Design of Rectangular Beams and One-Way Slabs - Design of Reinforced Concrete - page 87

Reinforcing bars. Note the supporting metal chairs. (Courtesy of Alabama Metal Industries
Corporation.)
cast and prestressed concrete or other concrete cast under plant control conditions requires
less cover, as described in Sections 7.7.2 and 7.7.3 of the ACI Code.
Notice the two #4 bars called hangers placed in the compression side of this beam.
Their purpose is to provide support for the stirrups and to hold the stirrups in position.
If concrete members are exposed to very harsh surroundings, such as smoke or acid
vapors, the cover should be increased above these minimums.
6. Minimum spacing of bars. The Code (7.6) states that the clear distance between
parallel bars cannot be less than 1 in.* or less than the nominal bar diameter. If the bars
are placed in more than one layer, those in the upper layers are required to be placed di-
rectly over the ones in the lower layers and the clear distance between the layers must be
not less than 1 in.
A major purpose of these requirements is to enable the concrete to pass between the
bars. The ACI Code further relates the spacing of the bars to the maximum aggregate sizes
for the same purpose. In Code Section 3.3.2, maximum permissible aggregate sizes are lim-
ited to the smallest of (a) one-fifth of the narrowest distance between side forms, (b) one-
third of slab depths, and (c) three-fourths of the minimum clear spacing between bars.
A reinforcing bar must extend an appreciable length in both directions from its point
of highest stress in order to develop its stress by bonding to the concrete. The shortest
length in which a bar's stress can be increased from 0 to f y is called its development length
or its anchorage length .
If the distance from the end of a bar to a point where it theoretically has a stress equal
to f y is less than its required development length, the bar may very well pull loose from
*25 mm in SI.