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RE: Subject: Re: roof truss daiphragm forces

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Tim,

Is this a heavy timber truss or a metal plate manufactured wood truss (which is why I would question why you had any input on the design)?  Is the ceiling or bottom chord of the truss flat or is it vaulted inside?  I am just wondering why the plan checker even asked for the straps unless he was concerned with out of plane forces against the bearing walls that may be caused by a heavy timber truss with a vaulted ceiling as the truss thrusts outward.

 

Am I way off base here. The reason I ask is that the only time I get involved in a roof truss system is if I am adding a drag truss or a girder truss and specifying where I want the trusses placed and providing lateral forces for the shear transfer from top chord to bottom chord on drag trusses. Other than that my only obligation is the connection of the metal plate mfg wood trusses to the bearing and shear walls.

 

Some time ago we had a discussion on adding straps across a discontinuity that occurs at the peak of the roof because the sheathing is not lapped or staggered. The discussion included a discussion of using a ridge cap to make the shear transfer. However, it works out that there is very little shear since the entire roof diaphragm is transfering shear at the edges making the moment greatest at the ridge parallel to the ridge line (but this was an insignifican force since the depth of the roof was so great that the deflection at the gable end is small. You might want to search the archives back about two or three years and use “ridge cap” as a search criteria. 

 

 

 

 

 

Dennis S. Wish, PE

California Professional Engineer

Structural Engineering Consultant

C-41250 (Exp. 3/31/09)

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

http://structuralist.spaces.live.com

http://www.structuralist.net (new site and services coming soon)

 

 

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From: Pinyon Engineering [mailto:Pinyonengineering(--nospam--at)hughes.net]
Sent: Thursday, May 17, 2007 5:16 PM
To: seaint(--nospam--at)seaint.org
Subject: Subject: Re: roof truss daiphragm forces

 

Yes, the comment was on Internal Diaphragm Stresses.  I have the chord designed on the gable end wall(and shed roof side).  I aways understood that that was sufficient.  the 2kip was figured based on the resulting shear diagram of the forces and simplified for all common trusses(way over designed for the center of the building). no drag trusses or internal shear walls it doesn't get simpler the building is a rectangle no popouts or architect stuff. The shears are not very high either.  - seems like this is one of thoes wood design things that works and shouldn't be thought about too much - like how variable in quality wood is

 

Tim

 

 

This is normally done on Blue houses only :-)

THis is for in-plane drag forces along the top chords of the trusses? A 2kip transfer via metal plates seems pretty high for a single truss, but I usually dbl up drag trusses.

Internal Diaphragm Stresses to me is a strange statement. Usually the ridges are at the lowest shears of the diaphragm (perp. to the trusses) and placing straps over the top does nothing. If the shears are high @ the ridges due to an internal shear resistance line, you need to create mini-shearwalls blockings between the trusses to get the load to the wall below...

-g