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RE: bouncy existing floor joists

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>I have A problem A custom home built in 1980's
>with 2x10 floor joist  at 12" on center it spans
>18 feet long the floor has 1 1/2" light weight concrete
>the owner try to remodel and want to stiffen the
>floor system because is very bouncy
>and vibrating ....  
AISC has a design guide for how to address floor vibrations during design and as a remediation, but it is specific to steel framing. Because it is now recognized that wood and concrete framed floors also suffer vibrations, the Applied Technocolgy Council created their Design Guide 1 Minimizing Floor Vibrations, which provides parallel recommendations for all three types of framing. See it here:
That link starts with "http://" and ends with "45103268689.aa". You may have to cut and paste it into the browser bar to get it to work if it wraps the screen. Or, go to , click online store, click floor vibrations (under ATC Reports and Other Products", and click ATC-DG1.
The principles are similar -- the right combination of mass and damping are the important characteristics. Extending partitions (which act as dampers) is often the easiest solution in a steel building, though I doubt that will be an option in your home. Stiffness does not always help much in resolving floor vibrations.
I've also seen solutions that utilize passive damping elements. Tom Murray came up with a tuned container housing bladders filled with varying amounts of liquid. These function a lot like the human body -- we feel the motion of vibrations because the organs inside of us are damping them out of the structure. An interesting story about Tom's passive device ... With engineering economic skill, he tested his first damping device, which was a container loaded with water-filled condoms. He had to switch from condoms to a more expensive bladder because of the 50% leakage rate!!!!!