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- To: <seaint(--nospam--at)seaint.org>
- Subject: re-roof 1950's building
- From: "Bob Freeman" <robert.freeman(--nospam--at)idsse.com>
- Date: Fri, 1 Feb 2008 12:59:25 -0800
Is the owner’s objective to get more slope on the roof?
If so, you may want to consider using new tapered rigid foam insulation over the lightweight concrete deck, then place roofing over the foam. It will change the slope by the amount you specify. It is not cheap, but there are several products which are computer cut for the slope you specify. Another advantage is increased energy insulation.
An advantage with this alternative is there is no significant additional structural loads to the existing structure. You may consider doing a calculation of the existing members.
If I understand your e-mail, however, for the system you are describing, I believe the Code requires a complete seismic analysis and possible seismic strengthening of the building. Logically, in an earthquake, the new roof structure you are describing can take the earthquake diaphragm loads, transfer them to the existing walls, and then into the footings. But, what about the existing steel structure? It may collapse, leaving the new wood roof structure in tact. Wood is much more flexible than steel deck with light-weight concrete fill. The steel with fill would probably be significantly more rigid and try to take the entire earthquake diaphragm load till failure, then the load would be transferred to the wood diaphragm. By then it may be too late.
With Joy and Hope,
Bob Freeman, AIA, EIT
IDS Group, Inc.
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