Bottom longitudinal reinforcement at supports for simply supp. beams

[almait]

Hi to every one.
I'm an italian engineer.
I'm studying the ACI 318 Code and at the moment I'm interested to shear desgin. Reading the code I didn't find any provision about bottom longitudinal reinforcement to place at supports and capable to withstand a tensile force equal to shear value V.
I saw some shop drawings of precast beam (Reverse T and double tee) and there's no similar reinforcement ...
Can someone explain me why the ACI Code doesn't prescribe this reinforcement at supports? Is it a difference of theories used to study the shear failure? It seems that this kind of reinforcement is needed for deep beams and not for "normal beams".
Thanks to every one wants to help me to understand better.
Sorry for my english.
Best regards

 

[kslee1000]

The shear crack in concrete is mainly due to the diagonal tension field normal to the shear plane exceeds the concrete tensile strength. The crack plane is usually less than 45 degrees measured from the bottom face, contrary to the deep beam, for which the crack plane is close to vertical (more than 75 degrees from the horizon), and for which, the presence of the horizontal reinforcement is crucial.

In calculations follow through the theories, the positive bars may not necessary to get into the support. However, since the design assumptions are flawed (Hook's law, homogeneous material), and the properties and behaviors are difficult to master even in the lab, not to mention in the real world. To cover these shortcomings, the ACI does have requirement for ectending the positivement as pointed out by JAE and others. I think this is holding true for all of the codes around the world, but just arranged and mentioned in different manners. As a final reminder, a few positive reinforcement shall be extended into the support, but not to make the interface too rigid thus losing ductility.

 

[cooperDBM]

I don't claim to be a shear expert but here goes ...

If you're trying to visualize the effect of the bottom longitudinal steel in shear keep in mind that almost all shear theories, especially ACI, use some sort of truss analogy. The bottom long. steel is the bottom chord and is vital for the stability of the system after cracking. ACI's shear provisions, which are now long-in-the tooth, doesn't explicitly check the strength of this "bottom chord" though newer codes do. Under ACI the design of the bottom steel is generally covered off by minimum steel and embedment requirements.

If the bottom long. steel is understrength then the axial strain, say mid-height, can increase, reducing the shear strength of the concrete through reduced interlock, etc.

There is now interest in the US, including ACI, in looking at the long. steel since it's recognized that increasing the long. steel, or even just accounting for it, can reduce the transverse steel requirements. They work together. It's also more important whether there isn't any transverse steel (i.e. hollowcore).

My two cents (three cents Canadian and rising)