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RE: Relative Stiffness of Wood Shearwalls

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Mike,

Just a couple of notes:

  1. There was discussion many years ago about mixing shearwall materials in the same line of shear. We were specifically debating the use of gypsum and plywood in the same line as well as shearwalls where sheathing was applied to one side while another was applied to two sides or the nailing was different within the same line of shear.

I think the point that I was trying to make was that while walls may vary in aspect ratio in the same line of shear, the capacity and potential drift of the wall is not dramatically different than if using dissimilar materials or sheathing each section more stiffly than others. Why push the potential for failure when you can have one wall reach a “limit state” (if I am using the term correctly) so that it then joins the drift potential of the next wall section. Therefore, as a system they will would in tandem.

While I disagreed with Keith’s comments about relative stiffness – he makes sense in pointing out the difference in deflection based on aspect ratio. However, the issue is to try and keep the walls as close in stiffness as possible to avoid failure of the stiffest which then cause all subsequent walls to fail.

  1. I believe Hardy specifically states in their ICBO report or ICC report that their frames should not be mixed (aspect ratio or capacity) in the same line of resistance. I don’t have a confirmation on this, but possibly someone else who is using Hardy can confirm this. I never have mixed them and as I noted, I do try to keep my shearwalls as close to the same aspect ratio’s as possible.

 

Dennis

 

Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

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

http://www.structuralist.net

 

-----Original Message-----
From: Mlcse(--nospam--at)aol.com [mailto:Mlcse(--nospam--at)aol.com]
Sent: Wednesday, May 19, 2004 10:17 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Relative Stiffness of Wood Shearwalls

 

Dennis...some comments below.

 

Mike Cochran

 

In a message dated 5/19/2004 9:00:05 PM Pacific Standard Time, dennis.wish(--nospam--at)verizon.net writes:

I'm getting into this discussion a little late but want to add some
thoughts:

1. The issue of relative stiffness using different materials is moot with me
- I simply won't do it. I have been searching for information in the 97 UBC
that I thought stipulated that relative stiffness could not be used in the
design of plywood shearwalls in one common line of shear. Possibly someone
can clear this up - simply, all walls in the same line of shear must be
sheathed with the same materials and the same nailing.

I don't think there is anything in the code that prohibits this, but using different nailing for different walls along the same wall line will result in the walls with the closer nail spacing most likely taking more load than the walls with larger nail spacing (depends on length of walls relative to nail spacing for each wall).  It makes more sense to have all walls in the same line to have the same nailing so there is less likely of a screw-up in the field by the person nailing the plywood shear mis-nailing the walls along that particular shear line.

Conversely, Hardy
specifically lets engineers know that you can not mix frame or panel sizes
in one line of shear. In Rigid analysis, I believe the opinion is that while
relative stiffness of walls can be balanced, they are not assumed to be
constructed in the same line of shear resistance.

I believe you can mix Hardy Panels in the same line, but you must distribute the load to the Hardy Panels by rigidity along that shear line.  End result being your not efficiently using the Hardy Panels (some take more load, others less than thier rated load so they all have the same deflection) as you would be if you were using all of the same size Hardy Panels in the same shear line



2. Stan's comparison between plywood shearwall and Hardy frame deflections
needs some understanding. The Hardy Panels have a higher capacity than their
Frames due to the light-gauge metal sheathing on one face that is welded to
the 12-gauge columns and top and bottom channels. The deflection of the
panel is also a function of the anchorage to the foundation - a 7/8"
diameter anchor bolt (Holddown) and a 1-1/8" diameter x 25" long anchor bolt
(Holddown) in each 12-gauge column. The capacity of the panels w/ the larger
diameter and longer anchor bolts has a higher capacity and larger
deflection. So the issue is what frame or panel is used and is the plywood
shear wall one highly loaded wall section or a couple of plywood shearwalls
in the same line of shear.
With this said, I don't feel comfortable designing a high load plywood
shearwall. I have designed walls with sheathing on both side, but rarely
design above 550-plf and typically when I have sufficient dead load to help
resist uplift and the stress at each end of the wood shearwall.
If push comes to shove, I would feel more comfortable designing a cold-form
steel braced frame or panel in lieu of a plywood wall - the materials are
delivered in monolithic form and are much more resistant to human error
considering the possibility of over-nailing, over-sizing hold-down bolt
holes or incorrectly splicing mudsills.  


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