Shear Design: Critical Section of a Short Beam

[umrce]

ACI & AASHTO specify the location of the critical shear section as typically being a distance d from the face of the support.

Say I have a bridge abutment that has a depth of 4.5 ft sitting on a 2ft dia. drilled shaft foundation that has a spacing of 5 ft so my span face to face is approx. 3 ft (ignoring the effects of being a circular shaft to simplify this example).

As I'm sure you already see where I'm going with this the clear span length is less than the depth of the member. What location along the beam should I use to determine my required design shear?

 

[ishvaaag]

Quite likely you may dismiss shear and directly pass to consider a strut and tie model. This is something within the connection influence area.

 

[hokie66]

ishvaaag is correct. The location of critical shear section is irrelevant in this case. Design as a deep beam, not a flexural member.

 

[csd72]

I believe that most codes insist that this span/depth is to be designed as strut and tie.

 

[rscassar]

If you load the member with a concentrated point load midspan it will strut at an angle greater than 45deg to the support. Strut-tie all thwe way.

 

[Teguci]

CRSI has example calculations for dealing with pile caps. In your case, their example may provide you with a starting point.

To directly answer your question, it is not the distance from the support that becomes critical in a deep beam but the angle the compression strut forms between your load and support.

As for circular columns/supports, I usually translate them to bearing lengths = 80% x diameter as suggested by ACI.

Make sure you consider mis-placement of the foundations. A 3" offset from design center is not rare.