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RE: 1970's Panelized Roof - Question

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Tony (and all the others)
Thank you for your fast replies. I too considered the mechanical units
as dead load but was not quite sure since they are replaceable. Tony, I
am assuming by your explanation of the king post that you are,
essentially, creating a column below the GLB that will be used like a
truss - relying on the cables strung from the top of the steel columns
and tensioned with turn-buckles, to force the King-post to push up on
the center of the Girder. Is this correct?
BTW, I did suggest to the contractor to place the units above the
columns so as to transfer the load as much axially into the columns as
possible. The problem is that he has a unit placed at 20-feet on center
which means that there is at least two units per Girder. There are only
four columns in the interior of the store as the girder spans are long.
I will discuss the King-post (truss) idea with the builder since the
beams (I've assumed what he calls out as a 7" x 26" GLB is actually a
6.75 X 25 or 27" GLB. Still, the GLB's are overstressed based on current
values even allowing for live load reductions.
I did not adjust for shape as Roger suggested and review my AITC manual
tomorrow as I have one that dates back to the 70's or early 80's (3rd
Edition??). Of course, I have no way of knowing if the GLB's were
installed with sufficient Camber to offset deflection when approaching
maximum stress.

Thank you all for your advice - it matches my opinions very closely
(although the King-post idea is wonderfully creative and I would not
have thought of this on my own).

I have one more question, but will put this in a separate post. Thank
you again.

Dennis S. Wish, PE
California Professional Engineer
The Structuralist.Net Information Infrastructure

Website:
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-----Original Message-----
From: LEDSTRUCTURES(--nospam--at)aol.com [mailto:LEDSTRUCTURES(--nospam--at)aol.com]
Sent: Monday, February 11, 2002 7:41 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: 1970's Panelized Roof - Question

hi Dennis,

your loading seems appropriate, 12 psf dead and 20 psf live reduceable
to
12psf at the girders. I would use the AC units as dead load, however i
wouuld
still reduce the live load since you will not have anyone standing on
top of
the unit. I would first try to locate the units as close as possible to
the
column locations, since your GLB's are most likely overstressed in
bending
and deflection. If you cannot get your GLB's to work that way then one
method
I have used in the past is a King post in the center of the beam with
cables
transferring the load back to the columns or to the end of the girder.

Tony Hoballah
LSI, Long Beach.

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