Structural Ridge Beam

[lumestruc]

Hello All, I need some clarification on this problem as I’m having some self doubts. Is the DL on plan of 0.166 kN/m2 correct (0.162/cos12)? Also would the rafters act as concentrated loads on the Ridge beam? Having some difficulty as the deflections due to using rafters and the live load is quite large. For example when I try a ridge beam of 4” x 10” the checks are failing substantially. Would the effective length be the full 4.28m or would the rafters break the span? Forgive me but I am a junior engineer fresh out of university and I need some clarity.

 

[jhnblgr]

Not sure what you standards require for min dead load by I typically use 20 psf. You can treat the rafters as an evenly distributed load typically unless your spacing varies or loads varies. Your live load is actually quite low. I tend to find conventional lumber doesn’t work for ridge beams. You might get a quadruple 2x12 to work if grade 1.

 

[BridgeSmith]

This would seem to be the place where you should be getting guidance from your senior/supervising engineer.

 

[Eng16080]

Also would the rafters act as concentrated loads on the Ridge beam?

Technically yes, but as noted above, it's common to assume the load is uniformly distributed. For most normal situations, both approaches should give very close results. While it would be ok to analyze each point load individually, this would be a more complicated and error prone analysis.

For example when I try a ridge beam of 4” x 10” the checks are failing substantially.

I didn't go through your numbers, so there may well be an error in your analysis. With that said, I commonly find that conventional lumber is inadequate as a ridge beam and that LVL material (engineered lumber) or steel is needed instead.

Would the effective length be the full 4.28m or would the rafters break the span?

The rafters would not break the span unless the rafters are somehow able to support the ridge beam, like if you had a tied roof. In that case, though, a structural ridge beam would generally not be needed.

 

[lexpatrie]

I think the question pertained more to bending effects on the ridge beam, if the rafters frame into the side of the ridge beam i think most engineers would consider i it braced in top side bending ever 2 feet or so. Effective length could then be determined from the tables (it's more than the spacing of the bracing in wood), for bending stress checking on the ridge beam.

Ridge beams wouldn't generally be viewed as receiving vertical support from rafters. That would require considerable justification, at which point it's more of a ridge board than a ridge beam.

 

[XR250]

I didn't go through your numbers, so there may well be an error in your analysis. With that said, I commonly find that conventional lumber is inadequate as a ridge beam and that LVL material (engineered lumber) or steel is needed instead.

I usually can get it to work for spans 13 ft. or less.