loading case for typical rafter sitting on two walls
loading case for typical rafter sitting on two walls
(OP)
I am designing a steel rafter (cold formed section SHS or RHS) to support roof loading with the worst case being about 10kpa upwards.
I have simply taken the contributing area for the line load on the rafter as 3.6m x 2.4m (room is 4.8x3.6m), rafter 3.6m long. So i have been conservative in taking the whole of the loading over the rafter but in fact the two walls that the rafter sit on are load bearing steel framed walls. I am thinking that these 2 walls may offer about 20 to 30% capacity to take the loads.
The design is not normal in that there is no roof space,ie, the roof panels sit directly on the walls.
How much can i downsize the rafter in taking the loading, is there a rule of thumb, a bending moment diagram for this?
Regards, Adam
I have simply taken the contributing area for the line load on the rafter as 3.6m x 2.4m (room is 4.8x3.6m), rafter 3.6m long. So i have been conservative in taking the whole of the loading over the rafter but in fact the two walls that the rafter sit on are load bearing steel framed walls. I am thinking that these 2 walls may offer about 20 to 30% capacity to take the loads.
The design is not normal in that there is no roof space,ie, the roof panels sit directly on the walls.
How much can i downsize the rafter in taking the loading, is there a rule of thumb, a bending moment diagram for this?
Regards, Adam






RE: loading case for typical rafter sitting on two walls
RE: loading case for typical rafter sitting on two walls
This is not the place to ask such fundamental questions, you should have a senior engineer that can answer these things.
Also you seem to be making a lot of assumptions about the surrounding structure that may get you into trouble.
RE: loading case for typical rafter sitting on two walls
RE: loading case for typical rafter sitting on two walls
But, as others have indicated, you need the help of a structural engineer on this.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: loading case for typical rafter sitting on two walls
The distribution of the loading on the beam depends on the type of structure bringing the load to the beam. If joists spanning 2.4 metre as single spans, it is just based on the contributing area. If joists which are continuous over the beam, the loading would be 25% greater. If a solid two way spanning deck, the load on the beam would be trapezoidal, but again increased if there is continuity at the beam.
RE: loading case for typical rafter sitting on two walls
RE: loading case for typical rafter sitting on two walls
As Hokie alluded to, some framing systems allow for a trapezoidal loading shape to be applied to the framing (sloping load shepe atht ehends of the trapezoids). This can reduce the shear some, but not much moment, usually. It approaches two way, but is really not. I too had a senior moment.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: loading case for typical rafter sitting on two walls
RE: loading case for typical rafter sitting on two walls
BA