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RE: Light Framing Wood - Room Addition Lateral question

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Thanks Stephen but the shear walls on the addition line up with the shear wall inside the hope, so it is not exactly a re-entrant corner as you find in an L-shape building. 


For an update, It turns out that I modeled the existing structure as well as the new structure being conservative on the roof live loads. Since seismic governs, I omitted the wind load analysis normal to the house. In most cases the shear in the line of drag to the existing shear walls (all new shear walls in the addition are within 1 to 2-feet (parallel) to the existing drag trusses. The new addition comes out only 8’feet while the existing structure is 50-feet deep.


What I found is that that actual shear was (with one exception of an interior shear wall of the addition that was more than 7-feet away from the nearest line of shear) is less than 800-pounds while the existing drag truss of 4300 pounds is much closer to the calculated actual shear in the line of resistance.


However, one thing struck me as odd – The plywood shear wall capacity far exceeded the actual shear that was designed into the structure. For example if the drag load was 4300 pounds, the plywood shear wall was designed for 6800 pounds. In one wall my  20 feet of existing shear  as specified by the original engineers schedule would be 9,200 pounds, but the actual shear in the wall is calculated to be around 5005 pounds of shear. It seems that they name have designed for stiffness an deflection rather than actual shear.


Years ago I did a remodel of a home designed by the same engineers (who are out of the area). I was able to obtain a set of calculations from them. They design custom high end homes in gated communities as if they were simple tracts and put only as much plywood as is necessary to cover the actual calculated load. Rarely do they pay attention to stiffness.


In short, there is sufficient reserve in the drag trusses or in the combination of all interior shear walls of the original existing structure to pick up two of the three short walls that are about 1-foot from a drag truss.


Personally, I think I am safe here. I played with the analysis of the existing loads. All of their designs are laid out similarly – they use a dead load for the roof of 18-psf while I am using a RDL or 24-psf (from my calculations). They also use a base shear of about 18.7% (0.187 Wd) base shear while I am closer to 21% based on being with 5KM of the nearest fault and with a soil profile for soil type D.


It appears that the existing home can take not only the addition, but then some if needed. I am adding one interior shear wall for the new addition and this is at a line of resistance much farther from a local line of shear – but this is a traditional shear wall less than 200 plf.


I think I am safe on this one, but I have not had many comments other than yours to see if I am missing something.


Thanks for your reply,



From: ECVAl3(--nospam--at) [mailto:ECVAl3(--nospam--at)]
Sent: Saturday, October 13, 2007 8:33 AM
To: seaint(--nospam--at)
Subject: Re: Light Framing Wood - Room Addition Lateral question


In a message dated 10/11/2007 9:29:48 P.M. Pacific Daylight Time, dennis.wish(--nospam--at) writes:


My gut tells me that if I design the addition as independent but make a positive connection, it will have a stiffness greater than the original residence shear walls by nature of the Proprietary panels I would need to use.


You could distribute the shear according to relative rigidities of the shear resisting elements.



Current code is still the 97 UBC here in Southern California.




Not for much longer! Don't forget to reduce the DSC allowable load by 1.33 for re-entrant corner conditions.


Stephen Macie, P.E.


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