overturning of headwall
overturning of headwall
(OP)
Hi, i am doing a headwall ( structual part), should i check it overturn at point "A", and use the same way as check a retaining wall?
thanks
thanks
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RE: overturning of headwall
Check the overturning and max soil stress at the intersection of the endwall with the slab (where the sewer pipe enters).
Your Overturning moment will be due to the lateral soil pressure, and your resisting moment from the vertical s0il pressure prism.
Mike McCann
MMC Engineering
RE: overturning of headwall
but I am not sure. the lateral force from soil, surcharge... will give the headwall a clockwise moment to overturn. how the soil pressure prism to balance it?
RE: overturning of headwall
Additionally, depending on the depth of the slab, you may have some passive resisting pressure at the toe of the structure that may help resist overturning.
P/A +- M/S (simplistically) will give you the max and min soil pressures.
Mike McCann
MMC Engineering
RE: overturning of headwall
or i misunderstood?
so appreciated, Mike
RE: overturning of headwall
RE: overturning of headwall
Isn't the overturning for this the same as any retaining wall or foundation?
Righting moment is just from the dead weight of the structure and the overturning from the lateral soil pressure, no?
If need be, depending on the construction of the sewer pipe and its attachment, you could also count on it + some % of the soil above the pipe in calculating the righting moment (probably not a good idea though).
RE: overturning of headwall
I'm just used to doing it at the heel, not the toe.
Mike McCann
MMC Engineering
RE: overturning of headwall
RE: overturning of headwall
Mike, why would you select the heel as the point to sum moment? The footing doesn't rotate about the heel.
RE: overturning of headwall
the prismatic soil pressure also gives a clockwise moment if rotate at "A".
RE: overturning of headwall
RE: overturning of headwall
There are many examples out there.
RE: overturning of headwall
you need to get a soil mechanics textbook or google retaining wall calculations. This is far too fundamental to answer on a forum.
Once you have done that, if you have a specific question with regards to the method then feel free to ask.
RE: overturning of headwall
EIT
RE: overturning of headwall
This discussion got me looking at the computer program I wrote 25 years ago for retaining walls, and you are right. I also used the toe when I wrote it. That being said though, the point should be able to be taken anywhere and get the same result. The toe is just the preferred point and easier to analyze.
In looking at the OP's problem though, if the overturning moments from the soil are greater than the resisting moments of the dead load of the structure, the heel should be extended under the soil to the rear of the outfall as needed to pick up the overturning resistance of the dead weight of that soil. It could be extended beyond the sidewalls too, but not with quite as effective of a result.
And really, the soil prism is just an overall summary of the total effect of the gravity and overturning forces on the base of the structure. It is best to keep the resultant of this prism within the kern of the structure, but it can vary from there if stability and soil bearing parameters are satisfied.
Mike McCann
MMC Engineering
RE: overturning of headwall
If you don't use the point about which the footing is assumed to rotate as it turns over, then I believe you will have to determine bearing pressures and use those in your OT calc...more work than is necessary.
RE: overturning of headwall
RE: overturning of headwall
I agree, but the only way to logically eliminate bearing pressure on the bottom of the footing is to assume rotation about the extreme end of the 'toe', right?
Besides, I'm inclined to believe that the conservative FOS used in checking overturning takes into account the point about which the calculation is done. I doubt any footing would actually rotate about the extreme edge unless on rock as you have noted.
RE: overturning of headwall