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  • 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 cos? 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.

  • #2
    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.
    Tony Bryer, Greentram Software Pty Ltd

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    • #3
      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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      • #4
        Re: Rafter Deflections

        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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        • #5
          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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          • #6
            Can someone please help.
            I am trying to understand about rafter deflection and how it has been implemented within Superbeam by others Sellam, Asutcliff in previous posts.
            I have understood "vertical load and span dimension on plan".

            The equations I used 0.003*4000*cos(45) and similar.
            Google: 0.003*4000*cos(45) = 6.30386386581
            Superbeam Calculator: 0.003*4000*cos(45) = 8.49

            Can someone please explain how the results of Google & Superbeam Calculator differ and with the result of say 8.49 would I put that in the span field with the note suggested by Sellam

            "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."

            Tony Bryer's post at the moment is a bit over my head but I would like to understand as you guys do.

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            • #7
              I figured out the difference between Google and Superbeam Calculator using Excel formula which is
              0.003*4000*COS(RADIANS(45)) = 8.485281374 for a 45 degree pitch so at the moment I am assuming that the figure of 8.485281374 would be the deflection to enter in the Span in the "TIMBER DESIGN PARAMETERS" in Superbeam.
              If someone could help me here it would be appreciated.

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              • #8
                just seen this thread and thought I would share my design of rafters - I use the calculation based on the plan area which of course has the load increased accordingly to compensate for the angled rafters then I check the size against the TRADA tables based on the angled dimension (so thats the rafter length measured between supports) and then use the TRADA size in the calculations - this means the section is checked for bending and as it complies with the TRADA table its OK in deflection

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                • #9
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                  Last edited by maxzimmer; 17 August 2021, 09:14 AM.

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                  • #10
                    I am trying to understand about rafter deflection and how it has been implemented within Superbeam by others Sellam, Asutcliff in previous posts.

                    ​mobdro 2022
                    Last edited by ghazestor; 24 December 2022, 04:58 PM.

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