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Re: Snow Load Factors - ASCE vs ACI

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The thing to be careful of is that ACI load factors are specifically
"calibrated" to their resistance factors.  IF you use other load factors
with the "normal" resistance factors in the ACI code, you will get
inaccurate results.

The alternate resistance factors that Bill P. mentioned in his post will
likely "fix" this problem.  HOWEVER, one note of caution...I am not sure
if this is an exact fix.  I do know that the 318 committee is in the
process of adjusting the resistance factors to allow the use of the load
factors in ASCE 7.  The end goal is that all the load factors WILL be
contained in the ASCE 7 and will be the same for ALL materials.  Then the
resistance factors will be adjusted for each material.

For Bill's benefit, I will point out that ACI had load factors LONG before
any other material.  I don't particularily blame them for being a little
resistant to making the change.  Why should ACI pay the price to switch
when they were the progressive thinkers?  Besides, after all the talk
about how much people have to pay for publications, etc, I will point out
that the new resistance factors cannot just be pulled out of mid-air.  I
would assume that it costs some money to preform statistical studies to
determine the new resistance factors.  Which is then "passed" on to the
buyers of the ACI code...

In case you want a more difinitive answer, you can call ACI about the
Appendix C resistance factors.  While the answer you will get will not be
an "official" response (that can only come by sending a written request to
the 318 committee), you can get a more informed response.  The staff
engineers at ACI will be more aware of the "circumstances" that the
resistance factors in Appen. C were originally considered.

Hope that helps.


On Fri, 17 Jan 2003, frp2000 wrote:

> Snow drift loads on adjacent sloped roofsI was given the following as loads on a double tee supported deck in Pennsylvania.
> DL = 5 psf
> LL = 50 psf
> SL = 30 psf
> and a note saying "all loads are cumulative"
> ACI-318, Chpt 9 has no mention whatsoever of the ultimate strength factor for snow loads. It has 1.4 D + 1.7 L
> ASCE 7-98, Chpt 2 has  1.2 D + 1.6 L + 0.5 S
> and 1.2 D + 1.6 S + 0.5 L
> For Pennsylvania, what load combinations should I use...???
> (My BOCA is in the storage)
> Thanks,
> jim
> sci