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Floor bounce

Started by bannerd, March 11, 2021, 09:23:58 PM

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bannerd

Howdy

Building a timber frame home with a full basement.  There is a slight bounce to the floor.  The two main beams are 8x8 that go down the center of the home.  The floor joists are 4x6 and span 10ft.  They are 24" on center and some are closer due to a 30" on center. Wood species is white pine.  With the snow on the sub floor I didn't notice it.  There was a few thousand pounds on the floor because of a tarp and water accumulation. With the hot weather the snow is gone and I'm starting to notice it.  I'm wondering if 4x6 were not the right piece?  When my kids jump on the sub floor I can see the 4x6 literally move.  The 8x8 doesn't seem to have play.

Don P

I'm getting a pass at 24" centers 10' span, around 1/4" deflection at full load of 50psf total, 1000 lbs per joist.

If they are joists supported on a girder that is also a spanning member vibration does go up.
If you want to check that, somewhere I have a calculator based on equations from Dr Woeste where he investigated that effect on floor vibration frequency.
Here it is;
Floor Vibration Frequency Calculator

bannerd

I think we found the issue, I have a scarf joint about 20 ft away.  These girders or support beams are 50ft long and have two scarfs in them.  The scarf is about 5 feet from the vertical 8x8 beam that is holding it up.  I always try to put a support beam under scarfs(Or a Knee) but this one was close I didn't think it would matter.  Since this area is where a pantry is going .. I plan on building some 2x6 walls and hopefully that will fix it as they will sit on the concrete slap-2-earth.

When the kids jump on the floor I can actually see that scarf moving and the vibration goes back to them.  Once I put a car jack under there it took the bounce right out.  When I jump on it there is some bounce but no where near as bad.  Thanks Don, That calculator is a nice piece.

This house is turning out to be a lot of work for a single person @ 30x50.  At least we're making progress.

Don P

Cool, glad you located the problem and it'll go away.
Just informational for those following.
It says something if we think a scarf needs to be supported to perform properly in bending. I also agree with that. A scarf in span is a particularly bad idea in a floor system, an engineer told me "never". Although we would like to think otherwise they do not have the same moment capacity or behavior as a continuous beam. The point of maximum moment is more variable in a floor, the waterbed moves around, kids jump wherever they want and we have a joint of unknown capacity. But then again I've seen a scarf in a roof drawn at a point of high bending moment without comment. Building is an exercise in humility :).




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