Cathedral Ceiling Roof Rafters Thrust 3

[RareBugTX]

I am designing a roof for a single family residence and the owners would like a cathedral ceiling WITHOUT tie rafters. I was thinking about designing the top plate rectangular frame (seen in plan) and its connections at all four corners of the wall as a rigid frame that will take the horizontal thrust of the roof rafters.The slope of my rafters is 10 in 12 and the span is 22 feet. Has anyone looked at this problem this way? Input is appreciated

 

[MiketheEngineer]

What is the length of the room?? We know it is 22' wide but how long??

I don't think you can make the top frame rigid enough. Maybe steel beams on their side?? Using say a 40 psf total load - check the side thrust generated. Usually very high.

I usually use a ridge beam that can carry the tributary area. There are are often quite sizable. Use Microlams or equal.

Just a thought.

 

[woodengineer]

I agree with MiketheEngineer. You want to avoid thrust on the outer wall. By sizing a ridge beam which may be posted down to bearing you can avoid your thrust issue.

woodengineer

 

[structuralaggie]

It sounds like you could design your top plate as a tension ring - but you will have to be careful detailing it. Usually there is a better or at least an easier option.

In this situation, if I can't hang it from a ridge beam or framing above it, I usually use a bent steel beam.

 

[Vinny7]

You colud try using pitched rafters with a moment connection at the ridge. At eaves fix one side down to a concrete padstone with anchor bolts and the other side fix down using slotted holes and slack anchor bolts, you would need this side to bear onto neoprene slip pads on the concrete padstone. When the dead load is ALL added to the roof tighten the connection. The rafter will have achieved its dead load deflection and you only need to worry about the thrust or deflection from the live load.

 

[COEngineeer]

More than likely the owner would be ok if you use scissor trusses. Depending on the pitch of your roof, you can have pretty good pitch on the celing. Sometimes you cant even tell it was not stick framed. You can even put faux beam on top of ceiling.

 

[pba]

Vinny7 refers to neoprene slip pads. I assume this reference should be to PTFE. Neoprene is an artificial rubber with no particular slip properties.

I take it this is a new building? Consider designing the roof with one side pinned and the other on a roller support (scissor trusses or something similar should work). You still need a ring beam as there will still be friction forces, but the horizontal thrust is much reduced due to the sliding joint. You also need to ensure that the finishes allow for the sliding joint...

Coefficients of friction between various materials are available in text books and from web sites. We tend to use 0.3 for greased steel to steel and 0.15 for PTFE bearings. Greased steel to steel requires regular maintenance PTFE although much more expensive, can usually be left alone.
For your roof also consider polyethylene sheet as a cheaper alternative, but make sure it is sourced from a reputable supplier and is not simply damp proof membrane...