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Re: PEMB's and lateral deflection

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> From: "Rich Lewis" <seaint02(--nospam--at)lewisengineering.com>

> and I think you have.  I am designing a foundation for a PEMB with a 475'
> span.  There are 3 interior column supports.  The rigid frame spans from
> the exterior wall, in 150' to a column on one side and 120' to a column
> on the other side, with the last interior column at the center ridge
> line.  I have a horizontal thrust load of 54k.  In typical PEMB's I use
> hairpins or tie rods to tie the 2 ends of the rigid frames.  Since this
> is 475' long this doesn't seem practical.  I see it as 2 options: 1)
> design the exterior footing for the overturning force.  This will make
> it a large footing.  2) tie the base of the frame to the first interior
> column, at one end 150' and the other end 120'.  Both have their
> advantages and disadvantages.
> 
> Which method have you typically used for large horizontal thrusts on
> long span rigid frames?

Sounds like a hangar. Maybe for UPS or something. I can't imagine USAF
doing this.

This type of project scares the crap out of me given that the PEMB
industry has some well deserved shame. There should be a significant
spec document and engineering coordination meetings before any steel is
fabricated. The foundation type should be pre-established at the early
stages of the project.

In addition to the regular forces with P-Delta included, etc., you
should also ask for the thermal reactions and acceptable foundation
movement (or rather, specify an amount that they should consider in
their design - e.g. +/- 1" vertical and horizontal). The classical PEMB
assumption is that foundations are rigidly static - forever.

When you get structures this big, the force numbers look really big.
Everything is relative and the stresses are still the same. So, expect
"big" foundations.

If you use hairpins, you have to ensure that the owner cannot ever cut
the floor that you rely on for the restraint. Even the best intentions
of a future renovation might be inadequate. You need to draw on a large
area for that much resistance. There simply is not enough redundancy to
cover the force magnitudes.

Tie-rods to the first interior column may get some benefit from
redundant lateral resistance at the interior foundation but compare the
cost to adding more concrete/steel at the sidewall. delta = PL/AE. Check
the tie-rod A required to keep delta < 1".

Thermal reactions can be significant for this span. Hell, most engineers
wouldn't allow that much distance between braced bays without an
expansion joint, in simple span structures. Imagine a 20 deg F temp
range (not unrealistic) at some point in the project life:
Assume 50 in^2 rafter for illustration;
delta unrestrained = 475' * 12 * 20deg * 0.0000065 = 0.74", 0 stress
P restrained = 29000 * 0.000065 * 20 * 50 = 174 kips, 0 elongation
Erected bare steel in the hot Texas sun could develop these forces.    

I'm inclined to suggest big, self-sustaining foundations for each
sidewall column.

Check out the foundations for the USAF Museum Eugene Kettering Building.
Profiled in Civil Engineering Dec 2003 (v73, #12). Post-tensioned
buttresses. Not a PEMB but there may be some ideas there for you. 

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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