How to span 40′ without LB wall under it
The other day I was thinking about the cost of a SIP-based home where the most simple of footprints was a rectangular box 40′ deep by 80′ wide. I had planned for this ‘thought’ experiment (something to keep me entertained while at work) to be a ranch-style on a full walk-out basement and got curious as to how to create the biggest space in that basement without lollies or other kinds of columns.
The idea is to create a big space for a formal home theater. Having something like 25’x40′ would be nice, be the idea of spanning up to 40′ really got me wondering even more. In this original experiment I wanted to use trusses to hide mechanicals (HVAC conduits, plumbing, electrical, etc.). The basement would need to have at least 10′ ceilings in my little make-believe world.
Ideas?
Replies
A basement without a lot of posts sure is nice. I don't know why more people don't seem to want that.
The house I live in now is 28' wide, and I used clear span floor trusses in part of it. I think it's great.
I've done wood webbed floors up to 32' clear span. (They were 24" deep) That would give you a heck of a basement. But I don't think I would be comfortable going beyond that with wood.
You could probably span 40' with steel bar joists. But I know almost nothing about them, so I have no idea how deep they'd have to be to work. Or how hard it is to attach floor ply to them.
Morning Boss,
If the desired one-room space is about 25'x40' would it be worth deliberate spanning of the shorter dimension, and then subdividing the rest of the basement accordingly? Its one thing to have 'other' rooms in the basement wih a column or load-bearing wall, but a home theater pretty much demands unobstructed openness.
I think you hit on the solution. A 25' span would be no problem with wood. Planinnig the remainder of the basement would then alow breaking the remaining floor trusses to be set in shoter spans also.
I have one 40' span in my basement plans, but with a storage room on one end of it, and a LVL beam on the other the longest span gets reduced to 26' (the same as the rest of the basement). That allows the same oreintation of the subfloor throughout.
If you could reduce the 25' to 24' the subfloor will break on the end of the trusses, where they are changing direction. That will save on some subfloor material waste.
On another note, I have set steel floor trusses, i.e. bar joist. for a 40' span on a residential job. They were top cord bearing and hung from the welding plates in the block wall. We thrue bolted 2x4 sleepers on top of the with 1/2" carriage bolts and fender washers. I don't remember the depth of the joist, but the finished height of the basement was 8'. We had to bolt strapping to the lower cord also to hang the dw. Labor intensive, but the archy owner planned every detail to get exactly what he wanted. We did the same thing for the first, second floor and the flat roof. The house was faced concrete block all the way up to the parapet wall surrounding the roof. He called it contemporary, but I thought the finished house looked like a sewage treatment plant for the highend neighborhood it was in :)
Dave
We had to bolt strapping to the lower cord also to hang the dw. Labor intensive, but the archy owner planned every detail to get exactly what he wanted.
Nice he didn't mind the labor I guess. If you'd shot the strapping, you'd have spent considerably less time. Like most things, there's a less, and more, labor intensive way of assembly. The fault was your client's, not the bar joists.PAHS Designer/Builder- Bury it!
"If the desired one-room space is about 25'x40' would it be worth deliberate spanning of the shorter dimension, and then subdividing the rest of the basement accordingly?"
Absolutely. A 25' span with some 18" deep floor trusses would give you a stiff floor, but be deep enough to get lots of mechanicals in them.
BTW - Ignore GHR. He claims to be an engineer, but has as of yet given nothing but poor advice and smart-assed cracks. (Like the one in this thread)
Can I pay off my Visa with my Mastercard?
40' bar joists are no problem, nor are 60', other than handling. 40'ers can be as short as 20" and give 127 plf total load. A powder actuated gun with a washer head pin will shoot down the plywood easily. Just don't ever try to get 'em out.
I did a 40' span on the last house that got a 300 psf roof. Those were only 30" tall bar joists. Lot of weight resting on those outside walls.
Edit: Forgot to mention that these were considerably cheaper than traditional wood trusses, according to the crane guy. I've never priced trusses. We did 2000 sq ft of 300 psf for $8400, including decking and crossbracing. Halving the load would have almost halved the cost. Would be closer to $12k today. Still inexpensive.
PAHS Designer/Builder- Bury it!
Edited 10/29/2005 10:08 am ET by VaTom
'Bar joists'?
Boss,You seem to infer 32' is the practical limit for wood trusses.If you successfully spanned 32' with 24" trusses than why could you not span 40' with 30" trusses? I realize that handling those monsters could be an issue but anything capable of spanning that distance is going to be heavy. What are there reasons wood cannot span 40, 50, or 60 feet?
Jon Blakemore RappahannockINC.com Fredericksburg, VA
Well, spanning farther with wood webbed trusses is certainly POSSIBLE. Whether it's a good idea or not is another matter entirely. The biggest issue with long span trusses seems to be the depth. People holler if their floor systems need to be more than 12 or 14" deep. They "don't want their house to be way out of the ground". A really deep floor system means more steps into the house, and from the main level down to the basement. Going with long clear span really deep floors also concerns me because I have no real-life experience with them. I won't recommend something I have no experience with unless I have something else to back it up with. (Like recommendations from a trusted engineer)Cost is also an issue. Deeper floor trusses cost more money to build. Since people don't value wide open spaces, the extra expense isn't justified.
Q: What's the difference between light and hard?
A: You can go to sleep with a light on.
Depends on what you mean by wood. Remember, they make domed stadiums out of wood.
40' clear span. Yowzer. I was gonna say floor trusses, but if the resident truss guru says no-go..... well, ya got me. But I do know that when it comes to building..... nothing is impossible.... except maybe paying the bill.
I'm curious to see if it's possible and somewhat cost effective. Hope someone has an idea or two for you.
Hang the floor from the roof trusses ... somehow ...
I've heard this before. I might have posted on this same topic before, in general, and someone mentioned this months (years) ago. I can see the construction-load lender taking a nervous swallow on that idea.
Can the first floor walls be sheathed in plywood and act as beams from which the floor system is hung somehow?
BillW ---Are you saying to build 8' deep girders to support the floor joists?That is a wonderful idea. There might be better.
Yes, that's what I'm suggesting - the 'beams' (are these called box beams?) would be above the joists you are trying to support instead of below. My father had a house built (archy designed) with a post in a terrible location in one room - I always wondered if the architect could have turned the interior partition above the post into a giant beam and eliminated the post.
For an added dimension in this discussion (no pun intended), I was also thinking about using 4" thick SIP panels for flooring over whatever joist system. This would provide some natural thermal benefits, and if I use a thicker panel over the section of the basement where this desire space is located then I could also get some acoustical benefits as well.
I hadn't thought of using SIP panels as a flooring structural product until this year when that large TOH project in MA used them.
That is a good idea Bill. A beam does not neccessarily have to go under what is being supported, it can also go over.The connection to the floor (in tension) ends up being a little trickier, but it can be done.Box beams are fun to work with. I've built engineered components out of wood I-joists and plywood, and been amazed at the result.
"Can the first floor walls be sheathed in plywood and act as beams from which the floor system is hung somehow? "
I was thinking about this a bit over the weekend. It might be possible, but I'm not sure it's practical.
Juts adding plywood to a wall wouldn't be enough to span very far or carry much weight. In this case, you'd be dealing with a TREMENDOUS amount of weight if you were trying to carry a lot of floor load. No way could you get enough fasteners in the plywood to resist the forces involved. One or 2 top and/or bottom plates wouldn't transfer much load either.
Assuming you COULD find a way to build a "beam wall", you'd still have to find a way to make doorways in it, or it would cut your house in half.
It could also make routing mechanicals difficult.
All in all, I think it's an interesting idea, but not really practical.
An ounce of mother is worth a pound of clergy. [Spanish proverb]
I'm surprised by your comments Boss but i know this is your area of expertice. I would have thought that an 8' x 40' wall, sheathed in plywood (with glue and lots o'nails), would easily support the floor. But you make a good point about the door openings ... Maybe if the wall extended all the way to the roof it would work, even with door openings?
And Cat, there's no doubt that the 'hanging' mechanism is non-trivial ...
Edited 10/31/2005 9:52 am ET by BillW
"I would have thought that an 8' x 40' wall, sheathed in plywood (with glue and lots o'nails), would easily support the floor."
Structural stuff is all about connections. And you're simply connecting the edge of some 2X4s with plywood. There just isn't enough material there to make the numbers work.
"Maybe if the wall extended all the way to the roof it would work, even with door openings?"
That might be possible. But I still think it would be so difficult as to be impractical. And you still have the problems with connections.
History never repeats itself. Historians just repeat each other.
I get your point .... not to beat this to death, but could you build the wall like a truss, with diagonal members, to achieve a very rigid beam spanning 40'?
"..could you build the wall like a truss, with diagonal members, to achieve a very rigid beam spanning 40'?"
Yup. But you'd still have problems with doorways. And it would brobably have to be pre-fabricated as a truss - Not built on site.
If you built it to go up into the attic (over dooryays) as you mentioned, you run into limits for how tall a truss can be built in one piece.
So you're still in the "possible but not practical" scenario.
When I was in grade school I painted just like Picasso. The teacher gave me an F.
FWIW;
Brothers house needed a 36 ft beam to hold the 2nd floor and roof up, span was over a section of sloped shed/skylight windows on the first floor, no posts.
Designed him a box beam for that, 3ea 2x6s for top and bottom chords, 36 inches deep, glued/screwed 3/4 ply shear panels plus some diagonals inside at the ends, installed wiring, outlets as it went.
Brother's main comment was that the effort to build the thing with the glue and all it would have been more cost effective (counting his time) to have bought a glue-lam .
One big 40x80 floor deck without any intermediate supports underneath? It can be done, somehow, but at an expense I'll bet you won't want to bear.
Imagine the section depth of one single joist, whatever its material, OK to handle that span for a floor load. Then, imagine the equipment and labor required to set an 80-foot run of them.
If your only real requirement down in that walkout basement is the 25x40 room with no posts, and you can design for post locations in the rest of the 40x80 footprint, then put two posts in that 40-foot-wall that runs longwall to longwall, at the 1/3 span points.
Those columns can pick up relatively small steel wide flange flush beams, that will span the 25 feet across your theater, and your floor joists, only needing to span 13 feet, can be hung from the beams. #2 SPF 2x10s will do for your floor joists.
This is all presuming you've no bearing walls above your 25x40 theater area, other than above where your two intermediate steel floorbeams are.
3200sqft basement implies at least 3200sqft upstairs. For 6000+sqft you can afford an engineer to solve your problem.
(I could even do it.)
Wow, just because I thought of a way to keep my mind tasked this is your response? So much for sharing in the experience (and experiment). lol
Why can't you run a I-beam with no colums the 25' way and span I-joist 20'?
My friends house had two 40' I-beams with no columns in between but it was 24" high and 13-3/4" wide. Yours at 25' span wouyld be as high so you wouldn't have to loose to much head room in the basement.
Joe Carola
Edited 10/29/2005 11:55 am ET by Framer
I-beams and I-joists present obstacles in passing mechanicals through them. I've seen I-joists abused to the max, where the radius (they were using flex-duct) cut into the I-joist actually exceeded the manufacturers maximum recommendation and started into the stiles. Scary for anyone that bought that home!
This is why I like trusses. I did Google 'bar joists' and understand now. Very interesting! I will definitely keep them in mind!
This is why I like trusses. I did Google 'bar joists' and understand now. Very interesting! I will definitely keep them in mind!
There you go... Certainly not for every application, but for long spans, fantastic. Watch out that your walls can take the load. Concrete, no problem.
No reason not to have your construction repetoire a little more complete. Pif's concerns are valid, and easily accounted for. Here's our place, 30' span, no dark corners:PAHS Designer/Builder- Bury it!
Think of a wide covered bridge. Make the 2 long walls like old bridge trusses. They are pretty, and there are many types of them. Slap some sips on the outside and leave the truses exposed. I imagine it would add tremendously to the construction cost.
Not only would this be a tremendously expensive item, but the size is terrible for human comfort regarding proportion and light.
I'm not speaking of the basement, but of the rooms above. 32' to 36' is the absoilute maximum width you want for a house unless yuou are going to use 12' cileings. The light from windows on either side simply will not penetrate in any comfortable way. Spacial clusterings for people to group in always range to no more than 14' so two of those plus a pasageway = no more than 32' I can think of one room which is 28' x 28' with full windows on two opposite walls and it makes the 9' cieling feel too low.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!
Nuke, after what Pifin said about scale and light, are you still thinking of this 40x80 house?
Sure. I'll just make the interior halls 8' wide instead of the tiny 40" wide halls I have now.
From a 'shell' perspective in using SIPs, having 10' vertical walls that transition into vaulted ceiling elevations doesn't seem to unreasonable. I had originally planned for 10-12' walls to begin with on both floors. The elevation will help the three-dimensional volume perception.
BTW, the 40'x80' was just an example, and not a hard reality. When a SIP shell appears to cost within reasonable amounts I thought the idea of 'box' being the cheapest and quickest into-place design would be afforded. This area also includes garages, workships, etc. on the lower levels.
Nuke, what about Hambro trusses?
I've used them in commercial work, they were pretty impressive.
It's a steel truss, top chord bearing, with a concrete slab over top. The top chord beomes part of the slab.
I don't know what's the max span, but we're only thinking out loud anyway, so it couldn't hurt to check their web page.