Civil Engineering Reference
In-Depth Information
away from the support to the interior of the beam. This is contrary to the
usual expectation. However, it should be remembered that the deflection in
deep beams are substantially influenced by shear and, as such, location of
the opening in the region of high shear and intercepting the critical path is
understandable. The openings should invariably be provided with some loop
reinforcement in their periphery to avoid possible stress concentration.
3.6
Effects of main and web reinforcements
It was probably for the first time that Kong and his associates, in 1970-72,
considered the main reinforcement as an integral part of the shear
reinforcement for calculation purposes. The main steel not only acts as tension
reinforcement in flexure, but contributes substantially to the shear strength of
beams. Further, web reinforcement controls crack widths and deflection.
However, first cracking is generally not influenced by its provision. Of all
types of web reinforcement, the inclined type placed perpendicular to the
plane of rupture (critical diagonal crack) has been found to be the most
effective arrangement to offer resistance to sliding (Ray, 1980; 1982a, b 1983;
1984). The next practical and effective type is the horizontal web steel which
with nominal vertical web steel may further increase the effectiveness of the
beam and so its strength. It was observed (Ray, 1980; 1982a, b; 1983; 1984)
that in beams with web openings, horizontal web reinforcement distributed
equally on either side of the opening location showed better results. In beams
with unusually high web reinforcement, special attention should be paid to the
detailing of anchorage and bearings at the load and support points. Otherwise,
web steel must be limited to a certain amount.
Failure will be gradual and slow in beams with web reinforcement, while
it is sudden in beams without web reinforcement. A vertical web
reinforcement placed near the vertical edge of a beam with web opening
located in its neighbourhood, guards against any premature failure due to
rotation of the corner of the beam. From electrical strain measurements on
main steel it was observed (Ray, 1980; 1982) that the general trend of the
stress-strain characteristics under different load levels resembled stress-
strain behaviour of steel but shear failure occurred at steel strains below the
yield-point values normally expected in shear failures. It was further seen
(Ray, 1980; 1982) that after cracking of the beams the steel strain rapidly
increased at the location near the supports and the steel strain in the flexural
zone remained almost constant (i.e. tension was uniform). The inclined
cracks began to develop at higher loads.
3.7
Diagonal mode of shear failure load
The failure of reinforced concrete deep beams occurs under a state of biaxial
stress. It is assumed that the diagonal mode of failure, more commonly
