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RE: Narrow Garage Piers - Residential

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Dave,
I wouldn't consider this issue as related to conventional construction as it refers to the use of alternate braced frames and defines the minimun H/b ratio for non-engineered alternate braced panels.
 
In the discussion of full compliance design, you have calculated the shear to the front of the garage. Whatever it is, the full-compliance code does not have a provisions that I know of for shear panels less than 2:1 ratio. With that said, engineering judgment, tells me that an engineer who can design a panel with less than a 2:1 ratio and can adequetly evaluate the piers deflection (stiffness) and sufficient connection of the panel to an adequate foundation , then the plan checker would accept the panel as he/she would a proprietary shearwall such as the Hardy Panels or Simpson Strong-walls which come down to a 12-inch width to resist shear.
 
If one panel is sufficient to resist the shear applied to the front of the garage, then you may drag the diaphragm shear into the one panel. However, intuition (I keep mentioning intuition because I believe we have stopped thinking in practical application of the code methods and think only in terms of what is theoretical) tells me that I would want to provide two piers - one on each side of the opening and keep the stiffness of the piers identical so as to uniformly distribute the diaphragm shear into two piers. The other reason to do this is that many believe (now that the 97 UBC is codified) that it is correct to design an open-front or soft story structures by rotation fo the diaphragm. In this case, there is a relative stiffness that differs greatly between adjacent lines of shear (the back of the garage consider 90% or more sand the front of the garage where short, but tall panels exist.
 
To summarize - if  you are following full-compliance (engineered) design, then the table is not followed (as it is prescriptive) and you work with the shear you have evaluated according to the provisions within chapter 16.
 
 

Sincerely,

Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

http://www.structuralist.net

dennis.wish(--nospam--at)verizon.net

 

-----Original Message-----
From: Dave Adams [mailto:davea(--nospam--at)laneengineers.com]
Sent: Wednesday, May 07, 2003 6:49 AM
To: seaint(--nospam--at)seaint.org
Subject: Narrow Garage Piers - Residential

Hello List,
 
I'm looking for some clarification on the wording of 1997 UBC Table 23-II-G, Footnote 3 (revised per errata January 2001):
 
"In Seismic Zone 4, the maximum ratio (...height-width...) may be 3-1/2:1 for walls not exceeding 10 feet in height on one side of the door to a one-story Group U Occupancy."
 
The part that I'm getting tripped up on is the "...one side of the door" phrase:  Does it mean that you can only use ONE SIDE, even if BOTH sides meet this criteria, or does it mean IF ANY ONE SIDE ("either side") meets the criteria, you may use the stated ratio.  It doesn't make sense to only allow this criteria for one side if both sides meet it -- maybe there's some history behind the footnote that someone can share.
 
I'm talking about engineered construction (as opposed to "conventional") and I know there are realistic limitations to the behavior of these short panels, but I'm just looking to see what everyone's understanding of that footnote is.
 
Thank you,
Dave Adams, S.E.
Lane Engineers, Inc.
Tulare, CA