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Unread 11-24-2020, 08:14 PM   #3
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 92,231
Welcome back, Austin.

While cross bracing is sometimes specified for those engineered joists, they're frequently designed with a full ceiling below that might help the "feel" of the floor. My first recommendation would be to contact the manufacturer with your concern and see what advice they have. You can be sure yours would not be the first such call.

And while your intended beam might certainly dampen the vibration you don't like, I think it might be overkill. Were I to experiment without contacting the manufacturer, the first thing I'd try would be to install some long 1x4s screwed into the bottoms of and perpendicular to the joists. I'd run two rows at the 1/3rd points of the joist span and see if it made any difference.

The manufacturers of those joist systems sometimes specify some cross bracing, which would be better than the struts I suggested, but not as easy and quick to try. Again, talking to the manufacturer would be a better first step.

I would not recommend the sistering recommended by Mike above. What you'd be doing, if I understand his recommendation, is double sistering engineered josits with dimension lumber, which is not gonna do anything significant for the existing joists. While it might have some dampening effect, it's a hellofa difficult and expensive method to experiment with, 'specially since you don't need the additional structure for your deflection requirements.

The metal strapping across the bottom of the joists is the same principle as the 1x4 "stuts" I recommended.

My opinion; worth price charged.

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