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Re: MBMA Code and Wind Loads

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There seems to be much confusion concerning the changes to wind loads 
in ASCE 7-95.  This confusion is reflected in the submissions to this 
thread.  In addition to the standard (ASCE 7-95) and the 
corresponding commentary you would do well to refer to the "Guide to 
the Use of the Wind Load Provisions of ASCE 7-95" which was written 
by Kishor Mehta (chair of the ASCE 7 Task Committee on Wind Loads) 
and Richard Marshall.

In addition to the significant change of averaging time basis (from 
fastest-mile to 3-second gust) they list eight other changes.  One of 
these is "an alternate procedure...for determining external loads on 
main wind-force resisting systems of low-rise buildings."  Through 
personal communication with other members of the ASCE 7 committee I 
learned that this new procedure is based (at least in part) on 
MBMA-sponsored research.  While MBMA was involved in the formulation 
of (and lobbied for) the new method, it is applicable to all low-rise 
buildings (which are defined in ASCE 7-95 as having a "mean roof 
height h less than or equal to 60 ft" and "mean roof height h does 
not exceed least horizontal dimension").

When this alternate method is used, both the pressure and suction 
design values are based on qh which is "the velocity pressure at mean 
roof height using exposure category C, irrespective of actual 
surrounding terrain."  This means that the vertical distribution of  
BOTH the pressure and suction values are CONSTANT.  Mehta's "Guide" 
indicates that the new provisions are based on wind tunnel studies 
(which are referenced).

My comparisons of base shear for the buildings between 15 and 60 feet 
high (with fairly flat roofs) using the standard and the alternate 
(low-rise) procedure show that the low-rise procedure produces base 
shears that are smaller by a factor of 1.6 to 2.2.  For the 
dimensions (24'x60'x20'high), exposure(C), and wind speed (140 
mph 3-second gust) that you gave I calculate a net shear 
(pressure plus suction) of 32.8 psf.  If your design wind speed is 
140 mph fastest-mile and you convert to 3-second gust (158 mph) the 
net shear would be even higher (41.8 psf).

It sounds like the metal building designer used something like the 
alternate procedure of ASCE 7-95, but with the wrong wind speed 
and/or exposure.

> I do not believe that the source of the ASCE 7-95 wind load provisions is
> MBMA.  I attended a seminar given by Dale Perry (one of the authors of ASCE 7)
> shortly after ASCE 7-95 came out.  My understanding is that ASCE 7 now uses
> three second gusts rather than fastest mile wind speeds because that is the
> more prevalent way of measuring wind speeds  these days and that
> internationally most codes are written around three second gust speeds.  The
> change to three second gusts meant changing a lot of factors so that the
> pressures derived came out about equal to ASCE 7-93.  They also increased some
> factors, resulting in higher wind loads, based on research or experience. 

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Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201