Concrete Span Definition for Shear 1

[Nick6781]

The old ACI 318-11 section 8.9 clearly defined the span length of concrete beams/slabs for determination of moment:

I believe the newer versions do not have this definition anymore. However, my question is related to shear.

We know that if a concentrated load occurs between the face of the support and a distance d away from it, the shear demand must be evaluated at the face of the support. In this case, regardless of how the span is defined, the maximum shear to be considered occurs at the same location.

However, if the support is considerably wide, using the center-to-center span (L1) to evaluate shear results in a lower shear force at the face of the support compared to using the clear span (L2). Which definition of span should be used?

 

[KootK]

However, if the support is considerably wide, using the center-to-center span (L1) to evaluate shear results in a lower shear force at the face of the support compared to using the clear span (L2). Which definition of span should be used?

Interesting question. I would say that shear ought to be based on L2. Fundamentally, you want to be capturing the real -- or perhaps conservative -- behavior of the thing and, for shear, the center to center span is unlikely to be the best representation of that.

The ACI stuff is based on columns of "normal" proportions relative to the beam depths. Where you are dealing with very wide supports (wallumns) it will be prudent to consider what effect that may have on the reality of things. And that appears to be just what you are doing here so good on you.

 

[Nick6781]

Interesting question. I would say that shear ought to be based on L2. Fundamentally, you want to be capturing the real -- or perhaps conservative -- behavior of the thing and, for shear, the center to center span is unlikely to be the best representation of that.

The ACI stuff is based on columns of "normal" proportions relative to the beam depths. Where you are dealing with very wide supports (wallumns) it will be prudent to consider what effect that may have on the reality of things. And that appears to be just what you are doing here so good on you.

Appreciate your insights! I have a few follow-up questions.

My issue is that the load is actually a moving load. If I use L2 as the span, I would essentially have to consider the entire load as the shear force when it is infinitesimally close to the face of the support. In that case, I can't make my one-way slab work without adding stirrups.

Do you think using L1 as the span length—and thus reducing the shear force at the face of the support—is justified when considering the influence cone of the point load?



Also, when the point load is infinitely close to the face of the left support, how much beam behavior is actually present? Should I be concerned about the interface shear instead, since a potential crack might form at that location?

 

[KootK]

In that case, I can't make my one-way slab work without adding stirrups.

If this situation is a moving, concentrated load on a one way slab, then I've got the perfect paper for you. Basically a permutation of punching shear.