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Safety factors in Pre-engineered Metal Buildings

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I have one safety concern and one gripe regarding PEMB's.

First, the safety concern:  It has been a few years since I scrutinized
calcs for a pre-fab metal building.  The following practice may not be as
prevalent as it used to, but I believe it merits some attention.

The issue is "common" practice in the PEMB industry for sizing diagonal
cable bracing to resist wind or earthquake forces.  Several designs I
have seen took 1/2 of the ultimate (breaking) strength of the cable and then
multiplied
it by 1.33 to get the allowable load.

ASCE 17 (not sure that's the right number--whatever the standard is for use
osf steel cables for buildings, and is adopted by at least the Calif Bldg
Code) allows the following load in a cable used to resist wind or earthquake
loads:
 1/2 of the ultimate load,
OR  1.33 times the ASD load

This practice stated above seems to combine two separate provisions,
resulting in an inadequate (in my opinion) safety factor.

I spoke with an engineer at the ICBO about this, and he with agreed with my
opinion.  To get a written opinion from the ICBO would have cost me $125.

A designer at a PEMB manufacturer told me last week that they design for
ultimate (breaking) strength divided by a 2.35 safety factor, and then
increase that by 1.33 for wind or EQ, which gives a safety factor of
slightly less than two (bad, in my opinion).   A designer at a structural
engineering firm who designs PEMB's told me he uses one OR the other method,
but not both (good).

If I come across use of a safety factor of less than two, I will put the
burden of
proof on the PEMB manufacturer, requiring that THEY provide a written
opinion from ICBO on this practice.  Or the mfr. could just spend $10 or so
for a modest sized building
and bump the bracing cables up a size.

The gripe I have involves sizing column base plates.  All the PEMB mfr
designs I have seen for baseplates use a concrete strength of 3,000 psi,
which results in a slightly smaller baseplate than using 2,500 psi concrete.
Strict adherance to the Code requires that concrete over 2500 psi have
special inspection, cylinders taken, etc.   So to save a few ounces of
baseplate material, the PEMB mfr. increases inspection and testing costs for
the concrete by several hundred dollars.

How about assuming 2,500 psi concrete for the baseplate design unless told
otherwise by the engineer of record or foundation engineer for the project?

Thor





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