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Old 29th April 2011, 11:10 AM   #1
philjepson
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Location: Southampton UK
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Default Rafter Deflections

Wondering if anybody else has come up with this one.
When designing a simple timber rafter, the requirements for strength are based on the vertical load and span dimension on plan. However, when it comes to deflection the calculation based on the span on plan dimension will be incorrect. If I have got this right, to obtain the correct value of deflection normal to the rafter, one needs to take the value calculated from the span on plan dimension and divide this by cosē of the pitch angle. In the case of a 45 degree pitch roof this will double the value of deflection calculated from span on plan. Hope this makes sense.
If there is agreement it would seem to be a useful addition to SuperBeam if this modification could be incorporated.
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Old 3rd May 2011, 03:40 AM   #2
Tony Bryer
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Default Re: Rafter Deflections

The way I would analyse a rafter would be to calculate the load (which acts vertically of course), then resolve it into two components, one acting normal to the member and causing bending, the other acting in the line of the member and causing compression. The member then needs to be checked under combined deflection and compression, and for deflection - possibly not so critical as elsewhere unless there's a sloping ceiling under the rafter. Eurocode 5 suggests deflection limits for timber members of L/250 if there is a plastered ceiling under, L/150 otherwise - note that these include an allowance for long term creep deflection so the L/250 isn't very different to the BS5268 0.003L.
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Old 16th September 2011, 11:13 AM   #3
sellam
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Default Re: Rafter Deflections

The way I deal with this is to add a note to the end of the page and limit the allowable deflection:

"Plan loading and plan span has been used to calculate the bending stress.

The axial loading has not been included so the allowable bending stress/applied bending stress has been kept low to compensate.

The allowable deflection of 0.003L has been modified by 0.003L*Cos(roof pitch) to give deflection in the rafter."

Hope this helps.

Steve
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Old 1st November 2011, 12:30 AM   #4
Beau
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Unhappy Re: Rafter Deflections

Quote:
Originally Posted by philjepson View Post
Wondering if anybody else has come up with this one......If there is agreement it would seem to be a useful addition to SuperBeam if this modification could be incorporated.
Would be nice to have some roof function as with some American or Canadian softwares.
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Old 29th March 2012, 06:58 PM   #5
asutcliff
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Default Re: Rafter Deflections

I use a similar method to Sellam for rafters & hips / valleys & these seem to come out well. It would be just another really useful addition to this excellent software if rafter calculations could be added to the stable.
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