Building Design and Construction Handbook - Merritt - Ventech!

CONCRETE CONSTRUCTION 9.81
Minimum Reinforcement. For ƒ�c not greater than 4400 psi, reinforcement (both
positive and negative) with a yield strength ƒy should have an area equal to or
greater than 200/ƒ y times the concrete area of the rib bwd, where bw is the rib width
and d � rib depth. For exceeding 4400 psi, the area of reinforcement should be
ƒ� c
y
at least equal to 3�ƒ�b d/ƒ . Less reinforcement can be used, however, if the areas
c w
of both the positive and negative reinforcement at every section are one-third greater
than the amount required by analysis. (See also Art. 9.55.)
Maximum Reinforcement. Positive- and negative-moment reinforcement ratios
must not be greater than three-quarters of the ratio that produces balanced conditions
(Art. 9.46). The positive-moment reinforcement ratio is based on the width
of the top flange, and the negative-moment reinforcement ratio is based on the
width of the rib b w.
Reinforcement for one-way concrete-joist construction consists of straight top
and bottom bars, cut off as required for moment.
For top-slab reinforcement, straight top- and bottom-bar arrangements provide
more flexibility in attaining uniform distribution of top bars to control cracking in
the slab than straight and bent bars.
Requirements for structural integrity included in the ACI 318 Building Code
affect detailing of the bottom bars in the ribs. Over supports, at least one bottom
bar should be continuous or lap spliced to a bottom bar in the adjacent span with
a Class A tension lap splice (Art. 9.49.7). At exterior supports, one bottom bar
should be terminated with a standard hook.
For development (bond) of reinforcement, see Art. 9.49.
Figure 9.24b shows rebar quantities, lb/ft 2 of floor or roof area, for continuous
interior spans of one-way concrete-joist construction made with normal-weight concrete
for superimposed factored live load of 170 psf, for preliminary estimates.
9.57 SHEAR IN JOISTS
The factored shear force V u at a section without shear reinforcement should not
exceed
V � �V � �(2.2�ƒ�b d) (9.70)
u c c w
where V c � nominal shear strength of the concrete
� � strength-reduction factor (Art. 9.44) � 0.85
d � distance, in from extreme compression surface to centroid of tension
steel
b w � rib width, in.
Based on satisfactory performance of joist construction, the ACI 318 Building Code
allows the nominal shear strength V c for concrete in joists to be taken 10% greater
than for beams or slabs. The width b w can be taken as the average of the width of
joist at the compression face and the width at the tension reinforcement. The slope
of the vertical taper of ribs formed with removable steel pans can safely be assumed
as 1 in 12. For permanent concrete block fillers, the shell of the block can be
included as part of b w, if the compressive strength of the masonry is equal to or
greater than that of the concrete.