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RE: ASCE 7-95 question...

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I talked to someone at Texas Tech about this.  He agreed that the values
for Figure 6-4 don't make sense for a flat roof.  He offered the same
suggestions that you did (and that I had decided to do anyway prior to
talking to him)...use the method for all building heights which uses
Figure 6-3.

He did have an explanation as to why the odd values.  Apparently, the wind
tunnels tests done for Figure 6-4 were done on a gabled roof (i.e. NOT
flat).  The values were intended to be used for just gabled roof
structures but ultimately the decision was decided to use it for all
situations (not just gabled roofs).  This incosistancy was not "adjusted".

Scott Maxwell, PE, SE

On Wed, 19 Jul 2000, Connor, John A NWK wrote:

> One thing to keep in mind, if your building is 60 feet or less, you are not
> forced to design the main force resisting system as a "low rise".  You are
> permitted to use figure 6-3, and design it as a building of "all heights"
> (non-low rise).  I find Fig 6-3 easier to work with than Fig 6-4, so I
> design all of my low-rise buildings this way.
> As for a reason why the coefficients are not equal, maybe it has to do with
> the wind component that is coming in from the skewed direction (fig 6-4).
> Fig 6-3 only has wind in the directions of the principal axis.  
> Some FYI: in ASCE 7-98, Fig 6-4 is tabled the same as 7-95.
> I have never sat down and compared the results of fig 6-3 to 6-4.  Does
> anyone know if the results vary significantly between methods?
> John Connor, PE
> Kansas City, MO
> -----Original Message-----
> From: Scott E Maxwell [mailto:smaxwell(--nospam--at)]
> Sent: Wednesday, July 19, 2000 3:11 PM
> To: seaint(--nospam--at)
> Subject: ASCE 7-95 question...
> I have a question for anyone out there who is familiar with ASCE 7-95.  I
> am computing the main wind force loads for a building that is under 60 ft
> (low rise).  So I am using ASCE 7-95's "low rise" method.  I am in the
> process of determining the External Pressure Coefficients (GCpf) per
> Figure 6-4.  I am looking at CASE A.  My roof slope is 0 degrees.  So per
> Figure 6-4, the coefficient would be -0.69 for zone 2 (windward roof) and
> -0.37 for zone 3 (leeward roof).  Now this does not make sense to me.  If
> my roof is flat (i.e. 0 degrees) shouldn't both of these coefficients be
> the SAME?  This is not consist with the method for non-low rise buildings
> (the cofficients are the same for roof with 10 degrees or less).
> Also, another dumb does one obtain a roof with a slope of
> 90 degrees?  <grin>
> Thanks,
> Scott Maxwell, PE, SE

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