Longest practical unsupported floor span?

[MushCreek]

In struggling with house designs (yes, I'm still at it) one of the factors is what shape to build the house. I'm attracted to 16' widths because everything is small, making it easy for a solo DIY'er. Narrow widths, though, make it a challenge to make sufficient room inside, so you wind up with a lot of wings, which I like, but they're aren't very efficient. Going in the other direction, it becomes easy to fit everything in a neat rectangle if you make it wide enough. At what point should you split up the support for the floors? 16' is easy to span without any heroics, but as you go up, you either add a lot of wood, or go to very deep trusses. I know you can span 30' or even more if you want to spend enough money, but at that point, or sooner, wouldn't it make more sense to have intermediate support? The reason this is an issue is that I will have a basement under my house, and would rather not have a forest of columns down there.

One of my favorite designs is 28' wide, or a little under 27' between the foundation walls. Would you run supported beams across the 28', then run floor joists perpendicular to them, or run supported beams lengthwise (about 40') with the joists perpendicular to them? Or what about steel? Can I run steel I-beams across the 28' without support? I'm trying to minimize cost, effort, and support poles, in that order. Or should I keep working the 16' wide angle? Can I easily go 18' or 20' unsupported? I can look this stuff up in span tables, but they don't tell you whether a given span is practical from the standpoint of cost or ease of build.

[Windpower]

 
Our architect is very big on floor trusses like these. 24 feet spans are easy and economical. the open space makes running the utilities a snap too (maybe snap isn't a good word to use talking about floor trusses




http://www.toolbase.org/Technology-Inventory/Whole-House-Systems/trimmable-open-web-floor-truss

We have seen a couple of his designs with huge open spans -- our garage will be a 34 X 24 with the floor trusses spanning the 24 foot making the bonus room upstairs open and no posts in the garage to run into either.

[glenn kangiser]

For permits steel is always going to require an engineer, although I do a lot of it with bigger spans than that... but it gets real expensive compared to wood.

I think it is still possible to find 20' wood that is decent or manufactured wood such as TJI's are used a lot now.

[MushCreek]

I priced cold formed steel, and it comes with the engineering required, but it is spendy. They could span 32' unsupported, but at a cost. Seems like 20' might be the cut-off point at which it's cheaper to go with mid-span support? I was quoted $4.78 per foot for steel joists that would span 28', for example, where the ones to span 24' were $3.90 per foot. Doing some quick math, I would need 744' to do the main gable in a 24' wide house, or nearly $3000 just for the joists. I'm thinking wood would be somewhat cheaper, although I haven't actually calculated it out. The cold formed joists are lighter, though, and probably result in a very flat and stable floor.

[glenn kangiser]

Yes -I  think 20 feet is about it, but then if you go to a mid span support you can go to lighter joists also, so I think the wood is hard to beat no matter what you do.

[Squirl]

2 x 12 SPF No. 2 with a spacing of 12 in has a span of 20 ft 7 in.

[Don_P]

You're in SYP territory, 2x12 will go 20' @ 16".
TJI's or open web is the way to go IMO, engineered as well, but all of these engineered products come with engineering included.

[TheWire]

I did a 40 psf floor over a 26' x 26' with 16" deep I-Joists 16" OC.  Its a solid floor.

Jerry