Foundation settlement under 3 sec gust wind loading

[JayRod]

Hi,

Low plasticity CLAY.
Square raft foundation 9m x 9m x 1m deep
Dead load ~400kN
Overturning moment for wind load case (1 in 50 years probability, 3 second gust wind speed) ~10 000 kNm

Question is about settlement.
The settlement difference when wind load is applies is excessive.
However this is temporary transient load. It will load the foundation once in maybe several years for few seconds in cycles over maybe 1 maybe 2 days at most.
Also the load may happen from different directions, so it may reduce the differential settlement to some degree.

I'm struggling to find any research on foundation settlement under this nature of load - acting for few second in cycles and then stopped.
Found some research on transient load, but the time was in milliseconds, too short period.
Other I found was on cyclical load 0.5s load 0.5 no-load. Without stop. Eventually the settlement developed like the load was pernament (after >10k cycles)

But none of this cases is actually comparable to my problem.

Maybe you have some insight?

 

[Ron]

You have a clayey material. Pore pressure does not dissipate quickly in clayey soils, therefore I would not expect any settlement from a wind gust event.

For overturning, your load will be transient. Depending on the bearing pressure you have used for your foundation, you will get a higher pressure on one part of the foundation and a lower pressure on the other side, depending on the location of the kern as the rotation develops.

 

[JayRod]

Thank you for your reply

I am aware it is transient, and it will not produce a lot of settlement difference because it is transient, can act from different directions and it is cohesive soil, so water won’t have time to dissipate.

I am designing foundation so the bearing capacity will be ok even under this transient load.

But to meet settlement difference requirement with that load, the foundation would need to be much bigger. It is radar supporting structure with very tight deflection requirement.

I’m looking for some research papers that I can reference. I can’t just say based on engineering judgment

Regards,
Jay

 

[r13]

Consider use pile or pier foundation if applicable.

 

[aeoliantexan]

If the clay is unsaturated, it can respond as the pore air compresses; it doesn't have to wait for pore water to be expelled. Think of a plate load test or consolidation test on unsaturated clay - I think there would be considerable deflection in a few seconds.

I don't have what you need, but I agree that you can't afford to be wrong. Perhaps you can get some information from a book on unsaturated soil behavior.

I you have a bearing stratum for piles at reasonable depth, I suspect that the cost of piles would be justified.

 

[BigH]

This paper by Poulos might be of interest - he talks about wind loading - I have only "glanced" at it.

https://www.jstage.jst.go.jp/article/jgssp/2/2/2_ESD-KL-2/_pdf

 

[aeoliantexan]

Are you sure of your numbers? I may be wrong, but my calcs say your foundation will almost flip over unless you bury it a few feet deep.