Bouncy and Springy Floors
Dear Tim: I live in a new town house where Trusjoist beams were used in my unheated basement. They were used within code for the span ... 16-17 ft. However in my bedroom the handles on my dresser rattle when I walk across a carpeted floor. I am a normal weight 130 lbs., and am going crazy with this problem.
Truthfully, my builder has made some half baked efforts to correct the problem, even Trusjoist people looked at it. Of course, they're not going to say it's been put up right, but also wrong for my house. Do you have a suggestion? Alice S., Delmar, NY
Dear Alice: There are tens of thousands of people out there like you who suffer from bouncy and springy floors. As crazy as this sounds, the floor may indeed be built completely to code and to the manufacturer's specifications. But that doesn't mean you need to suffer.
Floor joists, whether they be solid wood, engineered lumber like yours or even steel, can deflect or bend under a given amount of weight and a given amount of span. If you take a 2x10 solid lumber floor joist that is spanning 16 feet and place a 94-pound sack of cement in mid span, the floor joist will bend downwards a given distance.
But if you remove the sack of cement and shorten the span down to 8 feet and then place the bag of cement at mid span, the floor joist might not deflect a bit or such a small amount as to be hard to measure.
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This deflection is allowed and is not unsafe if it falls within limits. The building code and most engineering books allow a 1 in 360 deflection. This means that the floor joist is permitted to deflect 1 unit of measure for every 360 units of the same measure of the span. But keep in mind, this is a minimum standard!
The sad thing is the floor could have been designed to meet a stronger standard - the 1 in 480 deflection standard. This simply means the floor joists needed to be the next size up or even taller for the given span you have. You can sometimes achieve the tougher standard by decreasing the spacing between floor joists. This means instead of placing them 16 inches on center you put them 12 inches on center.
The company that made your floor joists have published booklets for years showing both standards to builders. Your builder chose to use the minimum standard.
The floor can be fixed by placing a new beam at mid span down in the basement. This will make the floor as solid as if it were concrete poured over solid rock.