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RE: ASCE 7-98 Wind Pressure Questions

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A while back I attended an AISC seminar where they suggested that a drift limit of L/400 for a return period of 10 years was appropriate, while strength design would obviously be based on 50 or 100 year recurrence.  They reference "Serviceability Limit States Under Wind Load" by Lawrence G. Griffis, AISC EJ, 1st qtr. 1993.
-----Original Message-----
From: Tim Allison [mailto:AllisoT(--nospam--at)trusjoist.com]
Sent: Tuesday, October 22, 2002 6:04 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: ASCE 7-98 Wind Pressure Questions

I have seen increases used for the return period, but I have not calculated a shorter return period.  I know of apartment buildings that are designed for sub-50-year periods since they are on a college campus and it is expected that the building will need to be replaced frequently.  My personal feeling is that the increased risk of failure does not warrant dropping below a 50-year return.  I would also guess that there is not much cost benefit in most circumstances since you still have to contend with other loads and the common size of materials available, though I am sure there are exceptions.
 
I understand there is some effort being made to design buildings for 100 years of serviceability or more, at least in the Chicago area?  I guess for right now a longer return period would only be a value-added design.
 
Since I only have used 7-98, I'll have to defer the latter question to a more seasoned engineer.


>>> David.Dickey(--nospam--at)mhgrp.com 10/22/02 05:00PM >>>
In C6.5.5 of ASCE 7-98, it states that design for applications of serviceability, design using maximum likely events, or other applications, wind speeds associated with the mean recurrence intervals other than 50 years may be used by multiplying the design pressures by the factors in Table C6-3.  Has anyone used this clause to justify using smaller (or larger) pressures? 
 
Because ASCE 7 doesn't specify serviceability requirements, it is common to adjust levels of acceptability.  For deflection, L/360 may be changed to L/240, or H/500 changed to H/250...for vibration, a reduced acceleration, etc...  But, what situations would warrant a change in the Return Period?
 
Another question, in ASCE 7-98, the directionality factor has been separated from the load factor and is now included in the equation for velocity pressure.  Consequently, the velocity pressure is reduced by 0.85 while the new load factor is increased from 1.3 (in ASCE 7-95) to 1.6.  My question is why weren't the ASD load combinations affected?  Are the wind pressures now lower for ASD combos with 7-98 than with 7-95? 
 
(I understand that wind speeds were adjusted in the hurricane zones because of new data, hurricane importance factors, etc..., but what about inland areas where the 90mph wind speeds stayed the same from 7-95 to 7-98?)
David Dickey