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RE: 1970's Panelized Roof - Question

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Option 1:

Post up off of the main building columns and provide self supporting roof
top steel frame.

Option 2:

Similar to #1 but engage exterior masonry parapet.

Option 3:

Post up off of slab-on-grade or new footings penetrating roof with new
columns to support RTU frames.

 -----Original Message-----
From: 	Dennis Wish [mailto:dennis.wish(--nospam--at)]
Sent:	Monday, February 11, 2002 9:30 PM
To:	seaint(--nospam--at)
Subject:	1970's Panelized Roof - Question

I have client who is adding 10 A/C units to the top of an existing building
constructed in the 70's. The roof is a panelized roof:
2x4 @ 16" o.c. DF #1 Sub-Purlins - 8'-0" maximum span
4x16 @ 8' o.c. DF #1 Purlins - 18'-9" maximum span
6x24" DF/DF GLB (exact size not yet verified) Beam at 18'-8" apart up to
47'-6" span.
7x26" DF/DF GLB (exact size not yet verified) Girders at 45' Average spacing
and 37'-6" maximum span

The problem is that I can't get the GLB's to work based on a 12-PSF dead
load and reducible 20-psf live load (16-psf on the Beam and 12-psf on the
Girder). I still have to add the A/C units to the Girders.

What have others done to calc out an old roof for additional mechanical
loads on an older Masonry bldg w/ panelized roof? The building was
originally a Thrifty Drug Store and is being remodeled into two stores?

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


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