# Thread: Floor Strength Question

1. Member
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## Floor Strength Question

Greetings all

The time has come to go shopping for a new bed, and SWMBO and I were out looking at the possibilities. We are both interested in a water bed of a particluar design. I do not really feel like typing out history of bed types, reasoning behind it, etc, since I want to keep this short and to the point. Translated, that means I don't really want to hear about different beds I should be considering. I want to know about how to calculate loads and load bearing capacity before we make a decision.

What I need to know is how to calculate the ability of our floor to bear the load of this particular bed. The total weight is going to be just under 1000 lbs. The floor is in an old stone farmhouse, and consists of old pine boards approximately 7/8" thick. The joists are set on 24" centers, and are mixed fir and chestnut, approximately 2 1/2" x 7 3/4". The main beam is red oak, 7 1/2" x 10". The joists are mortised into the main beam at the center of the house, and set on a stone sill on the outer wall.

If I had to make a guess I would say the floor would support the weight, but I would feel a little better with some objective figures. Having written that, if anyone here has the formula for figuring this stuff out, I would appreciate hearing about how to do it.

Thanks.

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What is the span of your 24" OC joists Bill? How far between the beams/girders that support the joists?

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Hi Mark

I forgot that bit! The span is just under 12 feet.

Bill

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Bill - There are some online span calculators that I've used in the past. Google span calculator and you should see them. You'll need to know what the load requirment is for the bed.

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Thanks Mike. I checked a couple of the span calculators out, but it is all Greek to me (ha ha). I am not sure what values to enter for dead load, live load, etc. I can figure out how much the bed weighs per square foot, and even with me and SWMBO in it, but when does dead turn to live, and what exactly does all that mean?

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bill, if it where me i`d shore up under where you intend to put the bed.....only cause it`s better safe than sorry......a pound of prevention ect....
i`d most likely do something simple like a couple of 2x10`s crossways to your joists solidly braced to the ground just for cheap insurance....tod

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It would work Tod, but unfortunately, it would also interfere with the living room floor plan.

In our last house, we had a full flow California hippy style big bag-o-water that must have weighed at least 2000 lbs. It was an older home too, with full 2" wide by 7" deep joists. They were set a little closer, at 16", but not all of them - i.e. the spacing was a little irregular. We had no problems with that bed or that floor for the almost 14 years it sat there.

There is a partition wall under what would be close to the foot of the bed, but it is non load bearing. Too bad, as all the studs are a full 2" x 3" red oak.

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To me it looks like you are a little maxed out there on your spans Bill. With people in/on the bed I get about 50 lb psf and my numbers for your floor come out to somewhere around 35 lb psf or so. Let me state though that I do not have the prized stamp that engineers have paid good money for and lots of time in school to get. And, I'm kind of a better safe than sorry guy like Tod mentioned as well. Can you call your local building department and get some info from them? This is the kind of thing that they deal with on a daily basis.

May I suggest that you don't ask the waterbed selling folks. They seem to have a tendancy to overstate the "light as a feather" issue of their beds.

hth
Last edited by Mark Rios; 12-26-2006 at 08:24 PM.

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ohhhh........2nd floor........over a living space ........i was thinking single floor over a basement/crawlspace.......soooo, 2x8`s 24" oc with a 12` span, single layer pine decking.....how much of the beam/joists was removed for the mortise? was it haunched? ......i`m only guessin` here bill but my guess is that the bed loaded would push the limits of your construction? only a guess......tod

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Thanks guys. I have been doing some reading on loads, and how to calculate them. According to what I have read so far, the load needs to be calculated as distributed across the entire floor, not just the square footage it occupies. That makes things a little better, but I need to do some more reading.

I doubt I could get any useful info out of a building person besides "no, don't do it". My experience with people like that is that they err way on the side of caution as a CYA measure. Besides, I am dealing with species with which people do not build anymore, as well as dimensions not used.

Tod, the tenons are haunched, and set in about 3" into the main beam. The tenon on each is approximately 4" wide. Each one is pegged with a trunnel. The other end of the joists seem to be sunk into the masonry about 3" -4" or thereabouts. The walls at this level are 18" thick!

Mark, we went through the waterbed salesperson routine years ago. Some girl very glibly told us that the load was less than a refridgerator. I asked her how many people put 10 fridges in their bedroom, a question that clearly startled her.

Edit in: I did some more calculating. Our last waterbed weighed in with 1496 lbs of water. This one will have less than half that. The room size is essentially the same, although the joists are spaced a little further apart. I don't watch that much TV in the living room anyway, so I only need to worry if the floor gives way in the middle of the night, right?
Last edited by Bill Grumbine; 12-26-2006 at 09:19 PM.

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