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Re: ASCE 7-95 question...

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Rick,

Opps!  Thanks for pointing that out to me.  I guess I either assumed that
the method by table 6-3 was similar to older methods like BOCA 1996 in
which the values are the same across the roof (which of course makes an
you know what of me) OR I suffered from a brief bought of selective
reading!

Thanks,

Scott Maxwell, PE, SE


On Thu, 20 Jul 2000, Rick Burch wrote:

> It looks to me like the roof coefficients for flat roofs in Fig.6-3 DO
> vary similarly to those in Fig. 6-4; i.e., there is not single
> coefficient for the flat roof case.
> 
> For example, in Fig. 6-3, for roof angles < 10, H/L <= .5, the Cp value
> varies as you go from the windward edge towards the leeward edge.  Cp
> varies as so: -0.9, -0.9, -0.5, -0.3 as shown in the table. For H/L >=
> 1.0, there are two Cp values: -1.3 and -0.7.
> 
> Rick Burch
> 
> 
> Scott E Maxwell wrote:
> > 
> > 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)engin.umich.edu]
> > > Sent: Wednesday, July 19, 2000 3:11 PM
> > > To: seaint(--nospam--at)seaint.org
> > > 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 question...how does one obtain a roof with a slope of
> > > 90 degrees?  <grin>
> > >
> > > Thanks,
> > >
> > > Scott Maxwell, PE, SE
> 
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