Structural Ridge Beam & Statics? 1

[Wheels77PE]

I am designing a cathedral ceiling and having a difficult time wrapping my head around the structural ridge beam concept. If we look at the statics of the sloped rafter member and draw a free body diagram, there is a vertical (Fy) and horizontal (Fx) component to the reaction. I understand the ridge beam is designed for the vertical component. How is the horizontal accounted for? Many articles talk about the outward thrust not being present if the rafters don't move but even if the structure doesn't move, the load is applied. If the load is applied the forces are there internally. To be in static equilibrium the reactions have to support the internal forces. Thoughts?

 

[woodman88]

As I see it, with a ridge beam the forces are resisted by vertical reactions. But as the ridge beam deflects down the lower bearing will move outward. With a ridge board, the rafter is supported by a vertical reaction at the low end and a horizontal reaction at the ridge board. So the collar tie is needed to remove the horizontal force at the lower bearing. Hope everybody agrees with this?

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[KootK]

I certainly agree. For OP's benefit, I'll also add:

1) For the usual case of wall support (sans collar tie) at the low end, we normally assume no horizontal reaction there.

2) if there is no horizontal reaction at the low end, statics dictates that there is also no static reaction at the high end.

1 + 2 leads us back to JAE's original response.

JAE's concerns regarding where the horizontal component of wind load goes in these situations has long nagged at me. That might be worthy of it's own thread.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.