Single Family Foundation question 9

[AlpineEngineer]

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.

 

[concretemasonry]

Just go back to your very basic mechanics of materials and look at the distibution of your loads. What even happens to Mohr's circle?

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

 

[cap4000]

1995 ACI 14.2.4 indicates the "effective wall length" equals the bearing plate width plus four times the wall thickness. Design a spread footing anyway. Good Luck

 

[JAE]

cap4000 - that 4T value is valid for the design of the wall itself, but what concretemasonry says above is more valid for the footing design.

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.

 

[concretemasonry]

JAE - Thanks for the confirmation. I thought I had gone "brain dead".

The concentrated load seems somewhat high, but the 9' wall height indicates it is not your typical home.

Dick

 

[AlpineEngineer]

Thanks so much for all the input. It does make sense about the load distributing but I was unable to find any support for it. I was hoping to find a table like the Boussinesq chart that would show the load distribution. I would think that wall thickness would be a function of how the load is distributed though and of course bearing plate area. But for now I will go with 30 degrees.

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.

 

[cap4000]

AlpineEngineer

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.

 

[csd72]

How did you design the wall for the point load if you did not spread it. Remember that there are two things you check for masonry point loads one is the stress at the top of the wall and the second is the capacity of the wall at mid heeight (to find the effective width you take the load spread out at 30 degrees to mid height.)

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.

 

[oldestguy]

I have a question. Is this a concrete wall? And is there any chance a little reinforcing steel can be added, or maybe it is there anyhow for crack control, etc.?

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.

 

[emmgjld]

For soft soils (allowable bearing 800 to 1500 psf) in residential construction, I tend to widen out the footing beneath heavy point loads. I also tend to balance the loads to the footing widths more than most contractors care for.

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.

 

[AlpineEngineer]

Just to clarify, is it 30 degrees from the point load (so 15 degrees either side of load) or is it 30 degrees each side of point load for a total of 60?
Thanks.

 

[concretemasonry]

Just a practical note-

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

 

[JAE]

AlpineEngineer,

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.

 

[UcfSE]

I agree with the above posts using a 30 degree spread. Another spread I often use is a 2 vert:1 horiz spread. Sometimes the calculation is a little easier and it's slightly more conservative.

Remember that just because the contractor has never done it doesn't mean you're wrong.

 

[tAvAnA]

if you know the soil bearing pressure you can then divide the point loat into it and square root of that will give you the footing square.

Point Load/ Soil bearing pressue= square root of the
pruduct

 

[COEngineeer]

Look at linear the load along the walls, if they are not close to 2500 psf (20" footing) then I would not worry about the point load. Even after you do the 30 degree distribution, your wall will act like a beam and distribute that load even farther. 25000 lb is not that big for 9' wall. I have a feeling since you have point load on this wall, probably most of the floor load bears on the wall running the other direction.