spongy/springy loft floors

JFrazee · 03-01-2010, 09:04 AM

We had a cabin built 3 years ago. This winter as the humidity went down and wood shrunk the floors in our loft are really springy. They are supported by 6X6 white pine beams, 24" 0C with a span on 16'. The floor is a single layer of 3/4" tongue and groove pine boards. I am looking for suggestions on ways to make it more rigid. A support beam below, is not practical since it would be right in the middle of our living quarters and walk way.

Ron6519 · 03-01-2010, 07:15 PM

Lack of humidity did not make the floors springy, inadequate framing is the issue. Bring in a structural engineer to the house for an onsite inspection.
Ron

JFrazee · 03-01-2010, 10:27 PM

Thanks for the advice. That was on my list of things to do if no one gave me any other ideas to look at.

Daniel Holzman · 03-01-2010, 11:59 PM

Assuming your beams are dressed size (5.25" x 5.25"), have an allowable maximum fiber stress of 1500 psi (typical for high quality white pine), span 16 feet as you indicated, and are 24 inches OC, then for a 40 psf live load and 10 psf dead load, you would have a factor of safety against bending failure of approximately 1.0. Not good. Also, your d/L would be approximately 120, which would account for the springy floor.

If you have a lower loading, or the wood is absolutely top quality, or the beams are full size, you would have a higher safety factor and a higher d/L ratio. You say this is a cabin, not a four season house, so that may account for the small size joists. Drying of the wood is not likely the problem, as previously noted.

As to how to repair the problem, you need to consult with a local architect or structural engineer, you may have some options available like doubling the joists (sistering), or adding on to the bottom of the joists to make them more rigid. A hands on inspection by a local engineer would make the most sense.

ccarlisle · 03-02-2010, 07:07 AM

There ya go, two excellent opinions from Ron and Daniel, just for the asking...

Know what? I'd send them each a cheque for the 'free' advice, you know like for a reasonable amount of money equivalent to professional fees, and augmented by an amount for saving your sorry cabin from a potential disaster, then I'd pat yourself on the back for having enough brain function to post this thread.

But I'd seriously wonder about the process that brought you here. Sure, there's a chance you inherited this cabin from a hermit uncle, or won it in a poker game between college drinking buddies, or bought it for one-tenth of it's value two years ago, thanks to the bursting of the real estate bubble you guys created - but just as likely there's a chance you built it.

If so, you put the cart before the horse. Find out first, then ask, then do. Not do, then find out, then ask. Ass-backwards thinking IMO.

Throwing money around isn't always the answer. Ask Lindsey Vonn....

To justify yourself, you'll probably tell us that you got a real deal on 6x6s from Walmart and saved $200 by building that way.

JFrazee · 03-04-2010, 09:18 PM

Quote:

Originally Posted by Daniel Holzman

Assuming your beams are dressed size (5.25" x 5.25"), have an allowable maximum fiber stress of 1500 psi (typical for high quality white pine), span 16 feet as you indicated, and are 24 inches OC, then for a 40 psf live load and 10 psf dead load, you would have a factor of safety against bending failure of approximately 1.0. Not good. Also, your d/L would be approximately 120, which would account for the springy floor.

If you have a lower loading, or the wood is absolutely top quality, or the beams are full size, you would have a higher safety factor and a higher d/L ratio. You say this is a cabin, not a four season house, so that may account for the small size joists. Drying of the wood is not likely the problem, as previously noted.

As to how to repair the problem, you need to consult with a local architect or structural engineer, you may have some options available like doubling the joists (sistering), or adding on to the bottom of the joists to make them more rigid. A hands on inspection by a local engineer would make the most sense.

Thanks Dan, I think I will try and find a structural engineer. Originally it was to simply be a cabin but we love it here and so it has become our primary home. I will just have to make some modifications. Thanks.