Civil Engineering Reference
In-Depth Information
Occasionally, however, space or aesthetic requirements limit beams to such small
sizes that compression steel is needed in addition to tensile steel. To increase the moment
capacity of a beam beyond that of a tensilely reinforced beam with the maximum percent-
age of steel (when
0.005) it is necessary to introduce another resisting couple in the
beam. This is done by adding steel in both the compression and tensile sides of the beam.
Compressive steel increases not only the resisting moments of concrete sections, but also
the amount of curvature that a member can take before flexural failure. This means that
the ductility of such sections will be appreciably increased. Though expensive, compres-
sion steel makes beams tough and ductile, enabling them to withstand large moments, de-
formations, and stress reversals such as might occur during earthquakes. As a result, many
building codes for earthquake zones require that certain minimum amounts of compres-
sion steel be included in flexural members.
Compression steel is very effective in reducing long-term deflections due to shrinkage
and plastic flow. In this regard you should note the effect of compression steel on the long-
term deflection expression in Section 9.5.2.5 of the Code (to be discussed in Chapter 6 of this
text). Continuous compression bars are also helpful for positioning stirrups (by tying them to
the compression bars) and keeping them in place during concrete placement and vibration.
Tests of doubly reinforced concrete beams have shown that even if the compression
concrete crushes, the beam may very well not collapse if the compression steel is en-
closed by stirrups. Once the compression concrete reaches its crushing strain, the concrete
cover spalls or splits off the bars, much as in columns (see Chapter 9). If the compression
bars are confined by closely spaced stirrups, the bars will not buckle until additional mo-
ment is applied. This additional moment cannot be considered in practice because beams
are not practically useful after part of their concrete breaks off. (Would you like to use a
building after some parts of the concrete beams have fallen on the floor?)
Section 7.11.1 of the ACI Code states that compression steel in beams must be en-
closed by ties or stirrups, or by welded wire fabric of equivalent area. In Section 7.10.5.1
the Code states that the ties must be at least #3 in size for longitudinal bars #10 and
smaller and at least #4 for larger longitudinal bars and bundled longitudinal bars. The ties
may not be spaced farther apart than 16 bar diameters, 48 tie diameters, or the least di-
mension of the beam cross section (Code 7.10.5.2).
For doubly reinforced beams an initial assumption is made that the compression steel
yields as well as the tensile steel. (The tensile steel is always assumed to yield because of
the ductile requirements of the ACI Code.) If the strain at the extreme fiber of the com-
pression concrete is assumed to equal 0.00300 and the compression steel is located
two-thirds of the distance from the neutral axis to the extreme concrete fiber, then the
strain in the compression steel equals
t
s
A
2
3
0.003
0.002. If this is greater than the strain
10
6
)
0.00172 for 50,000-psi steel, the steel has
yielded. It should be noted that actually the creep and shrinkage occurring in the compres-
sion concrete help the compression steel to yield.
Sometimes the neutral axis is quite close to the compression steel. As a matter of fact,
in some beams with low steel percentages, the neutral axis may be right at the compres-
sion steel. For such cases the addition of compression steel is probably a waste of time
and money.
When compression steel is used, the nominal resisting moment of the beam is as-
sumed to consist of two parts: the part due to the resistance of the compression concrete
in the steel at yield, as say 50,000/(29
