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Re: Safety factors in Pre-engineered Metal Buildings

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> From: "Thor Matteson, SE" <matteson(--nospam--at)yosemite.net>

> 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.

My distaste for cable bracing aside, this is not really as bad as it
sounds. Taking 1/2 of the ->ultimate<- load is really reducing the max
cable tension stress down to elastic range.

(e.g.) ASD Ft = 0.6*Fy = 0.6*(0.85*Fu) = 0.5*Fu
(there are some basically flawed assumptions in this simple presentation
but it achieves the same range that you stated)

The intent is to establish the force that will give the same FS as with
rods, angles, etc. Where the 1/3 stress increase is permitted for other
steel members, so it is also taken for cables, for the same reason.
Specific exclusions must also be considered.

> 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

Then the design simply does not comply with the code requirements. Was
the manufacturer's drawing sealed by an engineer licensed in the site
jurisdiction?

It is not an issue of industry practice (or malpractice) or some deviant
approach which permits metal building manufacturers to bypass the
requirements to which other designers must comply. Metal building
manufacturers are obligated to meet the same minimum standards as
everyone else. In some cases the analyses are not easily reproduced if
you are limiting your basis to the AISC steel design manual but the
manufacturer must be able to justify their practices with solid
engineering and research. There seems to be a limited number of
(smaller) manufacturers who don't feel this way - avoid them.

> 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

The burden of proof is already on the manufacturer. Make them comply.

> 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?

Other projects, in other jurisdictions, may not share the same
inspection/testing requirements. Make the owner aware of the cost
trade-offs. Put it in the project spec for the manufacturer's design.
They will quote the project accordingly. Generally, it will have zero
effect on the base plate.

If the manufacturer qualifies the bases for 3000 psi concrete, you could
ask them to restate the base capacity, which they may do for good will.
Alternately, you can easily confirm the base capacity for the lower
concrete stress and use whatever material you wish.

If you are the EOR for a project which includes a metal building, get
involved early and get related extra work defined in your contract.

-- 
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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