I’m considering building a deep, two bay garage with a suspended floor. About 20-25#/sqft dead load (~3″ concrete) and 20-25#/sqft live load. Bearing walls to be 36′ apart. My first thought is that bar joists would probably be the most efficient and cost effective means.
Anyone worked with spans and loads like that?
BTW, years ago I helped frame a home on a steep hillside in Oakland, CA. The garage was on the top level of the house, it’s floor framed with 2X12s. In fact, the short driveway was also supported by a 2X12 system, anchored to concrete piers next to the curb. The entire house, three stories framed with 2X4 and sheathed with 5/8″ CDX, was supported by four giant piers, two pairs tied together underground.
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Good friend of mine built what basically amounts to a bank barn. Grade in front and rear differs in elevation by one story. 36x72
Basement (open to front) is poured walls and slab with second floor being bar joists with slab poured over. He says it is engineered to support as many cars or horses as you'd like to reasonably put in there.
Anyway, third story (under roof) was done with wood web joist I'd say at least 24" in depth.
That almost covers what ever you'd want to do. It's a beautiful building. Basement storage and landscape equipment, sencond floor wood shop, third floor bath, pool hall, office and studio.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
Sounds like what I was imagining, except that I intend to support the bar joists with wood framed exterior walls, either 2X6 or 2X8.
I don't see why not. I think there have been similiar discussions here in the past.
His third story floor is the wood web joists on 2x6 exterior walls.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
Have you priced bar vs wood? I'm curoius what the difference would be and also are you planning on a wodd flooring system on top of the bar joists?
I didn't get that from your op.
"When the spirits are low, when the day appears dark, when work becomes monotonous, when hope hardly seems worth having, just mount a bicycle and go out for a spin down the road, without thought on anything but the ride you are taking." — Sherlock Holmes, 1896
I think he's talking bar joists, pan deck, and 3" conc
I think he's talking bar joists, pan deck, and 3" conc
Yes, that's the system I've been considering. The design idea is to make each bay deep enough for two standard sized cars. The limitations imposed by the site make it better to suspend the garage floor, similar to the Oakland house I described, preferably as a clear span.
It's interesting how something observed while working on someone else's project can become a solution to a problem encountered later. Symbiosis or karma or grace.
Grace is all over the place. Even when I'm not paying attention to matters of the spirit, grace is always looking out for me, so it seems. Expressing gratitude for that brings me closer to its source.
The philosopher might say that everything which comes to me is grace as karma. Therefore I should accept it all gratefully, in order to learn what it has to teach me, with ease.
Depending on what your engineer and architiect, as well as the building department say, a post tensioned concrete slab me be cheaper too. You will have to factor in what degree of fire proofing will be required on the steel structure.
As a note, bar joists and pan deck are not normally refered to as a suspended slab. What most people refer to as a suspended slab is a post tensioned concrete slab, with no underlying steel structure
Thanks for the suggestion and the correct terminology. I've considered pre-cast products but never priced them accurately. They may work for my driveway, which will probably be suspended as well.
"You will have to factor in what degree of fire proofing will be required on the steel structure."
I think there is a rec room underneath. The fire separation would be created by the rated ceiling drywall.
Too bad there wasn't a way to provide insulation and protect the joists while leaving them exposed. Bar joists with nice wood top and bottom chords look great.
Over the years I've had several people ask me about using wood webbed floor trusses for garage floors. But the engineering gets complicated and the cost gets pretty high. I've never had anyone go through with it.
Spancrete seems to be the most popular solution around here. But I don't know anything about the cost or if spancrete can span 36'.
Cutting the span in half with a beam would make it a lot easier to do, if that's an option.
Thanks Ron,
I've been hoping to hear from you on this question. I can put in a beam at 12' from one end, cutting the span down to 24', so I'll probably do that.
Peter
If you use a PT slab, be very careful of rotomammering. The cables can be easily pierced by the drill.
Bar joists and pan decking will probably need fireproofing
And I'm sure that if it is a garage floor, the engineer will design it for a much bigger load
All good points, thanks.
I'm going to have to approach the load question carefully with the engineer, so as not to end up busting the budget. Fortunately there are suppliers/fabricators who have their own engineers who are willing to help with calculations.
I'll probably end up with a couple of beams and some columns, to divide the span into thirds. Pretty standard approach.
One of the nicer things about bar joists is that they carry their loads on the top chord. That makes it possible to hide a supporting beam in the ceiling.
By placing two columns in the middle of the big walk-out basement rec room, it would be possible to use smaller beams bearing on 12' centers, without any framing conflict on the floors above. That would divide the floor into 6 load sections, each 12'X12'. That's pretty standard dimensions for bar joists and pan decking so I don't expect to have any trouble, getting an engineer to sign off on it.
The structural system you are describing sounds a lot like VaTom used on the roof of his house. As I recall it had a foot or so of soil on it, so the loading would probably be comparable - and it was remarkably cheap. He might be the one to ask.
You're right. Tom's roof is one example which made me think about doing a using clear span to avoid columns in middle of the basement.
As I recall, he was allowed to do his own engineering, by his rural VA town's building department. That was possible in my rural NY State township when I first moved there but that's a thing of the past.
Since beginning this thread it's become clear to me that the simpler and more economical method would be to make a couple of design changes in the room lay-outs below the garage to allow loads to be carried through interior non-bearing walls, down to four columns in the basement, two floors below.
Two of those columns could be next to the basement stairs, out of the way, and the other two would be in the middle of a large open space, intended as a recreation room. Typical way to carry floor loads, just one that I was hoping to avoid if possible. Although the long clear span can be done, the economics make the decision simple.
The street hockey players will just have to deal with a couple of obstacles. I can probably find some pads for those columns but passing the puck will require more precision. If they don't like that we can put up some lights on the BB court and they play outside.
You mentioned loading - The way I was taught to figure it was 50 PSF live load PLUS the added dead load for the concrete slab. Then the trusses had to be designed for 150% of the max wheel load anywhere on the floor. Figuring out just exactly what the "max wheel load" was supposed to be was kind of subjective. Would that be for your car, SUV, or an overloaded pickup truck?The "anywhere" load is what made the designs so difficult and made the trusses expensive. It's tough to design for, and forces the trusses to be pretty stout.
Thanks for those calculations Ron,
The actual dead load of 3" of concrete is about 25 PSF, max. A live load of 50 PSF is standard so I wouldn't debate that with the engineers.
But I don't understand why they want to use such high wheel loads when reinforced concrete will certainly spread loads out dramatically, even carry most of it to the bearing points, with little help from the trusses.
I hope that my architect has a friendly engineer to help with this.
Anyway, you've helped me decide to forget long spans. This garage floor will have bearing points on 12' squares. All bar joists will be 12' long, max., carried by exterior bearing walls or I-beams which will be supported on 12' centers.
The wheel load I mentioned is just what our engineers tell us to use. I can't quote you any code that specifies that, So it could just be their standard practice. BTW - I do think that concrete will spread load out SOME. But 3" of concrete won't spread it out much. Maybe double the thickness of the concrete or so would be my guess.
There's no real need to do housework -- after four years it doesn't get any worse.
You could look into Flex Core concreate hollow core panels. I Spanned 28' with 10" deep panels & 3 in cap of poured concreate. For wider spans the panels are deeper.
Edited 4/21/2009 8:10 pm ET by jimcco
Thanks. I suspect that cost would prohibit that option but I will check again before finalizing the plans.
In Iowa 1992 my 28 x 30 cost $2800 for the panels.
If you go this route be sure you use a ledge on the inside of the stem wall. Do not set them on top of the foundation then frame on top; the panels have hollow ends.
Only $2800 for that span, that large an area? Wow! Now you've got my attention!
Did that include delivery? How did you set them? Is there any way to fasten to them? Or to fasten them to each other? How did you form for the 3" slab on top?
Do you remember if there were any special circumstance, like proximity to the manufacturer, which kept the price down?
Ya, the price was delivered but not set; in Iowa manufacturer was not local but not not sure how far away. Remember that was 1992 cost.
You'll need a small crane (hydraulic wrecker size) to set them on the wall.
Panels were varied width to get corect total width. Largest was 4' wide
They get set on a strip of hard masonite and can be slid to tight fit with one guy on each end with a bar.
We used light fibre type concreat for the deck.
Be aware you cannot drill or cut into the panels. We set conduit & light fixtures with screws into the space between panels.
Just a guess your 36' span would probably need 14" or more thickness.
I can swap directions 90 degrees, add a 24' I beam and one supporting column, to make the span 24'. I suppose that work better and be more economical.
I'm even more interested in using those hollow guys for my suspended driveway. The span there is likely to be 28'-30'X24'. The garage floor can be divided up and supported without incurring any serious design penalities but the driveway would require a lot more foundation work without a full span system.
Just thinking out loud now...The problem with using these for the driveway is that it will need a curb/barrier on each side. I wonder if the manufacturer could cast that sort of thing into their products. Seems like something they'd be asked to do, fairly often.
RE: Curb I would guess that kind of modification would not be easy as I think they extrude the panels.
You might add thicker depth of narrow width (16") along side as a curb.
Try this web site http://www.strescore.com for some info. or search on "hollow core concrete"
Edited 4/22/2009 1:58 pm ET by jimcco