Replies

1. 
   Mongo | Nov 22, 2002 11:37pm | #1

   That's a trait of TJIs...though they're strong, they do on occasion display what you have described. It's commonly referred to as the "trampoline effect." Around here, at least.

   If the undersides of the TJIs are accessable from the basement, you can run 1-by strapping 16" oc, perpendicular to the flanges. A dab of PL at each fastening point will help.

   Some take it a step further and skin the bottom of the TJIs with sheets of half-inch ply. Use PL there as well. That'll really stiffen the floor up, though you lose easy access to the joist bays.

   I'd run the strapping first and see if that rectifies things for you.

2. 
   BossHog | Nov 22, 2002 11:50pm | #2

   How deep are the I-joists on this job?

   It most likely has nothing to do with the fact that they're Trusjoist brand - It's most likely the length/depth ratio.

   Here's a thread regarding floor vibration that you might find interesting:

   Floor Vibration

   Why do psychics have to ask you for your name?

1. 
   Junkman001 | Nov 23, 2002 02:12am | #3

   Thanks for the input. The joists are 12"

1. 
   BossHog | Nov 23, 2002 03:50am | #4

   12" deep ??? Are you sure the beam is centered? A 17' 6" span (half of 35') is well within what I would consider acceptable, and I'm conservative as heck on floors.

   Have any info on the center beam? Type of beam, and support spacing......Make it idiot-proof and someone will make a better idiot.

1. 
   Piffin | Nov 23, 2002 06:09am | #5

   It may be that some of this vibration is coming from the fact that it overlays a centerbeam. Walk across on one side of the beam, deflecting down there and you can deflect up on the other side of the beam. I definitely agree that adding strapping to the bottoms will stiffen the sandwhich effect but I also wonder if the subfloor was glued down or just nailed..

   Excellence is its own reward!

   "The first rule is to keep an untroubled spirit. The second is to look things in the face and know them for what they are." --Marcus Aurelius

3. 
   broadmeadow | Nov 24, 2002 11:40pm | #6

   These joists have very uniform performance characteristics which allows them to be used in engineered floor systems with greater spans than dimensional lumber. The problem arises from the 'allowable deflection' - this is always expressed in terms of a ratio of unsupported length - usually 1/360. Increase the span - allowable deformation increases. Floor span tables are all based on a stated deformation ratio.

   We framed a custom house with these things - the big span caused the floor to act much like the canvas floor in a boxing ring - lots of movement. You don't have the same span but the idea is the same. A clear span of fifteen feet gives an allowable deflection of 1/2" under uniform loading at 1/360 (usually expressed in terms of a load/ unit area)- but doing aerobics in the center of that floor may cause bending beyond the calculated deflection. My own state of the art deflection test (and likely yours too) is to stand in the middle of the floor and jump.

   Myself, I would tend to design floors on a total deflection figure rather than a ratio or reduce to 1/480 for longer spans. Solutions? My own idea would be to sister dimensional lumber to the side of the I-joist with construction adhesive to help stiffen the floor but I'm not sure that an engineer would like anyone nailing the snot out that tensioned bottom chord.

   I don't like to use them due to off-gassing and the fact that these wonder products all rely on adhesive strength - show me a product that doesn't decay with time.

   Lots of deflection is great though when you are practising wrestling manouvres with the kids or if you are prone to falling.

1. 
   Mongo | Nov 25, 2002 08:22am | #7

   Excellent observations.

   I spec out TJI floors at either 480 or 720. 480 is usually good enough for bedrooms, 720 for the party rooms on the first floor.