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Re: RESIDENTIAL: What A Drag

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Bill,
 
In earthquake country, interconnection of all of the parts of the building is important -- generally requiring "drag struts".  This is especially important in residential buildings in which major components of the mass of the building are not necessarily in the vicinity of available shear walls [or other vertical lateral-load resisting elements].
 
My first goal is to establish a common top-of-double-plate elevation for each story throughout the building, and keep the drag forces in the double plates.  When this goal becomes impractical, some other way of maintaining the interconnection is needed.  Sometimes plywood-sheathed shear panels that reach from the common plate level up to the "roof at some arbitrary elevation above" is needed.  I think that the plywood-sheathed panel serves the function that you had in mind for trusses -- sometimes a steel "column" in flexure can be the element that "reaches up".  You need to provide for overturning resistance for plywood panels.  Residential building designers don't make it easy -- persevere! -- it's important.
 
Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net
 
----- Original Message -----
Sent: Monday, August 30, 2004 8:15 AM
Subject: RESIDENTIAL: What A Drag

More of my mental meanderings herewith, as I attempt to continue my transition into the exotic world of residential structural design.

 

As many of you have seen by my posted example on an earlier thread, I’ve got a convoluted roof design that I’ve only just unraveled geometrically.

 

One thing I’m concerned about that I haven’t been able to resolve to my satisfaction as yet, is the need to tie the roof to the walls, and the use of DRAG STRUTS to accomplish same.

 

Unlike on a nice, quiet flat- or low-slope roof, where the diaphragm is conveniently “there” wherever you want to connect it, these three-dimensional roof geometries for residential construction mean, I assume, that I’ve got to have some sort of truss element or some such contraption to connect the “drag strut” at the top-of-wall level to the roof at some arbitrary elevation above. I guess the manufactured roof truss people deal with this all the time (but unfortunately manufactured roof trusses are considered “too expensive” in our market).

 

Has anyone dealt with this in “stick framing”? What sorts of solutions have you come up with?