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Old 12-08-2010, 09:45 PM   #16
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Sistering Bouncy Floor Joists


Yes there is both vertical shear and horizontal shear in a beam loaded with uniform vertical load such as live load on flooring. However, in virtually every typical circumstance in residential construction, the limiting design factor is the bending moment induced in the beam by the load, NOT the vertical shear.

Vertical shear is normally greatest at the supports, so if there were ever to be a need to increase vertical shear capacity, you would generally need to do it at the supports. However, as I noted, it is EXTREMELY rare that vertical shear is a limiting factor in the performance of joists and wooden beams in houses.

The reason that sistering a joist with either plywood or solid wood reduces the "bouncy" feeling of the floor is that adding material to the joist increases the moment of inertia of the joist, which reduces the deflection under a given load. You can also reduce deflection (improve stiffness) by shortening the span of a joist, i.e. adding an intermediate transverse beam, replacing the joist with a material of higher modulus of elasticity (substitute steel beam for wooden beam), or increase the moment of inertia by adding a steel plate or strip to the bottom of the joist (the straps referred to is one example of how to do this).

If you add a strap to the bottom of the joist, it is essential that you get the strap to work together (composite action) with the joist. To do this, you need to connect the two members together using nails, glue, bolts or similar fasteners, in order to overcome the horizontal shear that develops between the two pieces. Calculation of the horizontal shear is somewhat complex, and I am not going to present the formula and theory on a DIY site. That is why structural engineers go to school.

If you use an adequate number of fasteners, the two elements work together, and the composite moment of inertia is greater than the original moment of inertia, so you get a stiffer joist. If you use an insufficient number of fasteners, the composite beam is likely to break at the connection point.

PLYWOOD: Plywood is a complex material, which as was pointed out has every other ply oriented 90 degrees to the previous ply. The properties of plywood are difficult to compute mathematically, however the Forest Products Laboratory in Madison, WI has published formulas and guidelines for the computation of the strength of plywood when loaded parallel to the face plies, and perpendicular to the face plies. The publication Plywood Design Specification January 1997, published by the Engineered Wood Association (APA), discusses at length the appropriate strength and stiffness parameters for plywood. This is a lengthy document, not light reading, however if you want to know how a plywood-lumber composite beam is going to perform, this is the document you need to consult. It is inaccurate to state that plywood is equivalent to a solid wood beam of half the width of the plywood, in fact computing the equivalent size solid lumber beam to a 3/4 inch thick piece of plywood is not simple, and depends on the type of loading, the orientation of the load, the grade of plywood, and the method of attachment of the plywood to the solid lumber.

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Old 12-08-2010, 10:17 PM   #17
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Sistering Bouncy Floor Joists


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Originally Posted by Aggie67 View Post
Both planes.
Here's a visual of the shear that Daniel is describing.
So the plywood is essentially solid wood. Correct? Or did I fumble? j Never mind. Dan's reply came up after I posted this, so I'll ask a different question. In general, if plywood is used as a stiffening member with the face grain parallel to the floor joist, does it add approx the same stiffening as a same-thickness piece of solid wood, given that each are of the same length and located at the same place along the floor joist?

Last edited by jklingel; 12-08-2010 at 10:24 PM.
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Old 10-05-2012, 07:51 AM   #18
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Sistering Bouncy Floor Joists


I hate when I am searching out info on a project, knowing what each alternative is and trying to choose the cheapest, easiest, and most effective, how I never ever really get an answer.
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Old 10-05-2012, 08:21 AM   #19
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Sistering Bouncy Floor Joists


I'm sure by now, the OP has done what he thought was best and, hopefully, it has worked. Obviously in this case we're looking at stiffness rather than strength. I think, as a carpenter, it goes against my grain to install a full length 2x8 without resting it on the plates at each end. It makes no difference in this case, but i guess I'm being anal about the way it looks. The point I really meant to get across was that Plywood, even though you have to rip it (not really a big deal is it?) can be handled in 8' lengths, doesn't risk splitting as it dries out, can be cut and placed easier if there are wires/pipes in the way, and is lighter and more maneuverable in a tight space. And yes, because it's 8' lengths, you'd probably need to double up the thickness, although if the joists are only 12', it's possible applying plywood to the middle 8' would be enough, and maybe you could get away with just one layer. Another way is to strap the underside perpendicular to the joists, but I doubt it would be as effective as sistering the joists.
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Old 10-05-2012, 08:31 AM   #20
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So here is MY deal. Home was built in 1970. Upstairs floor joists 14' span, he used 2x8s 16" on center. I have the downstairs ceiling out right now and I am going to fix the bounce this weekend. So I research. At first I was going to sister every other joist with 2x8 and be done with it. Now I am considering the plywood and glue option as possibly the best option. I am going to go purchase the materials later this afternoon, so I need to make a decision. I have all the tools and help for each option, just want to choose the one that is best.
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Old 10-05-2012, 01:15 PM   #21
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Well you know my answer to this! Either way is good, but do you have to circumnavigate a lot of pipes or wires?
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Old 10-06-2012, 08:50 AM   #22
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Mr. Jackpine-
I don't know if this will reach yo in time, but here are the methods I used on my very old house with plenty of bounce & a few cracked joists:
I am not an engineer, nor a carpenter. So take this as "Might Be Crazy"

Background: Joists are 2x6, about 100 years old. Each has shrunk to about 1-3/4 x 5-1/8" (I would have expected actual 2 x 6). Centers are 16, 23, 26- Kind of randomly. Span is 9 feet.

Mehtod One: Sister 2x6 on one side. Wash!, glue & nail. Glue is wood glue, not construction adhesive for a bond stronger than the wood. Nails are 16d. Tow rows, staggered by 2" horizontally. Spaced 8" apart. Rows are 1" from top & bottom edge of the wood. Sisters land on main beam & outside wall. (House has no top plate).
Installed cross bracing in entire house- both sides of main beam at span center. I could not do solid blocking due to gas, hydronic, domestic piping and air handler ducts, plus conduit & more stuff in the way. It looks like a factory down here.
Result: No bounce at all. A couple of rooms are ceramic tiled with no tile nor grout cracking. (One room, I cheated & did a floating installation, like a sand bed- but used Ditra for less room-to-room height offset.)

Method Two: Plywood Sisters on one side of the joists. The plywood is subfloor rated, not sheathing. This has less voids (so I'm told). I did this on the second floor. Installation was: *Wash the wood! Glue (wood glue) Nail as above. The subfloor was 1x12 planks. It is now subfloor rated plywood; glued & screwed. This helps spread the load across many joists (I hope).
The sisters land on the ledges at each end. (House is balloon framed.)
Result: No bounce nor squeak at all. None.
Side Note: These are 2 x4 oak joists. The span was up to 16 feet.(Yep-16 feet) I also added a beam-in-soffit below & a bearing wall below- to a footing in the basement. The longest span is now 6 feet. I can't say if the sisters or shortening the span helped more. Still no where near by-the-book, but the house has been here over 100 years.

If I had to do it again:
I found the plywood much easier to install. I did have problems with the 2x6 when I tipped them in place. I had to rip them to the existing joist size & bevel one corner- then bang them in with block & sledge.

If I had a brain in my head: In Europe, it is common to lay a 2x4 flat on the bottom of each joist, then generously screw it into the joist. This creates a kind-sorta I Beam. If the subfloor is also well secured to each joist, you get a wood I Beam. But, blocking is required to prevent twisting. I learned this from my Made-In-Germany father who was an engineer. I did this in a crawl space house I owned over 25 years ago. It worked great!
I don't know why I did not think of it for my basement accessible joists here. (Could not do it on the 2nd floor without destroying beautiful plasterwork below.)

I hope this reaches you in time & helps you decide.
Again, I'm no pro but these are my results.
Paul
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Old 10-07-2012, 09:03 AM   #23
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Sistering Bouncy Floor Joists


[QUOTE=Bird Doo Head;1024965 I could not do solid blocking due to gas, hydronic, domestic piping and air handler ducts, plus conduit & more stuff in the way. It looks like a factory down here.
[/QUOTE]

Sounds like a thorough job. Just out of interest, if you had no room for solid bracing, how did you manage to get a one=piece 2x joist in there?
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Old 10-07-2012, 09:56 AM   #24
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Originally Posted by mrgins View Post
Sounds like a thorough job. Just out of interest, if you had no room for solid bracing, how did you manage to get a one=piece 2x joist in there?
I guess I should have added that detail. Oops!

For things perpendicular to the joists (only NM cable & some communications wire in my case); it was remove; attach new wood; re-drill & replace. Boring, but simple enough.

Some joists had nothing attached to (parallel with) one face & stuff on the other. Those were the lucky ones. I used the empty face.

For each & every joist with stuff on both sides, one at a time I removed anything attached to the existing joist's face. That meant removing conduit straps, gas lines, etc. If I had enough free-play, I'd use temp tie line to hold the offending item away from the face enough to get the sister in (at an angle) so I could bang it upright. If I couldn't get clearance, down it came!

For the conduits, it was simple to re-offset to match the new location- adding to compensate as needed. What a time consumer that was! (I was an industrial electrician for many years. Conduit's pretty intuitive for me. Most days, anyway!)

For gas, I re-rolled the end fittings, replacing 90's with 1/8th bend pairs (with nipples between) as necessary. Fortunately, I only had a couple. The hydronic is all copper, so it was simple to cut & re-work. Again- Time consuming.

The solid blocking problem is that, with so much mechanical attached to the long faces of the joists, I'd be notching forever. On some, it's stacked quite a bit, so I would not have much end left on the blocks. I also have some with, as odd as it sounds, plumbing running along the bottom of the subfloor half way between joists. Goofy, huh?

There actually a couple that I could not even get cross bracing in. For those, I wiggled in 1 x 3 toward the top & one toward the bottom to span between the joists. I doubt they do any good at all. (Flex?)

In retrospect, I wish my father's advice was in my mind. I would have tried the bottom 2x4 flat system for all but the cracked joists. (I wish his advice was in my mind for lots & lots of things I've done over the years!)
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Old 10-07-2012, 10:15 AM   #25
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Sistering Bouncy Floor Joists


2008 Flashback thread
OSB Stronger than Plywood?

Adding a 2x4 along the bottom to make a T increases strength to 1.777777% in a 2x10 existing joist.
Sistering it doubles the strength. But adding the 2x4 is a lot easier. Does nothing for straightness though.
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Old 10-07-2012, 10:47 AM   #26
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Originally Posted by Octopus View Post
You are fine with the additional 2x8. 3 - 16d nails in a vertical row @ 16" on center. No screws, no plywood, no glue. Solid blocking between joists ( same dimension as the joists themselves ) is a must @ half the distance of the span
X2. I prefer this Kiss method
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Old 10-07-2012, 04:08 PM   #27
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Quote:
Originally Posted by mae-ling View Post
2008 Flashback thread
OSB Stronger than Plywood?

Adding a 2x4 along the bottom to make a T increases strength to 1.777777% in a 2x10 existing joist.
Sistering it doubles the strength. But adding the 2x4 is a lot easier. Does nothing for straightness though.
Thanks for the data Mae-Ling. That's interesting to know.
Is the formula something a BDH like me can calculate? I curious about 2x4 on the bottom of 2x6's. That's what is in my 'grand estate'. (Code for: 'Little House Of Big Projects")
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Old 10-07-2012, 06:11 PM   #28
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Sistering Bouncy Floor Joists


Take total when added (5.5+1.5=7) divided by height of height of old joist (5.5) then cube it.
7 divided by 5.5= 1.2727272727272 now cube that 1.272727272727272 X 1.272727272727272 X 1.2727272727272 = 2.061607813673927.
Just over twice as strong, basically the same as doubling it up.

You have sorta made a 2x8 ( actually just slightly under) I punched 2x8 spruce into the span calculator and it says your good for about 12'.
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Old 10-08-2012, 05:52 AM   #29
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Thank you for the formula, Mae Ling. I enjoy the science behind just about everything. This entire series of entries is full of "save this" information, thanks to all of you. Personally, I learned a lot.
By now, I hope the original poster has a good start on his or her project.
Paul

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