Civil Engineering Reference
In-Depth Information
Lab Building, Portland Cement Association, Skokie, Illinois. (Courtesy of Portland
Cement Association.)
In this section the theoretical points of cutoff have been discussed. As will be seen in
subsequent sections of this chapter, the bars will have to be run additional distances be-
cause of variations of moment diagrams, anchorage requirements of the bars, and so on.
The discussion of cutting off or bending bars is continued in Section 7.11
7.2
BOND STRESSES
A basic assumption made for reinforced concrete design is that there must be absolutely
no slippage of the bars in relation to the surrounding concrete. In other words, the steel
and the concrete should stick together or
bond
so they will act as a unit. If there is no
bonding between the two materials and if the bars are not anchored at their ends, they will
pull out of the concrete. As a result, the concrete beam will act as an unreinforced mem-
ber and will be subject to sudden collapse as soon as the concrete cracks.
It is obvious that the magnitude of bond stresses will change in a reinforced concrete
beam as the bending moments in the beam change. The greater the rate of bending mo-
ment change (occurring at locations of high shear), the greater will be the rate of change
of bar tensions and thus bond stresses.
What may not be so obvious is the fact that bond stresses are also drastically affected
by the development of tension cracks in the concrete. At a point where a crack occurs, all
of the longitudinal tension will be resisted by the reinforcing bar. At a small distance
along the bar at a point away from the crack, the longitudinal tension will be resisted by
both the bar and the uncracked concrete. In this small distance there can be a large change
