From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Fri, 24 Mar 2000 09:35:38 -0500
I would not let a beam bear directly on a wall (or a column for that
matter). A beam will deflect and you will end up with edge bearing and that
will cause trouble.
Without looking in the code (since it is at the office), I typically use a
minimum of 1" (more for heavily loaded beams) between the edge of the wall
and the edge of the bearing plate/pad. The bearing area is, of course,
determined by the allowable bearing stress per ACI Code. (Beware of random
oriented fiber [ROF] neoprene bearing pads as the allowable bearing stress on
these can be higher than the allowable bearing stress on concrete.) Also, I
will rarely use the allowable increase in bearing stress permitted by the
code based on the area ratios of the bearing plate and truncated pyramid
below the bearing plate.
The beam should bear full length on the bearing plate/pad so that the
assumption of uniform stress under the bearing plate is not too far from
wrong. Oversize bearing plates/pads are worse than undersize as the concrete
doesn't know that the plate/pad isn't supposed to be so close to the edge of
the concrete. (I have some dramatic photos of concrete spalling off of
columns because the bearing pad was larger than specified, the beams
deflected [together with other factors], causing high edge loading, resulting
in the spalling.)
I don't know if this answers your question. The 1-1/2 inch bearing of the
beam is very small and appears to be the result of arithmetic dictating a
wrong answer and no one visualizing what is happening. It seems like a beam
stretcher is needed on this one!
Hope this helps.
A. Roger Turk, P.E.(Structural)
Bill King wrote:
>>The info in PCI is very close to the info in ACI318 section 10.17 so I will
give some more detail as to my question.
The question comes from a residential building where there is a steel beam
bearing on a concrete wall. The problem is that the beam only bears ~1.5" on
the wall. We are fixing the bearing problem but that is where my question
comes in. How do you determine how much bearing length you need for the beam
on concrete based on the edge distance?<<