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RE: Wind Loads-UBC vs. ASCE
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- Subject: RE: Wind Loads-UBC vs. ASCE
- From: "dcarroll" <dcarroll(--nospam--at)bfmengr.com>
- Date: Fri, 16 Mar 2001 17:47:07 -0500
what is wrt? -----Original Message----- From: T. Eric Gillham PE [mailto:teric(--nospam--at)gk2guam.com] Sent: Wednesday, March 07, 2001 7:38 PM To: seaint(--nospam--at)seaint.org Subject: RE: Wind Loads-UBC vs. ASCE *******A few comments below: -----Original Message----- From: Padmanabhan Rajendran [mailto:rakamaka(--nospam--at)yahoo.com] Sent: Thursday, March 08, 2001 12:16 AM To: seaint(--nospam--at)seaint.org 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 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 __________________________________________________ Do You Yahoo!? Get email at your own domain with Yahoo! Mail. http://personal.mail.yahoo.com/
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- RE: Wind Loads-UBC vs. ASCE
- From: T. Eric Gillham PE
- RE: Wind Loads-UBC vs. ASCE
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