Subject: Re: Pre-Engineered Metal Building-New Thread
From: ad026(--nospam--at)hwcn.org (Paul Ransom)
Date: Sun, 20 May 2001 10:55:18 -0400
> From: MSSROLLO(--nospam--at)aol.com
> Based on the previous Pre-engineered building threads I have read, it seems
> there is a lot of skepticism in general with the usage of them and what we as
> engineers should do when working with PEMB on a project. As I have stated
> Role #3-Specifying, Evaluating Bids, Foundation Design, and Monitoring
> Construction (basically everything except actually designing the PEMB)
Include erector issues like designing the lift (long-span beams,
partially assembled roof segments, etc.)
> Role #7-Attaching concentrated loads to a cold-formed Roof System that was
> designed based on a "pound per square foot" loading.
Get the original spec's right: specifying a 5 psf UDL over the entire
roof for quote/design is not equivalent to locating a 200 plf cable tray
down the middle of a bay and dividing by the roof area. Properly
designed cold-formed members could handle it but you may not want to use
Sub-trades need to understand why concentrated collateral loads should
NEVER be attached to the flange of a cold-formed sction (or even to one
leg of an angle in the bottom chord of an OWSJ). I hear the dreaded,
"we've always done it this way" on this one aspect more than any other
(in fact there is third-party hardware devoted to this).
> From: h.d.richardson(--nospam--at)home.com
> I would like to suggest a modification to your item 7. You might
> consider adding loads to COLUMNS as well as to roof members. I just
> helped a client add jib cranes to the columns of of a PEMB and it turned
> out to be more work than I anticipated. To give you an idea of how
> closely the building was designed, the building met code when designed
> (1.7 kPa snow loading) but did not meet present code (1.8 kPa snow
> loading). For those not on metric, 1 kPa = 20.7 p.s.f.
I think this is more appropriate in
Role #4-Investigation/retrofitting of existing PEMB for additional
You are comparing a 6% change in code loads. This is round-off error,
well within the conservative analysis inherent in S16.1 and any safety
level of the factored load effects. I am sure that you could find many
examples of conventional structures, designed/built prior to a code
change, that had parts or portions which were similarly "overloaded" by
the change in SL. Considering the life-span of structures I anticipate
code requirements to become progressively more conservative but I don't
design for potential code changes.
Was the new code analysis conducted by the original manufacturer or
somebody making their own assumptions about the response of a frame with
non-prismatic elements? Did the original manufacturer design to S16.1
or, alternately, claim raw ASD as S16.1 compliant?
The fact that metal buildings are close to code tolerance is not a
reason to be critical of the industry and their design practices. I
anticipate that the further development and use of software such as
RamSteel, which will design steel structures to the limit in the same
way that metal building software permits, means that you will see more
conventional structures near the limit with future renovations.
Metal building design engineers are not restricted to designing at the
minimum code requirements. Frequently design is based on something other
than strength limits but a) the designers need to know the issues when
the building is ordered b) the spec writer must actually understand the
boiler-plate that they submit and c) the bid-evaluator must understand
the differences between proprietary products as well as qualified
inclusions and exemptions. Otherwise, the owner gets a pile of
scrap-steel through no fault of the fabricator.
Celebrate the fact that you can always get a bigger fee for renovating a
metal building than you can for renovating a conventional low-rise
building. I do.
Paul Ransom, P. Eng.
Burlington, Ontario, Canada
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