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RE: Point Loads on Strip Footings

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The masonry code only allows you to distribute the concentrated load over a
wall width equal to the bearing area plus four times the wall thickness (ACI
530, Section 5.12.1). Less if block is not laid in running bond. This is for
wall design, and you can argue whether this is way too conservative for
foundation design (I think it is, if the masonry is reinforced.) 

In the extreme, if the block is held together by some steel, then the wall
is a hugely rigid beam and you could estimate the distribution of the
foundation reaction just like you would for a rigid footing.

The more interesting question is whether your wall is going to like that
extra 11 kips, especially if it's an eccentric load. 

-----Original Message-----
From: npitera(--nospam--at)mmm.com [mailto:npitera(--nospam--at)mmm.com]
Sent: Wednesday, August 21, 2002 1:08 PM
To: seaint(--nospam--at)seaint.org
Subject: Point Loads on Strip Footings





A strip footing that supports a masonry block wall there is the potential
for
adding about 11 kips of point load at the top of the masonry wall. For load
distribution, I think I can take up to 1/2 the wall height which would be
4'-6".
With the 11kip load spread over 4'-6" feet, the  future bearing pressure be
3.5
ksf from about 1.0 ksf. The  allowable soil bearing capacity is unknown, the
building is about 50 years old and shows no signs of settlement.

Typically, when the allowable bearing pressure is unknown, I try to keep the
bearing pressure below 2.5 ksf. I would hate to underpin the strip footing
and
place a spread footing for fear of the a sudden settlement, or screw up by
the
contractor. I think that underpinning a standard strip footing may be a cure
that is as risky as the disease.
Doweling into the side of 12" thick strip footing to enlarge the footing I
think
is just as risky.

Are there any other options?

I would appreciate any comments on this issue.
 I thank you in advance.

Regards,

Nick.




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