How to Deal with an Inclined Rafter
How to Deal with an Inclined Rafter
(OP)
Ok, lets say you have a rafter which is 3m long in plan and inclined at a 40 degree angle, how do you account for the incline. I normal adjust the loading, say 1KN/m by dividing by cos(40) so 1/(cos40) = 1.31 KN/m and then design it. However, would it be better to adjust the length instead of the loading. So dividing 3/cos(40) to give you 3.92m in length.
This makes a bit of difference when working with timber and trying to calculate deflection. Would be really grateful if I could hear your opinions. Thanks
This makes a bit of difference when working with timber and trying to calculate deflection. Would be really grateful if I could hear your opinions. Thanks






RE: How to Deal with an Inclined Rafter
w = 1*cos2(40) and L = 3/cos(40).
To calculate bending moment, it makes no difference whether you consider the horizontal span or the sloping span. Either way it is 1*32/8.
BA
RE: How to Deal with an Inclined Rafter
Could you explain how you cos^2(40)?
RE: How to Deal with an Inclined Rafter
but if your rafter is 3m and sloping 40 deg, then the horizontal distance between supports would be cos40*3 = 2.3m. Use this for bending moment for vertical loads. For deflection, use the actual length of the member, 3m, and apply the component of your loads that is perpendicular to the rafter.
RE: How to Deal with an Inclined Rafter
Screenshots are from the book's free preview on Amazon.
RE: How to Deal with an Inclined Rafter
You stated that your load was 1 kn/m. I took that to mean a load per lineal meter of horizontal surface. So, when we consider a sloping surface, 1 kn of gravity load is spread over a greater length of sloping surface, namely 1/cos40. To state this a different way, the gravity load on the sloping surface is 1*cos40 per lineal meter. Resolving this load into axial and normal components gives 1.cos40.sin40 and 1.cos240 respectively.
BA
RE: How to Deal with an Inclined Rafter