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Grommet 03-29-2007 01:41 AM

% total roof load on interior bearing wall?

Looking to improve my understanding of the forces involved in choosing a doubled/tripled LVL beam to replace a 14 ft segment of interior load bearing wall. I will be asking the LVL supplier to run figures, or may need to consult a structural engineer, but for planning I would like to understnd the process better.

House is single story, frame on slab, with 2 x 4 wood trusses spanning 30 ft with 1.5 ft overhangs. Roof is single layer of composition shingles over 1/2" ply. The 2 x 4 framed interior wall is 12 ft from one exterior wall and 17 ft from the other exterior wall, and perpendicular to the trusses.

Live Load is 20 psf, Dead Load is 15 psf, No snow load, Tributary Area for each truss is 63 sq ft. Reaction for each truss should be 2,205 lbs.
Can anyone guide me to the correct way to figure the proportion of this weight that is carried by the interior wall? In an earlier posting Phil related that in a simplified model using beams rather than trusses crossing the interior wall in question, the answer would be 5/8 of the total roof load (that would be 689 lbs per lineal ft for my case). He said trusses however act different than beams. Can anyone take us a little further?

Header span tables I have checked either don't specify exterior vs interior, or assume floor loads, or don't give spans appropriate to LVLs. Uniform roof load tables might be the answer, but I need to be able to know the actual lbs per ft., how deflection affects this, etc.



gary 03-29-2007 07:37 AM

A truss is simply a bridge, the load is being transfered to your outside walls. Each piece of a truss is placed to maximize this bridging. A beam on the other hand has to in affect surport it self so without some kind surport under it will sag and can't even begin to match a truss for clear spanning. A truss can be thought of as the rafters, collar ties and beam all rolled into one. To answer your other question on how much force or load bearing try googling under truss manufacturing or specs. Ideally you would want most of the load bearing on the outside walls, the engineered wood being manufactured is lighter and stronger than conventional stick framing.

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