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Re: uplift in footings

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Andrew,
I became very concerned about uplift on pre-eng buildings a few years ago, so I tried to do a bit of research and found very little information. What little I found was that if you are located in cohesive soils. you should use an angle of 30 degrees from the vertical (assuming that failure would occur on a 45 deg line) and in non-cohesive soils to use a smaller angle, but not to exceed 20 deg from vert. These recommendations were based on no slab in place, as could occur when construction is not complete and you may not have all the dead load in place and no slab yet. Hope this helps.
Gary

Andrew Kester, PE wrote:
In Florida just about all of our canopy footings and interior columns on single story construction (retail box with steel deck and joists and HSS columns) are controlled by UPLIFT design, usually not even close to gravity. We put the top of our footings 1'-4" down and use the soil and slab and footing weight. We have always used 45 degree angle from the TOF for tributary area, unless we reinforce the slab we assume a little more. I doubt there is anything published, but this is the way many different engineers I have talked to treat this situation, just an engineering judgement thing. In my forensic work, FWIW, I have seen very little building failures precipitated by a footing pulling out of the ground :) Chances are , the TEK screws holding the deck will fail first. But then again, after I recently saw those failed 3ft deep steel girders and deck intact from a tornado, I had second thoughts... Follow up to that: Take an average steel moment frame with a beam to column moment connection. What if the inside flange of the column buckled, is this not from a positive moment in the beam from a downward force? This would cause an "inward moment" on the column that would buckle the inside flange of the column. Because this was the case with one of the columns, which is indicating a large downward pressure, right?? Andrew Kester, PE
ADK Structural Engineering, PLLC
Lake Mary, FL

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