Continuous deep beams - Reinforced Concrete Deep Beams - page 102

beam without web reinforcement. The reductions were generally minor and
were believed to be due to the direct compression strut being pulled apart by
the web reinforcement as the steel strained. This pulling apart reduces the
effective concrete strut capacity.
4.2.3 Continuous deep beams vs simple span deep beams
There are some distinguishing features of continuous deep beams which
render empirical equations based on simple span tests less than useful,
i) In a continuous deep beam, the point of contraflexure often occurs near the
critical section for shear. This situation causes difficulty with some empirical
equations. In the ACI procedures the ratio of moment to shear at the critical
section is a main parameter in the shear strength prediction equation.
Unfortunately, this ratio changes drastically when the point of contraflexure is
near the critical section for shear which produces wildly varying strength
predictions. If the point of contraflexure coincided with the critical section
(moment equals zero), the ACI equations would require division by zero!
ii) At an interior support in a continuous beam, the region of high shear and
high negative bending moment coincide. In simple span beams the region of
high shear coincides with a region of low bending moment. These
differences cast further doubt on the usefulness of empirical equations based
on simple span test data.
iii) In the tests by Rogowsky et al. (1986) horizontal web reinforcement was
found to have little influence on the ultimate strength of the continuous beams.
The amounts of horizontal web reinforcement used were relatively light (typical
of minimum reinforcement used in practice). Had greater amounts of horizontal
web reinforcement been used it is possible that an observable strength increase
might have resulted. It will be shown later that for beams with proportions
similar to those tested, the addition of horizontal web reinforcement is not a
particularly efficient method of increasing shear strength.
4.3 Capacity predictions by various methods
Several of the methods available for analysing deep beams are discussed.
They were used to predict the ultimate shear strength for the continuous
deep beams tested by Rogowsky et al . (1986). The comparisons of
prediction accuracy are not intended as a criticism of the various methods.
The comparisons are intended to illustrate the difficulty of extrapolating
methods developed from or for simple span deep beams to continuous deep
beams.
4.3.1 Elastic analysis
The discussion in this section pertains primarily to the classic elastic flexural
problem associated with deep beams. It has long been recognised that in