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Re: Pre-Eng. Steel Bldg Moment Ftg.w/ Tie-Rods;& Base PL. Shear

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> From: "Ed Fasula" <tibbits2(--nospam--at)metro.lakes.com>

> We are using dropped & encased tie-rods in combination with a moment =
> footing to resist 55k horizontal.  The rods are dropped about 2', so the =

> Assuming that all works, so much movement of a frame seems excessive.  =
> On the other hand, if it were a (more common) 80' building, we would =
> still have .79".  Has that been a problem?

Ask the pre-eng designer what lateral deflection is acceptable at the
base. They are probably working to something around h/200 or more
flexible so your base movement may not be an issue, if they can figure
it out.

> shear.  Even without the clamping action of the anchor bolts, the =
> horizontal thrust would nearly be accounted for by the simultaneous =
> vertical loads.  With 55 kips, it seems a little out of common =

There will be a small amount of base rotation or uneven stress
distribution possibly coupled with a flexible plate. Clamping, at best
will not be evenly distributed. Friction, at worst will be averageable
up to localized shear failure of the concrete surface.

The pre-eng base plate design will probably be based on even
distribution of shear on the anchor bolts. Adequate for small stuff in a
2D world where anchor bolts are placed perfectly and friction/clamping
are useful redundancies.

> I ran the numbers on a shear lug, and that method would require 1" thick =
> steel for the same force.  The text mentioned previously states you =
> can't use the lug in combination with the friction procedure.  Would =
> there be any merit to adding a 3/4" lug for added safety, or might it be =
> counter-productive?

If you're only guessing at the 75% effect then you are not prepared for
the possibility that the shear lug will actually be required to resist
more than 75% of the shear (e.g. fails). You would need to confirm the
relative stiffness between the shear lug and the base friction. Don't
waste your time writing that particular PhD thesis.

Go one way or the other or both (conservative but not necessarily
expensive) but don't try to go part way on each.

-- 
Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ad026(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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