×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Cathedral Ceiling Roof Rafters Thrust
3

Cathedral Ceiling Roof Rafters Thrust

Cathedral Ceiling Roof Rafters Thrust

(OP)
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

RE: Cathedral Ceiling Roof Rafters Thrust

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.

RE: Cathedral Ceiling Roof Rafters Thrust

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

RE: Cathedral Ceiling Roof Rafters Thrust

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.

RE: Cathedral Ceiling Roof Rafters Thrust

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.

RE: Cathedral Ceiling Roof Rafters Thrust

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.

RE: Cathedral Ceiling Roof Rafters Thrust

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...

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources