Return to index: [Subject] [Thread] [Date] [Author]

RE: Load Combinations for ASD 9th

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Title: RE: Load Combinations for ASD 9th

Those load combinations are out of date. The latest version if service-load load combinations can be found in ASCE 7-98. The old 1/3 stress increase was intended for the purpose you seek. However, in ASCE 7-98, it's use has been restricted in favor of a reciprocal 3/4 load reduction factor that gets you close to what you used to get, but not as close as you might hope. Read on for why.

In the old-days, the 1/3 stress increase accounted for loads in combination by increasing the allowable stress on the material side. This was an across the board increase whether the load was dead load, live load, wind or earthquake. Watch out, though, because in the latest version of ASCE 7 (98) because the utility of the historic 1/3 stress increase has been significantly reduced. First, they have prohibited its use on the material side in lieu of the use of a reciprocal 3/4 load reduction factor. Then, the use of that 3/4 load reduction factor is limited to cases that involve multiple transient loads but not the dead load in any combination. That use is further limited because the total load that results can't be less than that due to the dead load in combination with any one of the transient loads unreduced.

If you actually work out the language for service-load combinations in ASCE 7 into equivalent equations for load combinations, you'll find that there are now more load combinations that must be checked in service-loads than in factored loads. So much for the claimed simplicity of ASD load combinations.

Charlie

-----Original Message-----
From: Michael Zaitz [mailto:mzaitz(--nospam--at)hgbd.com]
Sent: Friday, August 18, 2000 9:12 AM
To: seaint(--nospam--at)seaint.org
Subject: Load Combinations for ASD 9th


Hello,

The load combinations are referenced as ANSI 58.1 "Minimum Design Loads for
Buildings and Other Structures".  The oldest copy I have is the ASCE 7-95 that
gives the following combinations:

1. D
2. D+L+F+H+T+(Lr or S or R)
3. D+ (W or E)
4. D+L+(Lr or S or R) + (W or E)

What I am wondering about is how people handle the 4th combination with respect to
one story buildings?  It seems to me that it is highly unlikely that the roof of a
one story building will be fully loaded with its live load capacity at the same
time a hurricane comes through (I can see it being reroofed when a earthquake
hits).  How do others handle this?  Is there anything I am missing due to having a
newer ASCE manual?

Thanks,

Mike