wood rafter thrust problem 5

[structuralnerd]

I have a wood framed building with a very tall and steep pitched roof. Unfortunately, the client wants the ceiling vaulted, so we cannot use trusses to frame the roof, only rafters with one intermediate brace toward the top of the roof. This results in quite a bit of thrust force at the bottom of these rafters. To take this thrust force out, I was thinking about resolving it into the sill. By analysis, this results in a really big sill plate. I was thinking of the sill plate as a simply supported beam. Is this the right way to go about this thrust problem? Would I need a pretty substantial connection at the end of this sill plate? Any other suggestions? Thanks in advance.

 

[WmacG]

StructuralEIT
For a real roller support, the vertical reaction at each end of a sloped beam would not be in line with the center of the roller axis. The horizontal distance would cause a moment and would cause the roller to rotate untill the beam fell off the rollers.

 

[Lion06]

WmacG-
That is only true if the bottom of the sloped rafter is not notched to bear on the top plate (roller), correct? If it is notched, then you will not have the resulting eccentricity that you are talking about, correct?

 

[WmacG]

Agreed

 

[DRC1]

StructuralEIT

If you had two roller supports on the rafter, and the rafter supporting vertical load, the rafter woul have an unbalanced horizotal reaction and the rafter would roll away.

There are not two roller supports, but unless the rafters conections are rigid, they would be modeled as pinned. Thus the sills will need to reesist a horizotal reaction.

 

[apsix]

Just to counter some recent posts I'll make a few broad statements. Feel free to enlighten me if they are wrong.

If there is a roller support there can be no horizontal reaction.
A sloping beam on rigid supports, with vertical loads, will only have vertical reactions.
Only if there is a vertical displacement of the top support (such as deflection of the ridge beam) will there be a horizontal reaction (or displacement) at the lower support.
If the top support is a vertical roller and the bottom support is a horizontal roller only then the beam will roll away ie. ladder on skateboard.

 

[Lion06]

DRC1-
As I said in a previous post, the "rolling away" of the beam will only happen if the beam is not notched to bear vertically on the wall (roller).


I agree with apsix.

 

[miecz]

apsix

CSD72 said essentially the same thing earlier in this thread, but with some of the recent posts, I think it needed to be repeated. I think you are right on target.

On your third point, I believe that, for a ridge supported rafter, normally the result at the rafter plate is horizontal displacement, and the horizontal reaction is small enough to be ignored.