From: Jim Kestner <jkestner(--nospam--at)somervilleinc.com>
Date: Thu, 10 Feb 2000 09:43:05 -0600
What William Sherman suggested is certainly conservative and should work
well, provided that there are no other concentrated or linear loads
within a distance equal to the effective width from your load. If there
are additional concentrated or linear loads in the vicinity, you would
have to add in those overlapping effects. Please note that if your load
is placed near the edge of the slab, the effective width would be
dramatically reduced, but it might be simpler to place a beam along the
edge to carry the full load.
Many years ago, I did a little study (based on Westergaard's original
work of Concentrated loads on Slabs) to actually see what the effective
width was for a linear load. I took the linear load and analyzed it as
if it were 10 concentrated loads and added up the effects. The effective
width turned out to be wider than a typical concentrated load (this is
what I would have expected). I, however, feel a little uneasy using this
wider width and therefore I take a more conservative approach, similar
to William Sherman.
One thing that many building engineers ignore is the lateral
distribution steel required to spread out the concentrated load to the
effective width. Westergaard's original paper has a neat little method
to calculate this. Obviously for precast plank, there is no steel in
that lateral direction, but the effective width that precasters use is
lower than those for monolithic slabs. For slabs on metal deck, the
lateral spreading out of the load should be carryfully evaluated, since
the stiffness and strength is substantially lower at right angles to the
One of Westergaard's later papers also has a method for calculating the
maximum negative moment produced over an interior beam due to a nearby
I believe some of the requirements for concentrated loads in the AASHTO
spec of today are based on some of Westergaard's studies in the 30's and
40's for the Bureau of Public Roads.
I believe Paul Rice of CRSI produced a study a number of years ago about
concentrated loads on one way concrete joists.
For those of you that don't have these papers, I highly recommend that
you obtain a copy from your local library for your reference.
I hope this helps.
Jim Kestner, P.E.
Green Bay, Wi.