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Re: ASCE 7-95 question...

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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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