Pitched roof as diaphragm

[vmirat]

Can a pitched roof with wood trusses, such as a roof on a house, be used as a diaphragm to transfer lateral loads to shears walls? If so, do the bottom chords of the trusses have to be designed to resist compression?

 

[Mattman]

Yes, a pitched roof with wooden trusses can be designed as a diaphragm. The bracing between trusses must be designed to resist the out-of-plane loads. Web and chord members may require bracing.

 

[phuduhudu]

Yes it is quite normal to use the roof plane to resist lateral loads at the head of the wall. The roof sheets alone cannot normally act as a shear diaphragm but the purlins and bracing in the plane of the roof normally act as a diaphragm. In normal domestic construction there are standard details and rules for bracing and purlins which ensure that there is sufficient diaphragm action so that calculations are not required. These are normally contained in prescriptive roofing codes. However for buildings not covered by these codes calculations would be required. Carl Bauer

http://www.bauerconsultbotswana.com

 

[DaveAtkins]

I have designed many wood framed commercial buildings with pitched roof diaphragms. I check the roof sheathing (OSB or plywood) as if it were a flat diaphragm. At any interior wall which is used as a shear wall, I position a roof truss directly over the wall, and fasten the truss to the top plate of the wall for shear and overturning of this "drag strut truss". I have the truss supplier design this drag strut truss for the lateral force it must transfer from the roof sheathing to the shear wall. At outside shear walls, the "gable end truss" which occurs over the shear wall is normally covered with wall sheathing and must only be fastened to the shear wall for shear and overturning.

 

[boo1]

The Americal Forest and Paper Assoc. Wood Frame Construction Manual-SBC covers later diaphragm loads from wind for calculation In-plane Shear in Roof and Floor Diaphragms. You also can include the drywalls contribution to shear resistance.

Cheers

 

[Fatalxception]

What about high sloped (> 45 degree) plywood roofs that do not have trusses. Instead, they only have say 2x10 roof beams and no ceilings. How effective is the diaphragm in transmitting the horizontal loads on such a high angle, into say wood shear walls at each end?

 

[dennisthemenace]

i have roof trusses that lean, are not aligned, have bowed chords approaching 1 inch, and web joints that have from zero to 60 percent wood to wood contact held together by a truss plate. Can anyone help in suggesting a reference to look up formulas etc so I may calculate the strength of the trusses and the joints? thanks for your help, Tim.

 

[Trussdoc]

Dennis, try the following:
Wood Truss handbook, available from the Wood Truss council of America (

http://www.woodtruss.com)

ANSI TPI 1-1995 Design Specification...Wood Trusses
WTCA also has quality criteria that you can better evaluate your trusses by. I don't think a 1" bow is really that bad, however, joint gaps in compression members can be serious.
A truss may be outta plumb by 1/50 of the depth up to 2".

 

[RARWOOD]

You might want to call the American Plywood Association in Tacoma and talk to some of their people.They should also be able to refer you to research papers and other publications which address diaphragms for more complex applications such as a hip roof.

I think in general the biggest problem is making sure the structure is detailed so you can transfer the shear into the diaphragm chords and boundary elements.

I have also used trusses as drag struts with mixed results. I've tried specifing the load that the trusses must carry to the shear walls with very poor result. When I did this the truss plant providing the trusses did not have educated enough staff members to properly do the design.

Another thing I have done is require the truss supplier to set a truss directly over the shear wall below. Then I had them apply plywood to both sides of the tyruss.

 

[trussdude]

We regularly design trusses with drag loads that transfer from the roof diaphragm. I am wondering if anyone knows of a quick source to calculate drag forces on a particular building, say a rectangular approximation that can put us in the ball park. Sometimes the building designers specify humungous loads that the wood trusses can't be designed for or you end up with a very expensive design. Anyone know of a good source of calcs for shear/drag design?

Thanks

 

[youngstructural]

I'm not entirely sure I'm being helpful here, but I thought I would mention it... If you are in Canada, Commentary B of the National Building Code is all about Wind Loads, and is of course based on listed research and papers. Out of my own curiosity, do the American codes have similar guidance? I had heard a rumous that their codes are based on wind velocity and not on pressures. Seems kindof convoluted, but perhaps easier to adapt to field measurements. Just curious.

And another stab at answering your question would be to point you towards "Wind Loads on Structures" by Claës Dyrbye and Svend Ole Hansen Consulting Engineers. It's a decidely european text, but I have found it handy to have around.

 

[trussdude]

Wind design with velocities is not that far away from designing with pressures. After all I can use a modified Bernoulli's kind of equation to convert velocity to pressure. And yes the ultimate calcs in the American codes also is based on pressures. If you look at the contour maps published by ASCE they do give the pressure in parenthesis. Some of the design staff may or may not be able to calculate pressure so the software programs used for truss or building design do that for you. [highlight]ASCE[/highlight] publications are a very good source for the formulae to calculate this pressure based on exposure, building mean height, form and other factors. To the best of my knowledge, the form of the equation is similar to basic fluid flow pressure equations.

I am looking for some theory on drag collector truss design, shear wall design for truss based structures etc. Anyone know of any good reading material for this?
Thanks