Best floor joist or truss construction to span 26 ft
I am planning a home addition that will be 26′ x 26′, two floors above a walk out basement foundation. 26 ft can be clear spanned with I-Joists or wood floor trusses. I could also use a steel beam in the basement ceiling plus load bearing walls on the first floor to cut the 1st/2nd floor joist spans in half and use SYP 2X12s.
To achieve stiffness and strength equivalent to SYP 2x12s with 13′ span, I expect that wood floor trusses with 26′ span might have to be 18-20 inches deep.
Are there any tradeoffs related to impact sound transmission between floors if load bearing walls are eliminated ? If floor loads are carried only by perimeter walls, I wonder if there are less paths for impact sound compared to a system with additional interior load bearing beams, walls and columns ?
Thanks,
Mike
Replies
There's no reason to not have a steel beam instead of a load bearing wall for the upper floor as well. I'd say that you'll get more noise with the beams built to minimum standards since load bearing walls will dampen a lot of harmonics/vibrations, but if you go beyond minimum construction and include decent floor insulation, and strapping or resilant channels, and flooring with a solid subfloor (perhaps even thin concrete and radiant heat!) you can built either one to be better than "normal" variations of the other.
The 26' span can be open using TJI 360 or 560 floor joists on 16" OC... they are 16" deep.
Also keep in mind that as you go to longer spans you'll want a stiffer floor or it will feel "bouncy". L/360 stiffness would be horrible over 26'. At least one of the I joist makers (boise Cascade?) had an online caculator that gives you a number of relative felt stifness - can't say how close it comes to real life but it's an important concept if spongy floors bothers you.
I feel like I've beat this horse to death. But with any floor question like this one I refer people to the thread on floor vibration:
http://forums.finehomebuilding.com/breaktime/general-discussion/floor-vibration