Wuzzat?

Figure out how much deflection [floor stiffness] you want: L/240, L/360, etc.. For this application you can probably stand a springy floor.

The engineering toolbox website will help but it's a bit tricky converting lbs per sq. ft to lbs per linear inch distributed load on a joist.

Magus2727

I do have a couple main walls that run perpendicular to the trusses. I believed it is a load bearing wall as it carried from the second floor to the first with a large heavy I-Beam on the ceiling of the first floor to provide the open space on the first floor. So that is a center support that runs perpendicular along the entire length of where I would have floor. Would it be better to have the 2 x 6 "floating" above the drywall that is the ceiling? I figured that, that would remove the load from only being on the bottom chord and would reinforce the bottom chord and also put weight into the webbing. I would most likely still ad in some 2 x 4 reinforcements tying the 2 x 6 into the top chord by additional supports. But since in do have the wall in the middle that would provide a center support if I put the 2 x 6 flush with the ceiling or in other words the center wall support.

nealtw

How far is it from the outside wall to the bearing wall, if you could run joists all the way out, you would not effect the truss.

Magus2727

I am not sure... I think if I got 12' length 2 x 6 it might go edge to edge with a "joint" in the middle wall. I will have to get back up in the attic and measure my room walls. Appraisal documents show that I have 30' between front and rear exterior walls. So it may fit... I have bays and outcrops. If not including those than its 26'. So I would need to go out to the ceiling wall correct so some locations I would only need 26' of new beam and in other spots 30'. They don't make 15' or longer 2x 6 do they?

Edit: looks like they make 2x6 in lengths 8', 10', 12', 14', 16' and 20'... so should be able to span the whole room while having a "joint" in the middle.

nealtw

If you going from the exterior wall to a center bearing wall, you could use a 2x10 up to 16 ft for more than that you would want to go to 2x12s. Either way yould would want to this at 16" on center not the 24" that the trusses are set at. I'm not sure how you would get lumber like this in the attic but lumber is available more than twenty feet long.

Wuzzat?

The 16" will give the OP 1.5x the load bearing capacity of the 24".

Magus2727

What is the typical attic storage / loft storage dead weight (PSF) rated for? Dead weight would be higher than live weight right since your going to be storing more than living. I don't think I understand the PSF thing right being 180lbs and my "foot print" is close to 1 squr. foot.

Worst case I have runs that are about 13 feet from load bearing to load bearing. Looks like it would be best to run 2 x 10 every 24" for that? Calculator is telling me I could go 12' 11" close enough to the 13' for me. That's with a live load of 40psf and dead load of 10psf...

The PSF thing still leaves me clueless. I have dumbbells that apply more pressure statically then that.

Wuzzat?

You mean
distributed load
http://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html
vs.
concentrated load
http://www.engineeringtoolbox.com/beams-support-forces-d_1311.html
?

And
"Comment: In addition to a 30 psf live load requirement for attics where development of future
rooms is possible and a 20 psf live load for limited attic storage where development of future
rooms is not possible, "

I suppose a distributed load spans more than one joist and the load footprint is a significant portion of the beam span.

And if you press an ice pick tip into the floor it is probably thousands of PSI.

nealtw

There is no standard for attic storage in trusses unless they were designed for that. If you are going from bearing wall to bearing wall you are building a floor that just happens to be in the attic. If we use 20 lbs psf and your floor is 10ft x 13ft your load limit would be 13x10x20 for a total load.

Magus2727

Think the 2 x 8's every 16" will be sufficient. What about lateral anchoring? Should I just use "X" 2x4 braces at the ends of the 2x8's to maintain the 16" spacing ? I am sure there is a more "standard" way to do it?