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Wood Prefabricated Truss used as a drag strut

Wood Prefabricated Truss used as a drag strut

Wood Prefabricated Truss used as a drag strut

(OP)
Is a "drag truss" a truss in which the bottom chord is designed to accomodate tension/compression loads due to lateral loads in addition to tension/compression induced by vertical loads? My friend, a structural engineer, specified a "double drag truss" to be used in a single family single story home and the truss company said they had never heard of this.  

Just curious....  

RE: Wood Prefabricated Truss used as a drag strut

I can confirm that it is a rare occasion (in the Mid-Atlantic region) that residential roof trusses are specified to resist drag strut loads so most designers aren't aware of even what they are.  However, I know that both Alpine's and MiTek software is capable of applying these loads to their truss designs.  Have the designer contact their software support to figure out how to do it.

RE: Wood Prefabricated Truss used as a drag strut

Might the use of the roof trusses to transfer wind loads across a building be a more efficient method than using plywood diaphrams?  

Trusses spaced at 16" or 24" would result in much smaller loads and using Simpson type anchors to fasten the truss to the top plates would maked specifying nail patterns for plywood much easier.

Could this be one of the emperical mechanisms working to support windloads prior to the development of engineered diaphrams?

RE: Wood Prefabricated Truss used as a drag strut

Out here in Phoenix there are MANY houses with drag struts / Drag Trusses. Mostly with all the tract builders and some of the custom.
 I know there is a good reason for them, I just dont think that they are being applied properly.

Example .. 8000# drag load on a 14-0 gable ??? give me a break. So if I understand it right, that means that the forces transfered throught the roof as a whole into the gable will amount to 8K in drag force ? 4 tons +/- ??
come on now !!

JP
General Manager (now)
Truss Designer (10+ years)

RE: Wood Prefabricated Truss used as a drag strut

jheidt2543,

The idea of the roof diaphram is to get the loads out to a shear wall such that you can then transfer the loads to the foundation.  Using the trusses to transfer all of the wind load essentially puts the loading right back into the top of the wall.  
There must be some diaphram action to get the roof loads out to the shear walls.  

RE: Wood Prefabricated Truss used as a drag strut

Here in Utah we often specify a "drag truss" and provide the design lateral force (wind or seismic) that must be transferred from the top chord down to the bottom chord of the truss.  It is the responsibility of the truss manuf. to design the truss to transfer this force.  It it the responsibility of the EOR to detail the connection of the truss bottom chord to the interior parallel shear wall below.  Simpson A35 clips work well.  Often, the shear wall only runs for a small portion of the truss length, so this must also be shown on the drawings.  It is also certainly possible to extend the shear wall up through the trusses to the roof diaphragm, but the drag truss option is usually less costly.   Truss shop drawings are a "deferred submittal" and must be reviewed by the EOR for conformance with the deisgn parameters before the building dept. will allow them to be intalled.  The drag truss usually needs careful review to make sure it was designed properly.

RE: Wood Prefabricated Truss used as a drag strut

Lots of trusses in Florida get designed as drag struts. Only requirement is for the building designer to specify the drag load. A truss manufacturer does not have the ability to calculate this load. One large builder requests #1750 loaded on every girder truss. This load usually has very little effect on the truss design.

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