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RE: MBMA Code and Wind Loads

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The MBMA has funded research that provided some of the input into the development of the ASCE 7-95, but the net result in the ASCE 7-95 has been to produce more conservative (higher wind loads) in structures in hurricane areas specifically and in cladding loads in general.  I would not characterize this as succumbing.

The group responsible for developing the ASCE 7-95 changed many things including the fastest mile basis to the 3 second gust.  The probabilistic necessity for the switch to 3 second gust was long over due.  There are many practices allowed in the MBMA which I disagree with, but I just preclude them in my specifications.  A properly designed prefabricated metal building with good performance characteristics is obtainable with a well developed specification and an understanding of what is said in the MBMA.  

The entire area of wind engineering research has suffered for a long time regarding funding relative to seismic research, but in spite of this handicap they produced a very fine design tool in the ASCE 7-95. Researchers participating on the ASCE 7-95 have conducted many wind tunnel tests on all kinds of models including monitoring actual wind effects on full size structures in the field to corroborate wind tunnel models.

There have been failures of prefabricated metal buildings in high wind exposures, but the problem does not lie in the ASCE 7-95.  I have designed structures using the ASCE 7 and then had a wind tunnel test performed.  The ASCE 7 has generally proved to be conservative except in very odd and rare cases on cladding.

Harold Sprague
Krawinkler, Luth & Assoc.
4412 W. Eisenhower Blvd.
Loveland, CO 80537
Voice: 970 667-2426
Fax: 970 667-2493
Email: hsprague(--nospam--at)klaalov.com


-----Original Message-----
From:	Roger Turk [SMTP:73527.1356(--nospam--at)compuserve.com]
Sent:	Saturday, February 14, 1998 7:26 PM
To:	seaoc(--nospam--at)seaoc.org
Subject:	MBMA Code and Wind Loads

Unfortunately, ASCE 7 is succumbing to MBMA's wind load requirements, even 
using the same drawings and charts that appear and have appeared for years in 
MBMA's manual.  IMO the best wind load criteria for the U.S. was in ANSI 
A58.1-1982.  After that, it went downhill quickly.

For the best reference on wind loads on low rise structures, I refer to, 
"Wind Forces on Structures," Final Report, Task Committee on Wind Forces, 
published in ASCE Transactions, Vol. 126, 1961, Part 2.  This report includes 
charts from the Swiss Building Code that were developed from wind tunnel 
tests on low rise structures, probably the only wind tunnel tests that were 
conducted on low rise structures.  (These charts had been reproduced in the 
AITC Manual, 1st and 2nd editions, but dropped from subsequent editions.)

A paper in the Fourth Quarter issue of AISC's "Engineering Journal," 
"Low-Rise Wind Load Provisions --- Where Are We and Where Do We Need to Go?" 
by Gregory L. F. Chiu and Dale C. Perry, is another reference on wind 
loading.  The authors state, "Before the end of 1996 it is quite possible 
that 12 different documents will be available ..." for wind design of 
low-rise structures.

The behavior of metal buildings subjected to wind is well documented in 
reports, generally phrased something like, "As expected, pre-engineered metal 
buildings fared badly during ..."

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

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