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buoyancy design

buoyancy design

Buoyancy design for structure with conventional steel columns and rigid mat foundation. What safety factor for buoyancy design is required? To resist buoyancy forces, can floor live loads above be used or only dead load?

RE: buoyancy design

We use buoyancy factor of safety of 1.5.

Don't usually use live loads at all unless there is some assurance that part of the live load will definitely be there all the time....That usually makes it into a dead load.

RE: buoyancy design

I agree with JAE that live loads should not be used for resisting flotation loads.

In the UK we use a MINIMUM factor of safety of 1.1 against flotation based on the highest credible water level around the structure (more often 1.2 is used).  The reason for the low FoS is based on the fact that you know EXACTLY what the uplift force due to water load is.

Andy Machon


RE: buoyancy design

Andy:  that makes sense....the 1.5 factor was just out of my head related to the 1.5 SF against overturning.  But you're right...the buoyancy is quite well defined.

RE: buoyancy design


RE: buoyancy design

I have used a S.F. of 1.2 for foundations supporting non-building type structures (i.e., foundations for tanks, silos, transformers, etc.) and 1.5 up to 2.0 for foundations supporting buildings.

The following safety factors are recommended by "Duncan" in his foundation design book:

"S.F.=2.0 for basements and other parts of buildings subject to severe damage due to uplift"

"S.F.=1.5 for underground tanks and vaults not structurally connected to the building proper"

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