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Re: uplift in footings

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While we're on the subject of footing uplift, why is the 0.6D factor applied to the weight of concrete footings?  Isn't this factor primarily intended to cover the unpredictability of future conditions.  As long as footings are inspected for size, it can be considered a given fact that the footings will weigh so much and provide so much resistance. 
Jim Wilson, PE
Stroudsburg, PA

"Gary L. Hodgson and Assoc." <ghodgson(--nospam--at)> wrote:
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.

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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