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Converting conventional roof truss to Scissors style truss or rafters?

bldgmeister (Structural)
16 Nov 11 17:14
I have a residential remodeling client that wants to modify existing conventional 2x4 hem-fir triangular shaped trusses to scissors style trusses to create a vaulted ceiling. They do not want to remove the existing roof deck/roofing to install new trusses due to cost and weather. Is it practical or cost effective to modify these trusses to include new bottom chords and diagonal bracing along with gussets to create the scissors style truss? Or is it more practical and cost effective to design a conventional rafter system with or without a center bearing wall? Here is some existing data and a photo is attached:

Truss horiz. span to bearing walls = 26 feet
Truss vert. height at center      = 46 inches
Code live (snow) load is      40PSF above 7000 ft. elevation
Code dead load is             15PSF
Truss members are 2x4 hem-fir, 1968 construction, good condition.

an interior bearing wall (bearing on a slab on grade) is possible below the truss centerline since it is consistent with the proposed floor plan and would be perpendicular to the roof framing.

I've done a search function and found some similar discussion, but the result was to remove most of the roof decking, except near the bearing walls to allow installation of new engineered scissors trusses from the exterior.
msquared48 (Structural)
16 Nov 11 17:29
I would not recommend trying to modify the trusses but go to a standard sticl frame system using an interior bearing wall/beam system.  You will need additional interior footings too though.

Mike McCann
MMC Engineering

 

MiketheEngineer (Structural)
17 Nov 11 13:29
Msquared has it right - but it "might"(maybe) be possible to get away with the existing slab.  You are only getting about 715 plf at the center wall.  Assuming you have a good slab and base - it might be OK!!  Check it.

Since you will have a center wall - just re-frame it to meet your needs.  Make sure you keep the king post (at center-line) intact and secure to new framing or wall.

Done it more than once!!
KRW7 (Structural)
18 Nov 11 15:31
msquared is right..don't modify the existing trusses.  

Is the ceiling removed from wall to wall?  If so, you can do a field splice truss as long as you have about a 46 inch wide opening to get each half in the building, but then you would have to redo the existing ducts and wires.  You could probably get away with a 36 inch wide opening depending on how the field splice truss is designed.
MiketheEngineer (Structural)
18 Nov 11 17:02
This can easily be changed to stick and frame configuration - problem is that I doubt a 2x4 Hem-Fir with about 50 psf on it will check out.  If it was 2 x6 - maybe.

And you want to watch out for any lateral (side) loads at the top of the exterior walls.

Get somebody who can check this out for you.
 
woodman88 (Structural)
18 Nov 11 17:21
It is also possible to just stick frame the ceiling to support the roof. Connecting the truss to the ceiling framing before removing the truss below the new stick framing members. You will need someone to do the connections and the interior foundations designs.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

bldgmeister (Structural)
18 Nov 11 21:10
After some discussion with the client's framing contractor, I've decided to design a stick frame with ridge beam/columns/footings and rafters. The existing trusses have vertical members at the centerline from the bottom to the intersecting top chords.

What is a practical way to intall a new ridge beam without removing all of the trusses first? Does it make sense to cut out the vertical truss member temporarily to have room to install the beam under a portion of the top truss chord that could be left in place?

Or how about installing the beam in 1/2 lengths with a mid-span column and footing under a center splice?

Thanks,
Mark
bldgmeister (Structural)
18 Nov 11 21:22
Scratch that last part about a vertical truss member. The existing trusses have diagonal members only that intersect at the peak. Is there any reason I couldn't install a beam directly below the intersection of truss top chords and diagonals? And then hanger (or notch) rafters off of the beam and down to the wall top plates before cutting the trusss to create the ceiling vault? Again, a photo is attached of the existing truss, only visible from the bottom.
MiketheEngineer (Structural)
21 Nov 11 10:26
Buy an engineer who is familiar with this.  A couple of hundred bucks and all your problems are solved??!!

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