loading case for typical rafter sitting on two walls

[adwinter]

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

 

[rscassar]

Are you sure the loading is 10kPa upwards, that is a very high uplift.

 

[csd72]

Are you asking us to design the beam for you? Are you really a structural engineer?

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.

 

[Ron]

I agree with csd72 and kikflip...get some local help. The sizing of a simply supported rafter doesn't depend on its support.

 

[msquared48]

Have to disagree here Ron... He appears to be talking about is considered two way action. In certain situations that can happen and help reduce sections.

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

 

[hokie66]

Hard to figure what adwinter is asking. As kikflip says, 10 kPa is out of the range for uplift. Maybe he means 10 kN/m on the beam, or about 4.2 kPa. Still very high, but possible in cyclonic conditions.

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.

 

[Ron]

Mike...I must be having a senior moment 'cause I'm missing something...I don't see any two way action here, other than a simple load reversal.

 

[msquared48]

Ron:

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

 

[Ron]

Gotcha...didn't think of that.

 

[BAretired]

I think a sketch of the structure would be in order. I am not clear on the precise geometry of it.

BA