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Re: doubts in Pre-engineered metal building design

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my experince.
1.  Most of the problems are just plain bad coordination between the designer
and the building manufacturer.  The mfgr will send shop drawings that will
locate vertical X-bracing in the same location where the architect has located a
door opening.  No one will look at the shop drawings and correct it, and then
the contractor will never install the bracing, or will do something stupid like
weld the brace to the door jamb.
2.  There is a vast difference between the welded moment connections that AISC
and SAC are recommending for seismic and the flimsy bolted connections that MB
mfgrs use.  Their connection designs are based on empirical equations that were
developed from static, not dynamic, tests, and the designers often streach the
equations beyond the original test basis to reduce cost (by making the
connection across the diagonal of the beam-column joint instead of vertically,
so that the lever arm for bolt tension is longer).  On the other hand,
earthquake experience of metal buildings has been pretty good, provided that
required bracing, etc, is in place.  There is something to be said for a
building that weighs basically nothing.
A few days after the Loma Prieta earthquake I saw an MB in Hollister that was
hit by the killer tomatos (stacked cans of tomato paste that were thrown against
the wall).  It was absolutely incredible how the building had been blown apart
by the falling tomato cans - columns were distorted by 5 to 10 feet laterally,
and metal siding and girts were twisted out like they were blowing in the
breeze.  I guess this is the down side of a building that weighs basically
nothing.
3.  I agree that MB designers tend to cut things a bit too close.  When I used
to spec these buildings I used to include a 15 psf "collateral load" that I told
them was for anything I wanted to attach or suspend from the building in the
future, just so I could get a little more margin.  Generally, with a little
coordination we wound up with a pretty good building.
---- martin



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