In order to prevent shear failure, one must understand the mechanism that provides shear resistance to an RC structure. Understanding these can help us work on specific aspects and hence improving the design. Shear failure in RC structures has not been an easy nut to crack for researchers. Although beams have been designed against shear failure for over a century or even two, we hadn’t really understood it well enough until a decade ago. This is why resistant factor for bending is at 0.9 while that for shear is at 0.75 or something close depending on the code you use. The more closer they are to 1.0, the better we understand the mechanism. We got a better idea of the shear failure mechanism only recently. It started with the UCSD shear model which was later modified to the modified UCSD shear model. Here is a quick overview.
Shear resistance in RC structures is provided by three components.
Concrete contribution
Steel contribution
Axial load contribution
The concrete contribution can again be divided into three parts. The main mechanism through which plain concrete resists shear is aggregate interlocking. This is due to the presence of coarse aggregates and how they resist moving relative to each other because of interlocking. Sound aggregates and good mix design can improve this aspect.
The second mechanism is because of the presence of a compression zone in the section. At any section along the beam there would be a neutral axis (NA) and anything above NA will be in compression. Here’s an analogy to illustrate how this resists shear. If you hold a pile of books vertically with one hand on top and the other on the bottom, the pile is stable. If you tilt the pile horizontal, the books fall down. But if you apply a compressive force while holding it horizontal, the book pile stays horizontal. The compression you apply is resisting the vertical shear. In the same way, the compression zone in concrete contributes to resist shear. So, increasing the compressive strength can increase shear resistance.
The third mechanism is the dowell action which I don’t want to go into detail. Pleas google this one if you are interested. This depends on the strength of the longitudinal steel and any increase in that increases shear resistance. So, when all of the three combine, the concrete by itself resists some shear (although dowell action can be controversial whether it can be considered a concrete contribution).
Next is the steel contribution. This is the obvious one that everyone knows about. Providing stirrups or ties help resisting shear force. This contribution is proportional to the area of transverse reinforcement. Increasing this increases shear resistance.
Last, but not the least, any axial load helps resist shear. It can be prestressing in beams or the already existing axial loads in the columns. So, in your case, prestressing the beam increases shear resistance.
Shear resistance in RC structures is provided by three components.
Concrete contribution
Steel contribution
Axial load contribution
The concrete contribution can again be divided into three parts. The main mechanism through which plain concrete resists shear is aggregate interlocking. This is due to the presence of coarse aggregates and how they resist moving relative to each other because of interlocking. Sound aggregates and good mix design can improve this aspect.
The second mechanism is because of the presence of a compression zone in the section. At any section along the beam there would be a neutral axis (NA) and anything above NA will be in compression. Here’s an analogy to illustrate how this resists shear. If you hold a pile of books vertically with one hand on top and the other on the bottom, the pile is stable. If you tilt the pile horizontal, the books fall down. But if you apply a compressive force while holding it horizontal, the book pile stays horizontal. The compression you apply is resisting the vertical shear. In the same way, the compression zone in concrete contributes to resist shear. So, increasing the compressive strength can increase shear resistance.
The third mechanism is the dowell action which I don’t want to go into detail. Pleas google this one if you are interested. This depends on the strength of the longitudinal steel and any increase in that increases shear resistance. So, when all of the three combine, the concrete by itself resists some shear (although dowell action can be controversial whether it can be considered a concrete contribution).
Next is the steel contribution. This is the obvious one that everyone knows about. Providing stirrups or ties help resisting shear force. This contribution is proportional to the area of transverse reinforcement. Increasing this increases shear resistance.
Last, but not the least, any axial load helps resist shear. It can be prestressing in beams or the already existing axial loads in the columns. So, in your case, prestressing the beam increases shear resistance.
