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Transferring of wind loads by bracing roof trusses

Transferring of wind loads by bracing roof trusses

Transferring of wind loads by bracing roof trusses

(OP)
Is there any method of transferring bracing forces through the roof trusses if the gypsum board ceiling is ineffective. My building code only allows gypsum board to be used for spans up to 9.0m (30'). I have a light gauge metal sheet roof which cannot be assumed to act as a diaphragm. Is there any other method out there to transfer the wind loads?

 

RE: Transferring of wind loads by bracing roof trusses

You could consider a plywood diaphragm under the gypsum board, or you could try to retrofit a plywood diaphragm onto the top side of the bottom chords.  You could also deck the top surface then install the metal roof panels.

Outside of that, you'll have to get a bit more unconventional with blocking and x-bracing, but chasing the stress paths could be a nightmare.

RE: Transferring of wind loads by bracing roof trusses

Most mfgs of light metal gauge roofing do have published shear values.  You might check with them.

RE: Transferring of wind loads by bracing roof trusses

Is this a domestic application?  If so, why not engage more shear walls?  If the bracing has to span 9 metres, you could use horizontal trusses.

RE: Transferring of wind loads by bracing roof trusses

(OP)
Thanks for the input everyone, from the sounds of things there are methods to achieve bracing other than gypsum boards.

Plywood diaphragm to the underside of the roof trusses (this will be the method that I go with).

Provide blocking and strap bracing to the top chord to create bracing in the roof plane.

Horizontal Trusses which I assume will be fixed to the underside of the roof trusses.

RE: Transferring of wind loads by bracing roof trusses

If you aren't using a diaphragm at the roof level/top chord, how will the load get from the point of origin (roof) to where the load is collected?  Decking the attic floor (top of bottom chord) is good, but for wind perpendicular to the trusses, the load has to get to the attic floor.  Also, are the top chords braced against lateral movement?  This bracing is usually provided by the roof deck.

RE: Transferring of wind loads by bracing roof trusses

(OP)
The way I have been brought up is that light gauge corrugated metal roof sheeting fixed through the ridges cannot be used as a structural diaphragm.

Because in this instance the roof pitch is very flat (3 deg), the majority of the racking force comes from wind force on external walls.

I am having a major dispute with the builder over this one.

RE: Transferring of wind loads by bracing roof trusses

kikflip,
You have been brought up correctly.  Don't depend on the roofing.  It is to shed water, not to brace the building.  If you had a steeper roof, you could provide knee bracing to bring gable forces down to the ceiling.  I know that in the States, they use continuous sheathing on the roof which serves as a diaphragm, but the ceiling is the most logical place for bracing.  If you can't follow my earlier suggestion to engage more shear walls, try Ron's plywood ceiling level diaphragm or cross bracing.  

RE: Transferring of wind loads by bracing roof trusses

Corrugated metal roofing cannot be used as a diaphragm? Wow, that means that almost every factory in Australia should have fallen down by now!

I would trust metal roofing far more than gypsum board but definately not more than the 9m you mention..

http://www.pdhonline.org/courses/s184/s184content.pdf

That aside, if you do not want to trust it then why not use metal strap bracing. Pryda do some heavy duty load rated strap.

RE: Transferring of wind loads by bracing roof trusses

csd72,
Your reference is for metal roof deck, not for roofing.  I've been practicing in Australia for 27 years, and have never used roofing for diaphragm action, and have never seen it used in that way except for very small garden sheds and the like.

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