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Michael,

Another thing to be careful with is that ASCE 7 now has the directionality factor of 0.85 which at first makes it look like wind loads have decreased however look at the new load combinations/load factors and you will see that for wind it has changed from 1.3 to 1.6.

Thomas Hunt, S.E.
ABS Consulting




Michael Hemstad <mlhemstad(--nospam--at)yahoo.com>

02/25/2003 07:31 AM
Please respond to seaint

       
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        Subject:        wind



I'm trying to get familiar with the IBC wind load
provisions.  I have run a comparison between the IBC
provisions, ASCE 7-98, and the 1997 UBC.  The results
are a little dismaying, especially the IBC values.
Can someone tell me if I've made a mistake somewhere?

The analysis is for a fictitious 42 foot tall building
in Minnesota, Exposure C.  Iw = 1.0; topographic
constant 1.0.  The building is enclosed, with a flat
roof.

IBC 1609.6:
V = 90 mph
Table 1609.6.2.1(1) (Interior Zone Wall) gives
pressure = 8.5 psf
Table 1609.6.2.1(4) gives Ht+Exp coeff = 1.51
load factor = 1.6

w = 12.84 psf, x 1.6 = 20.5 psf

If instead I use the last columns of Table
1609.6.2.1(1), I get a pressure of 7.2 - (-5.8) = 13.0
psf. Then,

w = 19.63 psf, x 1.6 = 31.4 psf


ASCE 7-98 Simplified Procedure (shouldn't be used
since ht > 30 feet)
Fig. 6-1: v = 90 mph
Table 6-2 give p = 14 psf
Exp. C multiplier = 1.40
load factor = 1.6

w = 19.6 psf, x 1.6 = 31.4 psf


ASCE 7-98 Analytical Procedure
Figure 6-1: v = 90 mph
Table 6-6 gives Kd = 0.85
Table 6-5, Case 1 gives Kh = 1.05
Figure 6-4 gives GCpf = 0.40 - (-0.29) = 0.69
Table 6-7 gives GCpi = 0.18
qh = 18.51 psf
load factor = 1.6

w = 18.51 x (0.69 + 0.18) = 16.1 psf, x 1.6 = 25.8 psf


1997 UBC
Fig. 16-1: v = 80 mph (fastest-mile, not 3-second)
Table 16-F:  qs = 16.4 psf
Table 16-G: Ce = 1.32
Table 16-H: Cq = 0.8 + 0.5 = 1.3
load factor = 1.3

w = 28.14 psf, x 1.3 = 36.6 psf


Factored loads vary from 20.5 psf to 36.6 psf.  Nice
tight grouping.

Can anyone tell me if these values seem right?

In the IBC calcs, I assume the higher value is the one
to use.  What is the lower value for?  Where does it
say this?

In the ASCE 7 Analytical Procedure, footnote 3(ii) of
Table 6-7 indicates GCpi is applied to all interior
surfaces.  Thus, it should cancel out for the MWFRS.
However, Section 6.5.12.2 indicates it is additive to
the exterior pressures for the MWFRS.  Can someone
shed some light on that?

Thanks for any help.

Mike hemstad
TKDA
st. Paul, Minnesota