Deep beams with web openings - Reinforced Concrete Deep Beams - page 63

3.2 Factors influencing behaviour
The main factors affecting the behaviour and performance of deep beams
with web openings are
i) span to depth ratio;

ii) cross-sectional properties (i.e. rectangular section, Tee-section, etc.);

iii) amount and location of main longitudinal reinforcement;

iv) amount, type and location of web reinforcement;

v) properties of concrete and reinforcements;

vi) shear span to depth ratio;

vii) type and position of loading;

viii) size, shape and location of web opening etc.

3.3 General behaviour in shear failure (under two-point loading)

Concrete strain variation at mid-span section indicates that before first cracking,
the beam behaves elastically, shows non-linear distribution of strain and more than
one neutral axes ( Figure 3.2) . The number of neutral axes decreases with
incremental loads and at ultimate stage only one neutral axis is present. Concrete
strain variation at the plane of rupture shows the deep beam behaviour also before
cracking and persistance of diagonal tension till failure. However, the extent of
crack width and the deflection pose no problem at the service loads. If, however,
the crack width is limited to 0.3 mm, the corresponding load will be in the range of
60-70% of the ultimate load (Ray and Reddy, 1979; Ray, 1980, 1982).

3.3.1 Beam with rectangular web openings
The first visible inclined cracks normally appear in the support bearing regions
and from the opening corners at load varying levels of about 36-55% of the
ultimate loads ( Figure 3.3) . With incremental loads, these initial cracks of
short lengths tend to propagate in their forward diagonal direction slowly.
Some similar types of crack parallel to and alongside the initial ones also form
for short lengths and these are not much active in the formation of critical
diagonal crack. For the loading range of about 50-97% of the ultimate, typical
diagonal cracks longer than the initial ones (resembling the phenomenon of a
critical diagonal crack in a solid web deep beam) suddenly emerge with a
harsh noise in the upper and lower shear zones above and below the openings
but appreciably away from the openings and bearing points. These critical
diagonal cracks instantaneously propagate both ways towards the bearing
regions and opening corners, widen and announce the failure of the structure.

3.3.2 Beam with circular web openings
The first visible cracks normally appear at almost the same range of
percentages of ultimate loads as in the case of rectangular openings ( Figure
3.4) There are two main distinctive features.