1967 foundation design 1

[penpe]

A heater structure installed in 1967 at a refinery needs to be updated, which will increase the dead load slightly and increase the height somewhat. It's around 150 feet tall including the 70 foot stack, so wind loading is significant. They'd like to verify that the foundation is adequate. I have the records indicating what wind loads were imposed then according to UBC, but I don't have footing design notes. My initial analysis indicates that the eccentricity M/P puts the resultant force well outside of the middle third of the foundation, (and our foundation design software says the same thing). Is there a load factor that should be applied to wind loads before computing eccentricity?
The dead load is 422 kips, live load is 36 kips, total shear from wind 60 kips, moment at foundation base from wind is 3600 ft-k. Foundation is 20' x 36' x 4.5' with no soil on it. The footing dimension responsible for resisting the moment is the 20'. The soil net bearing capacity is 2100 psf.

I'd like to be able to prove that the foundation WAS adequate according to wind loading (verified UBC 1967) and foundation design standards of that time - before I try to update to today's loadings and design standards, but everything I try ends up failing due to wind loads. What am I missing?

Thanks!

 

[WinelandV]

Penpe, I do include the weight of the foundation (usually the foundation weight is where I'm getting my OT resistance from, but I do a lot of PEMB foundations).

Please note that is a "v" (as in Violin) not a "y".

 

[1503-44]

You could use all capital letters. V would be clear.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."

 

[penpe]

In my attempts to validate the existing design, I've used soil data included in a report from 1967. It's a long list of numbers in psf, varying with the foundation's least dimension, foundation shape, (strip, square, or octagon), foundation depth. One set of values are for "loadings with no increases", another for "loading with 50% increases: test, wind, and bundle pull". This strip footing's least dimension is 20' so gross allowable soil bearing is 2111 psf with no increases, and 2913 psf gross with 50% increases for wind. It's curious that the 50% increase isn't 50% greater than without the increase. Also, the previous page says that values are for "net" bearing pressure, the page with values contradicts the previous page by saying "gross allowable". Am I correct that "net allowable" is the value used where footing weight DOES NOT contribute to the bearing pressure?
If net bearing pressure doesn't include footing weight, is the eccentricity still computed using weight of footing as part of P in the formula e = M/P?
Then in the subsequent formula for qmax = 2P/(3 x (B/2-e) x L) does P include footing weight? Where allowable soil pressure is given as either "net" or "gross"?

Thanks again!

 

[1503-44]

Yes. "Net never changes."
Gross Allowable Soil bearing can increased by depth by adding soil density x depth to the net bearing capacity. There is no other effect on the calculations.

e is determined from M/P. P is total vertical load at the base of the foundation, including vessel, concrete and soil above the concrete.

Soil bearing allowable for intermittent loads verses permanent type loads is often modified by how much settlement under permanent load is affecting your soil bearing allowable. Example: High moisture content soils may suffer from long term settlement, but might be relatively unaffected by high short term loadings.

Einstein gave the same test to students every year. When asked why he would do something like that, "Because the answers had changed."