Roof Thrust load
Roof Thrust load
(OP)
What is equation for rafter tie load ( thrust) for uniformally loaded roof? If the load were applied at the ridge the thrust load should be the vertical load divided by the pitch i.e. 3 times vertical load for a 4:12 roof but would seem to be less ( like half) for a uniformally distributed load. Tables seem to show some odd multiple less than half.






RE: Roof Thrust load
For a load directly on the ridge, the lateral thrust is only as you have noted if that ridge beam is not doing any work and has no stiffness whatsoever.
If the ridge isn't doing any work, then the thrust should be (wl^2)/(2h); where h is the vertical distance between the ridge and the eave, w is the uniformly distributed load, and l is the horizontal distance between the ridge and the eave.
RE: Roof Thrust load
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Roof Thrust load
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Roof Thrust load
How will the sheathing restrain lateral thrust due to deflection of the ridge beam?
Ron
RE: Roof Thrust load
RE: Roof Thrust load
RE: Roof Thrust load
Anyway, if near a shear wall, thrust loads would be inherently mitigated....by the shear wall.
RE: Roof Thrust load
The inherent stiffness of the roof diaphragm would act like a deep beam, not only:
1. slightly reducing the live load deflection of the beam, not the dead load (I did say slightly), but also...
2. reducing the associated lateral deflection of the wall.
The greater the roof pitch, the greater the effect, assuming you do have shear walls at the end of the diaphragm to develop the stiffness as previously mewntioned. Wew never really consider this effect, but it is there.
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Roof Thrust load
We know that roof sheathing diaphragm action is there, assuming it is well connected at the end walls. But, isn't it true that a greater pitch will tend to reduce the ridge deflection, while a flatter pitch would be more effective in counteracting trust at the exterior walls?
RE: Roof Thrust load
BA
RE: Roof Thrust load
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Roof Thrust load
I have not dealt with wood design, correct me if there is mistake/mis-understanding.
RE: Roof Thrust load
In the present example, we have not yet determined whether or not there is a ridge beam. If there is, the ridge beam carries half of the uniform load between side walls. The side walls each carry one quarter of that load plus load on eave projection if any. Rafter ties are not required as each rafter is a simple span.
If there is no beam at the ridge, then the rafter tie tension is WL/8h where W is the total load per unit of length on the roof, L is the span between side walls and h is the vertical distance from the middle of tie to the middle of rafters at the ridge line.
In the absence of both a ridge beam and rafter ties, the roof must act as a folded plate.
BA