Single Family Foundation question
Single Family Foundation question
(OP)
First off, I must admit I don't do many foundation plans. I have a 9' tall single family basement foundation wall. At the top of this wall is a fairly significant point load from an I-beam (25,000lb point load). The typical footer for the home is 10" thick 20" wide spread footer. With a soil bearing capacity of 2500psf the 20" wide footer won't provide enough bearing area for this point load, so I designed a widened spread footer pad. Now, the foundation contractor wants me to remove the widened footers, he says he has never seen such a thing. His logic is that over the 9' tall stem wall the load is dispersed enough so that by the time the loads reach the footer enough bearing capacity is provided for in the 10"x20" spread footer. I have searched and find no such justification or equations to estimate how (or if) this point load will actually disperse through the 9' stem wall.
Does anyone have any imput on this?
Thanks so much.
Does anyone have any imput on this?
Thanks so much.





RE: Single Family Foundation question
I assume the beam is bearing on a wall and not a column. It sounds like you assuming a strict column behavior, which is unrealistic.
Since the beam is on a solid. contiguous structural element, the vertical load will be distributed according to the properties of the material (concrete). A conservative assumption would be for the load to be distributed at a 30 degree angle from the point of the bearing, although, I am sure there is a bearing plate. Some people even use 45 degrees.
Calculate the load on the distributed length of the wall and you will much more realistic and still conservative.
Dick
RE: Single Family Foundation question
RE: Single Family Foundation question
We typically start with the 30 degree spread (30 degrees each way, which for your 9 foot tall wall would spread the load out over 10.4 feet.
That puts your wall load at 2406 lbs/ft (from the concentrated load alone).
Add in any other wall loads from above and the self weight of your basement wall, and then check the continuous footing width for that total load.
RE: Single Family Foundation question
The concentrated load seems somewhat high, but the 9' wall height indicates it is not your typical home.
Dick
RE: Single Family Foundation question
Yes, the home is quite out of the ordinary, a lot of steel, this particular beam is supporting a garage floor load, the garage has living area below it.
Thanks again.
RE: Single Family Foundation question
I also have used the 30 degrees for foundation designs in soil or rock. The typical Boussinesq pressure curves look more like a "circular bulb" shape in that the load intensity varies radially and does not continue indefinetly out at 30 degrees. Its more concentrated within a smaller length. Nice tread.
RE: Single Family Foundation question
You will find mention of this in masonry codes, and any publication that talks about designing lintels such as
http://www.bia.org/html/frmset_thnt.htm
I know this digresses, but it is worth mentioning.
RE: Single Family Foundation question
If so, isn't this just a deep grade beam? In that case,the load cam be spread along the base any distance you want.
I'd even guess that the tensile strength (yes it has that) in the concrete might do it. But to satisfy codes and inspectors, put in a few rods near the bottom.
Pretty simple to explain and to design and the contractor is just being practical. We should take some lessons from those guys now and then.
RE: Single Family Foundation question
I try to estimate the realistic grade beam/foundation span (30 degrees is conservative) and widen the footing for that span length. If at all possible, I only widen 1 side of the footing, usually the inside. The less the contractors have to cut boards, the less they mutter at me.
RE: Single Family Foundation question
Thanks.
RE: Single Family Foundation question
Stepping the width of the footings makes it difficult to do a good job of placing drain tile along or slightly below the bottom of the footing. It is desireable to have straight runs of drain tile. Some good residential contractor prefer to use both interior and exterior drain tile since the cost is minimal at this point.
Dick
RE: Single Family Foundation question
30 degrees each way - a total of a 60 degree spread. The idea is that the wall will arch across the 60 degree width. In actualilty, if you were to model it with a finite element program, you'd see a vastly wider spread, but not uniformly loaded. So the 60 degree spread is a somewhat conservative approach.
RE: Single Family Foundation question
Remember that just because the contractor has never done it doesn't mean you're wrong.
RE: Single Family Foundation question
Point Load/ Soil bearing pressue= square root of the
pruduct
RE: Single Family Foundation question