Rafter with collar tie

[JStructsteel]

Having a discussion with some other folks (non engineers)on another forum and they are saying a LVL vs 2x12 is the same for a rafter with only a collar tie at the top 1/3 point. My thoughts are the LVL is stiffer, thus wont deflect as much, so is a better design. They have an engineers drawings, stamped, but they say its over designed. (why they got a design then is beyond me). As usual they only are looking at the bending of the rafter for the load, and not the whole system. With everything the same, would you use a LVL vs 2x12?

Rafter is about 18' span, call it 25 psf live load.

Thoughts?

 

[XR250]

Put it into a 2D frame program, load it up and see what happens.
Ironically, i was having a similar discussion with some engineers at lunch the other day. They say they check a free body diagram but have never checked deflection (spread) which always seems to control the design.

 

[JStructsteel]

I might have to do that. Trying to explain this to a non-engineer is futile. They just say 'The span table say my 2x12 is ok, so the engineer over-engineered it' Of course, the snow load they are using to check their '2x12' is wrong, so pointed that out to them.

Hopefully I can model it this afternoon.

 

[charliealphabravo]

An LVL is stiffer than a 2x12 and the closest size is 1-3/4"x11-7/8" so it is substantially larger as well.

The thing with rafter/collar-tie/ceiling-joist systems is that the load in the rafter and the design failure mode is highly dependent on stiffness of support conditions, roof angle, ridge beam size, collar tie location, presence of ceiling joists, etc. If you have a soft frame wall with low slope rafters, collar tie, and no ceiling joists, then the V/M/P diagrams will be a complete stress reversal from a steep roof configuration or one with ceiling joists or a stiff masonry wall. Are the rafters hanging from a proper ridge beam, or alternatively, are they holding up a less substantial ridge plate.

You probably need a 2D frame analysis unless it is a conventional system that you can pick off a prescriptive chart.

 

[beam4u]

Using LVL's for rafters is rather expensive. TJI's might me less expensive. Here in the southwest, they use a purlin near midspan transferring some load to the ceiling joists. In either case an analysis seems appropriate.

f

 

[JStructsteel]

@ Beam4u. This is a rehab....removing some trusses in a garage to get more ceiling height. its about 5 trusses that will be replaced.

 

[charliealphabravo]

Post a sketch of the cross section and plan if you can.

 

[beam4u]

So the "collar tie" is acting as a ceiling joist?? I've done this many times - if this is the case I think you'll be surprised by the connection requirement.

f