Expert: Daniel Humphrey - 10/23/2008

Question
I am now living in South Africa after many years in the US where I got to enjoy timber frame homes. I had a timber frame home built for me here. Upstairs in the bedroom I get a little rattle of the bedside tables when I walk about which is not something I was aware of in homes in the US.

Here is what the engineer specified was built (converted from metric):
Span: 5200mm = 17.06 feet  
Joists: 297 x 44 laminated beam = 11.7 x 1.73 inches
Spacing: Max 450 centers = 17.72 inches (mostly built at 410 to 420 which is 16.14 to 16.54 inches)

The floor is 21mm (0.82 inches) shutterboard on 44mm (1.73 inches) lightweight screed.

Is this correct in your opinion?

I add that the builder in his proposal spec'ed 220 x 70 joists on 400 centers which is 8.67 x 2.75 on 15.75 inch centers.

Anything I can do aside from walk lighter?

Thanks in advance, Ken  

Answer
Dear Ken,

A 17' floor span is beyond the capacity of 2x12"  joists on even 16" centers. It won't break or deflect much, but you will have some noticeable bounce and vibration.

Stiffness is calculated by multiplying the wood's strength by the width times the depth cubed:
S=s(w*d*d*d).

So you can see how the depth of the beam is much more significant than the width, in terms of stiffness. Your engineer's 44 x 297 joists are 1.5 times as stiff as the builder's 70 x 220 joists. For the builder's joists to match the stiffness of the engineer's joists, you would need to have them on 300 centers, so that you get three in the same space you would have two of the deeper joists.

Vibrational frequencies, aside from deflection, is a factor of mass and length. The longer the length, the bigger the vibrations. By adding mass at certain points along the vibrational cord, you can dampen it. In practice, this means putting on a heavier floor or distributing furniture evenly around the room. You can also add mass to the underside of the joists--an extra layer of sheetrock in the ceiling below.

You can also reduce the length of the free span by adding a saddle beam to the joists at their midpoint. Such a beam may be attached below each joist and pocketed into the wall. It doesn't really support any weight on its ends, but rather distributes the load and vibration among all the joists.

Daniel