[tdncz]

I built a 24'x24' garage with a bonus room above.
This includes 19 24' wood I beam joists without any additional center support.
The joists have 3 1/2" flanges, are 11 7/8 deep, and are placed 16" OCI.
The bonus room is a bit too "bouncy," so I am considering two options -- or both.
1. Running dryway perpendicular to joists to create the garage "ceiling" (also considering 3/4" plywood but concerned about the weight this will cause to the ceiling/floor).
2. Two (or more) rows of blocking at 8' and 16'.

A few questions:
1. Blocking: since each "cavity" is approximately 11 7/8" deep X 12 1/2" wide (to the edge of the flanges -- not to the web), should the size of each block be this size too? Thus, there would be 5 points of contact: screw and glue block to bottom flanges (2), screw and glue to top flanges (2), and glue block to above subfloor (1). This leaves gaps on either side of the block between the block and the joist webs.
2. MUST each block be flush with the bottom of each flange (thus, each block 11 7/8" long)? I was hoping to only have to buy 2x12's -- BUT they are only 11 1/2 inches wide (so 3/8" too short).
3. Do I need 1, 2, 3, or 4 blocking spans? (I think 2 will do)
4. Can I stagger each block - to better screw and glue or MUST they be ON LINE (thus requiring me to toenail one side)?
5. Should I ALSO lay 3/4" plywood as a garage "ceiling"? (seems like this will add a LOT of weight and may cause the whole thing to sag.

THANKS IN ADVANCE!

 

[GBrackins]

Welcome to the Forum!

Your I-joists seem short for the span. I would expect something more like 14". Of course I'm not aware of the mill manufacturer of your i-joists so I can't say for certain since I do know know their physical properties.

If I was in your situation I would contact the manufacturer and seek their advise. The mill should be listed on the web of the i-joists. Common manufacturer's are Boise, ILevel, Georgia Pacific, Louisiana Pacific, etc. Each manufacturer have different physical properties for their products, and know best how to address excessive bounce. Chances are very good they will assist you for the price of a telephone call. After all they want people happy with their products.

Don't know if this helps but I do wish you well.

 

[tdncz]

Thanks, that is good advice.

 

[Daniel Holzman]

Mr. Brackins has provided good advice. I would also contact the individual who sized the I joists. Were they aware that you planned to put a bonus room above, or did they size the joists based on minimum loading? Understand that the bounciness of the joists is controlled by the span, the moment of inertia of the I joist, and the modulus of elasticity of the wood used to manufacture the I joist, none of which will be affected by adding blocking. Blocking is primarily intended to reduce twisting of lumber, serves an additional purpose of reducing the chance of buckling during installation, and has only a minor impact on the "bounce" of the joist. If the joists are undersized, you may need to add additional joists, install an intermediate support beam, or do other major surgery to cure the problem.

 

[tdncz]

I THOUGHT blocking helped (or even "solved") much of the bounciness.
I am not an engineer.... I don't even play one....
Just my reading of the forums.
Seems like at a minimum "sistering" a joist would help -- something I dont WANT to do but might have to.
Also, my webbing has no manufacturer listed.
Thanks,

 

[GBrackins]

is there any printing on the i-joists?

 

[tdncz]

Surprisingly no. However, on the bottom of the subfloor (glued and screwed to the top flange -thus visible from below in the garage), there is a Georgia Pacific "23/33 CAT PLUS STURD-I-FLOOR" tag. So it MIGHT be a GP I Joist.

 

[tdncz]

It certainly looks like the "GPI-90" - same dimensions and laminated flanges. BUT, I see the GP products have a giant "WOOD-I-BEAM" painted on the webbing.

 

[ront02769]

If you want to block the imposts, just buy some extra I-jpists and use them for the blocking. No worries. Ron

 

[hand drive]

It certainly looks like the "GPI-90" - same dimensions and laminated flanges. BUT, I see the GP products have a giant "WOOD-I-BEAM" painted on the webbing.

I would consider just ripping 3/4" plywood the full height of the joist ( or a hair shy) and nail it to the sides all the way down the span of the joist maybe staggering joists and adding the ply at every other joist or every 32". use vertical blocks ( spacer blocks) spaced 16" or 24" centered down the span of the joist both sides that are cut to be flushed to the edge of the flange. this gives you something to nail the ply to along with nailing it to the flange. for a 3 1/2" flange with 1/2" webbing figure just using 2x4's as the spacer blocks as 1 1/2" would be the target thickness.

start on one side and run the full length with your ply strips and stagger the seam for the opposite side by at least 4'. be sure and lay out your spacer blocks so they break in center of the ply depending what direction you are going with the ply. this may seem complicated and I can explain it better if you think you will do the procedure

edit- use #8 ring shank nails to nail it all up and nail about every 6 to 8" apart minimum

 

[tdncz]

I think I understand.
However, are you advising that I do not need to do this for ALL 17 fully exposed joists?
Note: I have 19 joists. But, the far left (on far left wall of garage) and the far right (on far right side wall of garage) are obviously only HALF exposed (the other halves are covered by the respective walls). Thus I have 18 actually "cavities" (that are 11 7/8" deep) spanning 24 feet. (Have I over explained this??).

So, with that said, it appears you are advising that i only have to fully cover both sides of - say - every other joist (9 in all).

On each of these 9 joists, I would glue and screw (or nail) on one side of the joist 3 8' long by 11 3/4" high (1/8" less than the 11/7/8" depth of the joists) 3/4 inch plywood spans with 2 2x4 spacers at each butt and 2 2x4 spacers at both the beginning and end of each span.

On the opposite side of each of these 9 spans, I would glue and screw 2 4' x 11 3/4" spans (at both the beginning and end of each span) and 2 regular 8' x 11 3/4" spans (4 pieces total) so that they would stagger the first side (as described above). 5 spacers total for this side.

Is that accurate?

 

[ront02769]

One more try...and I can't understand why people are making this so difficult. Do it the right way. The easy way. The way you are supposed to. Buy another I joist. Cut the
pieces to size. Nail them in. Done and done. But should everyone want to make things more difficult.....have at it. Ron

 

[hand drive]

I think I understand.
However, are you advising that I do not need to do this for ALL 17 fully exposed joists?
Note: I have 19 joists. But, the far left (on far left wall of garage) and the far right (on far right side wall of garage) are obviously only HALF exposed (the other halves are covered by the respective walls). Thus I have 18 actually "cavities" (that are 11 7/8" deep) spanning 24 feet. (Have I over explained this??).

So, with that said, it appears you are advising that i only have to fully cover both sides of - say - every other joist (9 in all).

On each of these 9 joists, I would glue and screw (or nail) on one side of the joist 3 8' long by 11 3/4" high (1/8" less than the 11/7/8" depth of the joists) 3/4 inch plywood spans with 2 2x4 spacers at each butt and 2 2x4 spacers at both the beginning and end of each span.

On the opposite side of each of these 9 spans, I would glue and screw 2 4' x 11 3/4" spans (at both the beginning and end of each span) and 2 regular 8' x 11 3/4" spans (4 pieces total) so that they would stagger the first side (as described above). 5 spacers total for this side.

Is that accurate?

yes, that is really close to what I was thinking. you will want to space the 2x4's on each side about 16" apart and center them at the butt seams as well. and that is correct to do three 8' sections at one side of joist and then break the spans down at the ends to 4' for the opposite side of joist.

another possibility would be to just add a 2x ripped to fit in the web and add it in both sides staggering the seams. like 16' one side with an 8' finish out and then do the same one the other side but running in the opposite direction so the seams are not located directly opposite of each other. or even easier but more expensive is like Ron mentions and just add on a new joist to make your joists double joists.

I mentioned going every other joist because you may not need it done to every joist. I'd recommend using 1 1/8" T&G sub floor if you go every other joist but 3/4" sub floor could suffice as well. it is hard to say what exactly would be needed but you might end up having to do every joist...

 

[SeniorSitizen]

I THOUGHT blocking helped (or even "solved") much of the bounciness.
I am not an engineer.... I don't even play one....
Just my reading of the forums.
Seems like at a minimum "sistering" a joist would help -- something I dont WANT to do but might have to.
Also, my webbing has no manufacturer listed.
Thanks,

I not engineer either but I do know cross bracing of floor joists has been around to help prevent bounce nearly as long as wood floors have and blocking isn't recommended for that purpose.

As an afterthought it becomes difficult but I suspect it could be accomplished.

 

[Daniel Holzman]

I am not clear where the belief that cross bracing stiffens joists comes from. From a mechanics of material standpoint, the joist stiffness is determined by the moment of inertia of the joist, the load on the joist, and the material properties of the joist. Clearly cross bracing does nothing to change the properties of the joist, and has no effect on the load, so the claim that cross bracing reduces bounce (increases stiffness) must be based on the theory that the cross bracing adds to the moment of inertia of the joist. Considering that the cross bracing is typically nailed in, the cross bracing acts like pinned elements, and cannot transfer moment from the adjacent joists to the "bouncy" joist. The cross bracing certainly does reduce the effective length of the joist, thereby reducing buckling potential in the horizontal direction, but this does not stiffen the joist against vertical load.

I know this sounds techy, so I suggest the curious reader refer to a few alternative sites for further discussion. For example,
http://www.johnbridge.com/vbulletin/archive/index.php/t-107602.html

https://books.google.com/books?id=S...=does cross bracing reduce deflection&f=false

http://www.eng-tips.com/viewthread.cfm?qid=348144

I recommend checking out the third reference if you only have time for one. There is a LOT of misinformation, incorrect anecdotal evidence, and faulty logic about the purpose and effectiveness of cross bracing. Similar to the misinformation about the purpose and effectiveness of collar ties.

If you carefully read the literature, and think it through, you should reach the conclusion that cross bracing provides the useful functions of reducing torsion of the joist, and improving lateral buckling performance during installation, but DOES NOT stiffen the joist. There are really only a few ways to stiffen a joist. You can sister another one, either full length or partial length. You can add additional lumber or steel to the joists to increase the moment of inertia of the joist (an example of this is a flitch beam). You can replace the joist with a stiffer element.

There is nothing wrong with cross bracing, but bear in mind that most of the time a joist is covered on top with plywood or similar sheathing, which actually does add stiffness if it is nailed to the joist, and also increases torsional stiffness. So go ahead and cross brace, but be prepared to go to step two when the stiffness of the floor does not improve.

 

[Gary in WA]

You can add blocking panels, but as said, they do nothing for strength- only distribute a point load to other joists/rotation prevention. The web stiffeners add some for increased loads/stiffness; "Unlike squash blocks and blocking panels, web stiffeners do not transfer vertical load around a TJI joist. Instead, web stiffeners reinforce the web and provide additional bearing capacity to a TJI joist." from; http://www.woodbywy.com/2013/11/11/common-field-issues-mistakes-can-avoided/

If you do use blocking panels, use sub-floor adhesive as you did under the sheathing (and in the grooves) as per manu. recommendations;http://www.woodbywy.com/literature/TB-104.pdf

Gary

 

[Mingledtrash]

Thought this might help.

 

[going_commando]

You probably need a beam in the middle with posts to carry that load. I am working on a house that has a garage that's 26x26 with a room above with I-joists, and there is a 12 or 16" tall steel beam running down the center, with no bounce. You could probably do something with 2x12s and posts, but I would suggest talking to an engineer at a lumberyard or something to find out exactly what you need to do.

 

[Aggie67]

Thought this might help.

Thickening the web of a joist with plywood won't prevent bounce. A thicker web will resist more shear, but the problem here is bending. And in order to add bending capacity and increase stiffness, you either need something along the lines of a deeper joist with larger flanges, or doubling up the i-joists. Regardless, you're far outside the realm of prescriptive construction. You're better off calling someone in to check this out properly.

 

[ront02769]

Apologies for jumping to the easiest way to block your I-joists when the real issue is that they are undersized as someone else pointed out. Easiest at this point run a center beam front to back and some jack posts and just tighten them up enough to take the bounce out. Ron