with respect to
From: dcarroll [mailto:dcarroll(--nospam--at)bfmengr.com]
Sent: Friday, March 16, 2001 2:47 PM
Subject: RE: Wind Loads-UBC vs. ASCE
what is wrt?
From: T. Eric Gillham PE [mailto:teric(--nospam--at)gk2guam.com]
Sent: Wednesday, March 07, 2001 7:38 PM
Subject: RE: Wind Loads-UBC vs. ASCE
*******A few comments below:
From: Padmanabhan Rajendran [mailto:rakamaka(--nospam--at)yahoo.com]
Sent: Thursday, March 08, 2001 12:16 AM
Subject: Wind Loads-UBC vs. ASCE
ASCE 7-98 wind map shows maximum "3 sec. gust wind
speed" of 150 mph, whereas according to UBC 97, the
maximum "Basic Wind Speed" is limited to 130 mph. Is
there any correlation between the two terms? If not,
is there any formula for extrapolating wind stagnation
pressure for basic wind speed in excess of 130 mph,
while using UBC 97?
***********First point, the maximum wind speed per ASCE 7-98 is actually for
Guam, where I live and practice, and it is 170mph (page 35, notes at bottom
of map). Our design wind speed per the UBC (all recent editions) is 155mph.
The correlation between the two wind speeds is given in another
your post, but wrt the formula used to calculate the wind stagnation
pressure @ 33' for ANY wind velocity (including V>130mph) per the UBC
qs = .00256(V^2) where qs is in psf and V is in mph
As a side note, a modified form of this can be found on page 30 of
7-98, Eq. 6-13.
2. ASCE 7-88 showed the gust factors, gz, as a
function of the height 'z' along the height of the
structure. However, ASCE 7-98 gives a single value
for gz (Sec. 6.5.8). Quick calculation indicates that
the force due to wind would be less, if calculated
with 7-98 provisions. The reduction is not
insignificant. Were the provisions of 7-88 overly
conservative or is there something in error in 7-98?
*********Actually, you may need to look this over again:
Here is my interpretation of both 7-88 and 7-98:
1. ASCE 7-88 section 6.6 requires that "for MAIN WIND FORCE
SYSTEMS the value of the gust reponse factor Gh shall be determined from
Table 8 evaluated at the building or structure height h."
Next sentence explains that for COMPONENTS and CLADDING the value
with height. Note that Gh is used for the main WFRS (hence the subscript
indicating height of the building), while Gz is used for components and
cladding (hence the subscript z indicating variance with height).
2. BUT, if you look at Table 4 on page 10 it gives some insight.
Gh is used, then G is evaluated at the building height h. Gz means G
varies with height. For exterior pressures for building AND other
structure MWFRS, Gh is used.
Furthermore, for components and cladding, G is given combined with
buildings, and is NOT a function of height (only area, although 2
tables are used for h<=60 ft and h>60 feet, so that may be the height
variation you are looking at?)
3.From 7-88 Table 4, G is defined as Gz (varying with height) for
Structures only (not buildings). So in this case G does vary with
for components and cladding only (not the MWFRS).
4. Using 7-98 for buildings, for the MWFRS G does not vary with
(same as 7- 88), for cladding and components G is combined with Cp and
not vary with height (same as 7-88). For other structures (section
6.5.13),G is given by Table 6.5.8, and does not vary with height (same as
7-88). Based on a lack of direction on how to treat components and
cladding for other structures, I would say that the same procedure as
building c&c be used, which would mean this is the only (as far as I can
tell) difference between 7-88 and 7-98, since 7-88 varies G with height
while 7- 98 does not.
Soooooo, the way I see it, for building MWFRS and c&c, 7-88 and 7-98
For other structures MWFRS both agree, while for the case of c&c wrt
structures, 7- 98 does NOT vary G with height (because it is combined with
Cp and is not a function of height), while 7-88 DOES vary with
As a side note, for c&c, most of 7-88 (see above), all of 7-98 and
UBC provisions, demand that for leeward suction pressures used to design
MWFRS and c&c it is always based on the mean roof height, and does
vary (although positive pressure does).
Long winded, to be sure, but hope this helps.
T. Eric Gillham PE
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