Rafter Thrust

[slickdeals]

thread507-189340

I may be way off on this one, but the reference thread concludes that there is no thrust if you have a ridge beam.

Isn't the thrust a function of the vertical deflection in the ridge beam?

 

[hokie66]

You would either have a thrust force or a roller connection. For the vertical load only cases, the thrust would not be necessary for stability.

 

[Lion06]

Correct, the thrust is not necessary for stability.

Yes, slick, rhe horizontal movement or thrust (depending which you are dealing with) is a function of the ridge beam deflection).

Even though the thrust is not required for stability, if you assume a roller support at the eve then you need to detail to allow that movement.

 

[paddingtongreen]

The movement to relieve that thrust is small compared with the deflection of the ridge beam.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[apsix]

Michael, you'll have to explain that.

By my reckoning, to relieve the thrust, the horizontal eaves movement is equal to the ridge deflection at 45deg pitch.

 

[msquared48]

In that the rafters will deflect too, shortening their horizontal span, the thrust from the ridge deflection will be less than just the geometric deflection pattern.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

I believe the horizontal deflection, using a roller connection would be [Δ]tan[θ] where [Δ] is the deflection and [θ] is the roof slope. I would ignore the effect of the joist deflection.

For a roof slope of 45⁰ it would be equal to the roof deflection. For usual slopes, it would be less and for very steep slopes, it would be more, although the plate action of the roof would tend to hog load from the beam.

BA

 

[csd72]

But if your roof is sheeted then this thrust will transfer to the ends by diaphragm action.

 

[msquared48]

csd:

If all the force is transferred to the end SW, then there would be no deflection in the ridge beam, which is not going to happen.

Without collar ties at the top plate level of the wall, there will be lateral movement in the wall. There has to be.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[BAretired]

If the roof deck behaves as a diaphragm on each side of the ridge beam, it becomes a folded plate. The vertical deflection of the ridge and the horizontal deflection of the support would be much less than that of a ridge beam acting alone.

BA