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RW's picture

Without a span table, my suspicion is that the I joists in a home are right at the limit of acceptable or they would not have passed. That notwithstanding, when the homeowner walks through the living room, the TV moves, the china rattles, etc. I can go in the basement and look up as he walks and see the whole thing flex. With all the wiring and plumbing runs in there, adding joists probably isn't an option. It seems I read of a way of stiffening things up by ripping OSB, glue & screw. On the right track here? It seems plausable in theory. I'd like a reference if anyone has anything they've read or done. Muchas gracias

"The child is grown / The dream is gone / And I have become / Comfortably numb "      lyrics by Roger Waters

Real trucks dont have sparkplugs

(post #93788, reply #19 of 36)

That is a good point that wasn't addressed. How and where were any cut-outs through the joists performed?! I had a moron of a plumber cut right through the top cord of a TJI when attempting to install the drain for a bath tub. Another time I had a know-it-all "electrician" almost cut right through the entire flange from top cord to bottom cord. Whatever he was doing with a Sawz-all I still haven't figured out! The manufacturers detail specific places, shapes and sizes for cutting through TJI's. If you violate those specs, you void any and all warranty of those joists and come what may if they fail. If they are notched through improperly the only thing you can do is to either 1) add another joist next to them or 2) thoroughly sister them with plywood as I mentioned earlier. Adding a beam underneath still won't do the trick. Improperly notching through TJI's compromises their structural design integrity in the entire floor system.

(post #93788, reply #23 of 36)

My theory is that all you need to do is run plywood across and UNDER the trus-joist.  The deflection is more across the joist, proof is the rattleling furniture is on the wall parallel to the joists not the wall where the joist rest.

So just install the first sheet of plywood (the thicker, the better) in the center of the room, across the joist, jack it up to hold it-prestress the floor and then nail or screw.  Stagger the rest of the plywood and make cuts to avoid the mechanical projecting lower than the joist.

(post #93788, reply #24 of 36)

I don't have a reference, but your idea of stiffening up the joists by adding plywood or OSB (and thicker is better - do no less than 3/4-inch) has merit.  If you do, you should do the middle third of each joist - no need to really do the whole joist.  What you are doing is adding to the joist's section modulus and moment of inertia, which directly affects bending stresses and deflection.  The size and spacing of the screws also is important as these transfer stresses to your new web.  (In other words, screw and glue the hell out of it.)  IMHO, I would try this first on a few of the middle joists and have someone walk on that stiffened portion and see if it is satisfactory.  Adding straps as some have suggested will help some, but really, that is also adding weight, and not adding anything to your joist's structural properties.  (Strapping limits lateral movement of the bottom chord of the joists.)

Some of the posts have stated that your floor system should be good for the span/deflection characteristics of your joists.  This may be true for STATIC load cases.  However, what you are describing is DYNAMIC floor response, and that is something else entirely.  It depends mostly on span, mass and stiffness.  But there are some technical variables which may cause a resonance or amplification which you are probably experiencing.  What you want to do is stiffen your floor system.  The absolute best way is to add a middle support and shorten the span.  Short of that, what you suggest is also viable.  However, the web you add must be continuous, and it sounds like there are a lot of conduits and such passing through the joists (although, they should not be inthe middle third of your joists).

Edited 10/30/2003 12:23:13 PM ET by KAORISDAD

(post #93788, reply #25 of 36)

The ide behind diaphramming the bottom of the joist uses the concept of increasing the tensile resistance to stretching rather than adding mass.

Assuming that Mylar did not stretch, you would see a larger increase in stiffness from bonding a taut piece of Mylar to the bottom of the joists than after adding ¾" plywood to the Mylar diaphram (theoretical) that you already installed.

In other words, any plywood thicker than ½" or 7/16";"is aready on the downward side of the cost/benefit equation.

A better way to increase stiffness beyond ½" ply is to double 3/8"" and offset all joints in the 2 layers, keeping in mind that you need perimeter blocking for both layers and those are the seams you most want to offset.

Don't screw, screws have too little shear strength. Use 8d or 10d nails, 4"OC perimeter and 8"OC field, hand driven to pull the ply tight to the joist.

It does depend on diaphramming the entire area rather than just the middle 2/3 or 3/4.

Adding mass can be done in just the middle but you're better off adding ¾" ply to the side of the joist if you are just adding mass.

Unless there is a bow in the floor, you won't need to "jack it up". If the floor is bowed!?!?! don't jack it past straight if diaphramming.


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(post #93788, reply #26 of 36)

I believe the idea was adding a plywood web to the side of the I-joists to stiffen them.

(post #93788, reply #34 of 36)

Lots of good ideas here guys,  if you load the middle of the i joists with plywood, apply diagonal bracing and strap it, you will be golden.  The point is to add beef and create on "box" or beam which will be stronger.  Personally I would start with the strapping and diagonal blocks or straps as described earlier.  Good Luck.