• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Wind Loads-UBC vs. ASCE
• From: "T. Eric Gillham PE" <teric(--nospam--at)gk2guam.com>
• Date: Thu, 8 Mar 2001 10:38:01 +1000

-----Original Message-----
Sent: Thursday, March 08, 2001 12:16 AM
To: seaint(--nospam--at)seaint.org

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 response to
your 	post, but wrt the formula used to calculate the wind stagnation
pressure @ 33' for 	ANY wind velocity (including V>130mph) per the UBC it
is:

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 ASCE
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 RESISTING
SYSTEMS the 	value of the gust reponse factor Gh shall be determined from
Table 8 evaluated at the 	building 	or structure height h." (emphasis mine)

Next sentence explains that for COMPONENTS and CLADDING the value varies
with 	height.  Note that Gh is used for the main WFRS (hence the subscript h
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.  Whenever
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 Cp for
buildings, 	and is NOT a function of height (only area, although 2 different
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 Other
Structures 	only (not buildings).  So in this case G does vary with height
for components and 	cladding only (not the MWFRS).

4.  Using 7-98 for buildings, for the MWFRS G does not vary with height
(same as 7-	88), for cladding and components G is combined with Cp and does
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 for
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 agree.
For 	other structures MWFRS both agree, while for the case of c&c wrt other
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 height.

As a side note, for c&c, most of 7-88 (see above), all of 7-98 and also the
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 NOT
vary (although positive 	pressure 	does).

Long winded, to be sure, but hope this helps.

T. Eric Gillham PE

*****************************

Rajendran

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