Hi, I need to upgrade an existing 500mm strip foundation to support a new pier at one end that will support one end of a steel beam. The load at this point will be around 30Kn I am waiting for an architect to give me the detail but was wondering in principle how the foundation should look? I'm can't imagine how a strip foundation could carry this point load without danger of it shearing. Any thoughts?
Point Load on Strip Foundation
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Hi I am a structural engineer so this is my field.
If we are talking mass concrete unreinforced you should assume a 45 degree spread thru the strip. If the strip is reinforced the load can be spread further.
To advise further i would need to know the thickness of the strip and what the strip is sat on? Clay/sand etc.
If we are talking mass concrete unreinforced you should assume a 45 degree spread thru the strip. If the strip is reinforced the load can be spread further.
To advise further i would need to know the thickness of the strip and what the strip is sat on? Clay/sand etc.
If it's 500 wide then taking a square area will only give a bearing pressure of 120kN/m2 - unless you're on cr ap soils, you probably won't need to do anything at all with it. If it's a significant change over the existing loading and/or a change in the load pattern, a spot of localised underpin might be required, but that's about it. Nothing spectacular required, no need for convoluted SE smoke and mirrors design/details 
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Thanks guys (apart from Woody ) This job was originally going to be a single storey extension and has been left at dpc level for 10 years. I now want to complete it as a two storey extension which requires a double steel UB to support the 1st floor cavity wall & roof down one side. To support the beam I'll need to create a pier (200mm bearing) at one end of the adjacent strip and I guess I'll need to widen the foundation to re-centre the pier on the foundation? I may be making a fuss over nothing but was worried about creating an eccentric loading on the existing foundation?
The existing foundation is a 200mm thick strip 1m down onto firm clay, BCO has said that there should be a 200mm bearing for the beam so I need to get down to foundation level and build up again, just wondering if I need to widen the trench around the existing foundation(3.5m) and re-pour over the top and sides with another 200mm to take the widened pier? I'm making a big deal of this aren't I
) This job was originally going to be a single storey extension and has been left at dpc level for 10 years. I now want to complete it as a two storey extension which requires a double steel UB to support the 1st floor cavity wall & roof down one side. To support the beam I'll need to create a pier (200mm bearing) at one end of the adjacent strip and I guess I'll need to widen the foundation to re-centre the pier on the foundation? I may be making a fuss over nothing but was worried about creating an eccentric loading on the existing foundation?The existing foundation is a 200mm thick strip 1m down onto firm clay, BCO has said that there should be a 200mm bearing for the beam so I need to get down to foundation level and build up again, just wondering if I need to widen the trench around the existing foundation(3.5m) and re-pour over the top and sides with another 200mm to take the widened pier? I'm making a big deal of this aren't I
If you think about it, most strip foundations will be loaded eccentrically. After all, we stick a cavity wall nice and centrally to a footing, then bung most of the load on the inner leaf. But, to all intents and purposes, the total load is taken as acting in the middle and the bearing pressure underneath being the same across the width of the footing.
If you (or more likely the BCO) are worried about it, just dig down the side of the footing to increase the width by 300 or so x 600 along the footing then cast to 225 above the top and build your new pier off that.
If you (or more likely the BCO) are worried about it, just dig down the side of the footing to increase the width by 300 or so x 600 along the footing then cast to 225 above the top and build your new pier off that.
To be honest it seems quite a major supporting column this. I would advise get it checked out by a structural engineer. Better to do it before than 3 months down the line when its starting to crack.
hey, thanks for that, not looking forward to digging up the work I did 10 years ago but suspected it would be required. Mind you the BCO will be pleased' cause I'll be able to rebuild with a 100mm cavity rather than the old 50mm that is there now! Many thanks for your time.
He just did
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