1000 lb/ft.) With this value a beam
section was selected and its weight calculated to equal 619 lb/ft. With this value a very good
weight estimate was then made. The new section obviously would be a little larger than the
first one. So we estimated the weight a little above the 619 lb/ft value, recalculated the mo-
ment, selected a new section, and determined its weight. The results were very satisfactory.
1
2
%,
4.
Selection of bars.
After the required reinforcing area is calculated, Appendix
Table A.4 is used to select bars that provide the necessary area. For the usual situations,
bars of sizes #11 and smaller are practical. It is usually convenient to use bars of one size
only in a beam, although occasionally two sizes will be used. Otherwise the workmen
may become confused.
5.
Cover.
The reinforcing for concrete members must be protected from the surround-
ing environment; that is, fire and corrosion protection need to be provided. To do this the re-
inforcing is located at certain minimum distances from the surface of the concrete so that a
protective layer of concrete, called
cover
, is provided. In addition, the cover improves the
