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RE: Rho factor for wind and gravity loads?

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From: Steve Mickelson [mailto:smickelson(--nospam--at)northstareng.com]
Sent: Wednesday, July 26, 2006 10:01 AM
To: seaint(--nospam--at)seaint.org
Subject: Rho factor for wind and gravity loads?

 

My question is this . . . why is there not a rho factor for wind forces or gravity loads?


 

Because they are considered “static loads” and the approach is to RESIST the loads primarily in the elastic range—even for extreme events.

 

For example, here in Houston we calculate the force from wind assuming a 110 mph 3-second gust. This is an “extreme event” with something like a 50-year return interval (sorry if this is a bit imprecise, I’m doing this off the top of my head).

 

We design structural elements, connections, etc., assuming we will fully resist such forces; that when such an event occurs, the structure will maintain its full ability to resist loads—we could even have TWO such events occurring fairly close together, and the structure would be expected to resist all such forces connected with the events.

 

That is not the case with seismic events; the structures are expected to resist the forces in that case, well into the plastic range, sufficient to maintain the desired degree of integrity—but not necessarily to remain unaffected. Bulldozers will be arriving sometime thereafter (at least in some cases). Redundancy is another way of saying “overall system ductility.”