Sloped Roof Framing Design

[shepherd]

Trying to find design approaches taken for sloped roof steel rafters (W shape, with W shape structural ridge) under dead, live, and wind loads? I'm considering just using a horizontal projection of all the loads and applying to the horizontal projection of span length to determine moments and shears. Also, does thrust need to be considered on the horizontal walls (gable type roof, with no "ceiling joists&quot, or is it usual to neglect the thrust in design. Does anyone have a reference recommendation that discusses this matter further.

Thanks

 

[JAE]

All your gravity loads are applied to the structure in the vertical direction. To determine moments and shears from this you use the horizontal projection of the beam for the span....not the sloped length of the beam. (to prove this think about the reduced bending in a beam as you slowly move it to almost the vertical position).

For wind loads, the codes dictate that they are to be applied normal to the slope...thus, you use the full sloped length of the beam for your span in determining shears and moments.

The lateral thrust, for a beam supported by vertical supports is zero for gravity. But if your support is another sloping member (like at a roof ridge) there will be a thrust and you must consider it.

 

[sigma1]

How are the rafters connected at the ridge?
Do you have ties?

 

[haynewp]

Are you saying there is a continuous W shape at the ridge for support of the rafters?

 

[dicksewerrat]

Weld them or use bolted connections. And yes there is a lateral force on the outside walls.

 

[haynewp]

Without tension ties like ceiling joists, you need a structural ridge beam to support the rafters. The amount of movement at the top of the walls will be a function of the deflection of the ridge beam, but the rafters would stop pushing on the load bearing exterior walls when the ridge beam stops deflecting due to vertical loads.

You still of course need a diphragm, bracing, etc. for lateral loads.