Unique roof rafter configuration and the analysis thereof...

[gcs3pe]

Stop me if you've heard this before (but I'll need a picture to believe you). I'm reviewing the roof framing for a client with a 48' deep center hall colonial. The 10/12 pitch is formed with 11.88" LVL rafters at 16" on center, and they're tied at the top of the outside wall with continuous, 48' 11.88" LVL ceiling joists running parallel to the rafters. Halfway up into this cavernous space (10') there is an attic ceiling joist framed with (you guessed it) 11.88" LVLs. All connections are made with 4ea 1/2" bolts, except for the rafter tops, which are strapped to a non-structural 16" LVL ridge board (and each other).

Question: has anyone had any luck convincing a code official that an attic itself cannot have an attic? He wants me to apply significant live loading to the top of the attic ceiling joists because (let's face it) there's 10' of headroom up there.

Since the system appears to work without the ceiling joists (members, connections good), should I have the attic ceiling LVLs removed? Because the way I see it, those forces could (on paper) exceed the moment capacity of the rafters.

It seems a bit counterintuitive. Of course, as always, I may have missed something in my hastily drawn FBDs.

 

[msquared48]

This sounds like a two tier collar tie configuration - interesting.

If the lower floor joists are adequately connected to serve as collar ties, then the upper set probably has little effect on the rafters as the moment arm is so much greater than the upper ones.

You could remove the upper ones, but why? Just label the upper area as attic space, providing a small limited access to it, and the lower space - well apply the standard residential 40 psf figure.

I would be surprised if that did not work. Then again...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[MiketheEngineer]

Check IBC - attics with certain amounts of headroom must be capable of carrying a certain live load - I think 40 psf??

But check in the Chap 16

 

[ToadJones]

Bizarre...I worked on a project once where the roof configuration was nearly identical with the exception that LVL "ridge beams" also ran parallel to the main ridge and supported the "upper collar ties" where they tie into the rafters

The roof was so huge it was like we were framing an entire house upon a house.

I'll see if I can find photos.


P.S. the roof configuration was installed because trusses that were originally installed by another contractor, collapsed in a wind storm.

 

[ToadJones]

...during construction

 

[gcs3pe]

The IBC and IRC require 20 psf live loading for attics, but that's enough to sink the whole truss (if you can call it that). Hanging a 22' wide floor at 20/10 from the midpoint of a long set of rafters appears to overload them.

Any ideas? And it must add to the wall thrust, too.

 

[MiketheEngineer]

Design trusses - I did it for 20 years - no big deal!! For almost ANY loading condition. Might not be cheap but well designed, built and installed - the metal gusseted trusses have been around for about 50 years.

Bolted trusses have been use for well over 100 years - probably 200!!

This is not rocket science. The first guy did a lousy job!!

 

[gcs3pe]

Mike - If you mean I should design trusses that meet the client's requirements - well, that would be a fine idea if this big boy wasn't already built! If you meant that I should try and turn what's already there into trusses - it'd be possible, I suppose, but the clients have plans for the attic space that preclude a maze of diagonal and vertical members.

 

[MiketheEngineer]

OH -----

Well what kind of loads do they want to support. A plain attic for "lightweight" storage is 20 psf -- for "sleeping rooms" it is 30 psf.

Need a bit more info.

Slightly inclined to allow the bottom chord to carry any "attic" and/or ceiling load while the top chord carries snow and live loads.

But without dimensions and specifics - that is just a guess.!! Have done dozens of "room-in-attic" applications where there is a loft/bedroom/office built into the truss. Worked great!!