# Floor bounce joist sistering



## joehomeowner (Jan 1, 2009)

I have a problem with floor bouncing. I have 2x10s with a span of 15ft 16” on center. I would like to finish my basement and I’m afraid the bouncing will crack the drywall and/or I would just like to fix the annoying bounce. I’m looking into sistering the joists. I can’t get a 15’ 2x10 down into my basement maybe 10ft at most. Can I use two 8 footers glued and nailed to the original joist then a shorter piece over the joint. Would a 10 footer attached mid span work as well? Would an I-beam be better than wood? How would I attach an I-beam to the original joist? 
Thanks


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## Wildie (Jul 23, 2008)

joehomeowner said:


> I have a problem with floor bouncing. I have 2x10s with a span of 15ft 16” on center. I would like to finish my basement and I’m afraid the bouncing will crack the drywall and/or I would just like to fix the annoying bounce. I’m looking into sistering the joists. I can’t get a 15’ 2x10 down into my basement maybe 10ft at most. Can I use two 8 footers glued and nailed to the original joist then a shorter piece over the joint. Would a 10 footer attached mid span work as well? Would an I-beam be better than wood? How would I attach an I-beam to the original joist?
> Thanks


 I'm not a carpenter by trade, but my thought would be to sister 2X8X10's to the 2X10's. Centered, leaving 2 1/2 feet at each end, open! Two 8' would allow flexing in mid span, just at the point where stiffness is most important! I'll be looking forward to seeing the comments from our expert members!


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## joehomeowner (Jan 1, 2009)

Thanks for your comment. Would it buy me anything to also put the 2.5ft pieces at each end? What combination (s) of lengths or materials would be best? I’ve read about someone using steel or manufactured wood I-beams? Wood I-beams are lighter and would be easier to carry. I just can’t get a 15ft long board down into the basement. 
Any thoughts would be helpful!!


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## Wildie (Jul 23, 2008)

joehomeowner said:


> Thanks for your comment. Would it buy me anything to also put the 2.5ft pieces at each end? What combination (s) of lengths or materials would be best? I’ve read about someone using steel or manufactured wood I-beams? Wood I-beams are lighter and would be easier to carry. I just can’t get a 15ft long board down into the basement.
> Any thoughts would be helpful!!


 I doubt that a filler at each end would be of much good! One thought that I have had since, do you have any cross bracing (X) in place right now! You should have two rows of these, 5 feet out from the walls! Its amazing how X-bracing will stiffen a floor.


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## buletbob (May 9, 2008)

say Joe! what type off floor bridging do you have installed?? is it continuous between all beams right to the foundation or girder???. and also what is the finished floor above. that floor beam is rated at 15'- 2" for northern Fir. I would check the bridging before doubling up the joists. even put two more rows in if need be. BOB.


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## joehomeowner (Jan 1, 2009)

My basement is about 30ft across with a steal I-beam down the middle. The one half of the basement is below the family room where I notice the bouncing or give in the floor. The finished floor is carpet. There are cross bracing (x) (floor bridging?) between each joist about 5 ft from the one end near the steal I-beam. This is where the most foot traffic is above. Does x bracing really help the flex? One person I asked about x-bracing said that it does little to help flex or bounce it’s there to keep the joist from twist. I have also seen were people use solid wood 2x10s between the joists. Which is best X or solid wood? How many would be best??
Thanks!!


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## buletbob (May 9, 2008)

what type of bridging do you have wood cross bridging or is it metal cross bridging. the solid blocking would be your best bet. the bridging DOES help to support the floor. when the weight of the floor above is pushing down the bridging will help the floor joist from deflecting down and from having the bottom of the joist kicking sidewards. by installing the bridging you will dispurse the weight to the surrounding floor joists.


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## joehomeowner (Jan 1, 2009)

The x bridging is wood pieces approx 1x 4 stapled in place. If the solid blocking is best then that’s what I will go with. I may have to run some wires through since I’m adding recessed lights. 
Thanks for your ideas


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## buletbob (May 9, 2008)

try nailing the bottoms of the cross bridging tighter to the floor joist hammering them in a upwards direction. with the wood shrinking over the years from first being built they will loosen up as why I don't like using the metal cross bridging, I have witnessed the metal pinging as you walk across the floor. 
I personally would do three rows of bridging on a 2x10 15' span ,its over kill but for an extra 20 minutes a tighter floor will be appreciated. BOB


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## Wildie (Jul 23, 2008)

I like cross bracing, rather than solid blocking because downward movement of the floor is transfered across the diagonal brace to the bottom of the adjacent joist, therefore the weight is transferred to both joists on either side. Whereas, blocking is pulling on the side of the adjacent joists. X bracing does prevent twisting of the joists also! And I do agree with 'buletbob' about placing 3 rows of bracing! I would also suggest that the bracing would be screwed in place, rather than stapled! This may account for the limited advantage of your existing bracing!


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## ponch37300 (Nov 27, 2007)

What kind of windows do you have in your basement? You should be able to get 2x10s in from the outside threw the windows. I have done this with ceiling grid before.


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## joehomeowner (Jan 1, 2009)

None, there are no windows and the stairs is finished with a 90 deg bend.


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## ijs12fly (Jan 3, 2009)

Joe, Here is a great article. The metal strapping may be your easiest solution.

http://www.taunton.com/finehomebuilding/PDF/Free/021184090.pdf

I too have joist issues but my concern is cracking a travertine floor.

Do you have space below the joists? If so you can make an upside T with a 2x4 or 2x6 and really strengthen the joists.


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## joehomeowner (Jan 1, 2009)

Thanks for the link - good stuff! I was going to finish the basement so I was trying not to reduce head room any more than it is now 7' 8". The pdf shows gluing and nailing 2 layers of 8ft plywood and overlapping the seams. Is this article using plywood because it has better strength or just easier to work with?


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## Nestor_Kelebay (Jun 17, 2008)

ijs12fly said:


> Do you have space below the joists? If so you can make an upside T with a 2x4 or 2x6 and really strengthen the joists.


Ijs12fly:

I'm assuming you mean and "upside down T with a 2X4 or 2X6..."

If so, can you link to any online literature on doing that?

I graduated as a mechanical engineer, and doing exactly that makes perfect sense to me, but I've never come across any technical articles on it. What I have come across is people saying "Nope, you can't do that. I've worked in the business for 600 years, and every beam strengthening design I've ever seen involves adding wood to the SIDE of the joist, never the bottom of the joist. Ergo, it won't work."

It's like beating your head against a wall trying to get that person to understand that if the wood you add is as strong as the wood the beam is made of, and you use a glue that make a joint that's as strong or stronger than solid wood, what you have is a stronger beam.

But, there is a certain wisdom in knowing when to quit arguing.


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## ijs12fly (Jan 3, 2009)

Nester, 

Here is a where I learned how to calculate the Inertia: http://darkwing.uoregon.edu/~struct..._lecture28.html
http://darkwing.uoregon.edu/~struct...xample28-4.html.

You will then have to learn how to calculate the location of the centroid. This link will explain that:
http://physics.uwstout.edu/StatStr/...ams/bdsn51a.htm

And one more good source I found for deflection is http://books.google.com/books?id=yZ...&hl=en&sa=X&oi=book_result&resnum=6&ct=result

With this theory I came up with the following:
Moment of Inertia (I) : Support
99 : 2x10
198 : Two 2x10's
159 : 2x10 with two 2x4's
153 : 2x10 with one 2x8
167 : 2x10 with two 2x6's
210 : 2x10 with one T 2x4
250 : 2x10 with one T 2x6

So you can see that using a 2x4 T will give you a 2.1 benefit and a 2x6 will give you a 2.5 benefit if you start with a 2x10.

Let me show you how I did the T with a 2x4.

First I = bh^3 / 12. 
2x10, b = 1.5, h = 9.25. A = bxh = 13.88
2x4: b = 3.5, h = 1.5. A = bh = 5.25
Assuming the bottom of the 2x4 on an x,y plot is y = 0, then the centroid of the 2x4 is y = 0.75". For the 2x10, its center is 4.625, but its sitting on the 2x4 so its centroid distance is 4.625+1.5 = 6.125. Now to combined centroid you will use Yct = (A1*y1 + A2*y2)/(A1 + A2) = (13.88*6.125 + 5.25*.75) / (13.88 + 5.25) = 4.65. Now we need to determine the distance from each center to the combined centorid. The 2x4 is 4.65 - .75 = 3.90 and for the 2x10 its 6.125 - 4.65 = 1.475.

Now we calculate I = Ia + Aad^2 + Ib + Abd^2
I = 1/12(1.5)(9.25^3) + 13.88(1.475^2) + 1/12(3.5)(1.5^3) + 5.25(3.9^2) = 210

So you can see that the moment of inertia for a 2x4 added to a 2x10 takes it from 99 up to 210, or a 2.1 multiplier.

Deflection is proportional to 1/I. So if you double I, you reduce deflection by half.

Here is a link to my major problem that I have not yet solved on another forum: http://www.johnbridge.com/vbulletin/showthread.php?p=744445#post744445

Regards

Steve


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## Nestor_Kelebay (Jun 17, 2008)

Steve:

Your U of Oregon links don't seem to work for me, so I wasn't able to follow your calculation because I forget how to calculate the effective moment of inertia of a composite beam. However, I know this to be a valid way of calculating the effective Moment of Inertia because I did these calculations in the 2nd year of Mechanical Engineering at the U of Manitoba.

Also, the results you're getting are in good agreement with the results I get when calculating the effective I of a 2X10 if you glue and screw a 2X3 to the bottom of it to effectively make it into a 2 X 12. (Almost the same thing as making a 2X10 "grow" into a 2X12, really.) The built up beam will have an I value of about twice the 2X10, or you get about the same increase in strength from that 2X3 as sistering the 2X10 with a second 2X10.

So, you're preaching to the choir. I agree that gluing wood to the bottom of a beam will increase the effective value of I, and therefore reduce the deflection of the beam under any given load.

The problem rears it's ugly head in this thread from last September: (can you skim it over?)

http://www.diychatroom.com/showthread.php?t=27278

Specifically, when thekctermite says:

"Sorry Nestor, but that is not a factual statement unless you've devised a way to grow 2x12's from 2x10's. Tensile stress will occur at the bottom of the loaded beam, and lumber added to the bottoms of joists would have to be fastened in a way that would resist the tensile stress to lower the joists' tensile chord...Not easily done. I've seen and inspected hundreds upon hundreds of engineers' designs and fixes for damaged joists and excessive deflection, and not one of them included the addition of wood to the underside of a joist. Nearly all incorporated addition of wood to the sides.

The linked thread was muddied by engineering gobbledy-**** that doesn't necessarily hold water in the real world, and generally isn't promulgated on a DIY site because it really isn't of benefit to the average poster. We need to give out advice that is practical and usable. I spent 5 years in college dealing with theory of structures, and assure you that trying to loosely educate most DIYers (and a lot of carpenters for that matter) is not going to accomplish anything but to frustrate them."

What thekctermite is saying is: "Every plan to increase joist strength I've seen involves adding wood to the side(s) of the joists, and therefore your gameplan to add wood to the bottoms of the joists won't work. If it did, people would do it. And don't bother with the technical gobbeldy-****, we're supposed to be giving practical advice in here, not trying to confuse the natives."

My position is that as long as:

A) the wood you add is as strong or stronger than the wood the joist is made of, and

B) the glued joint between the new wood and old wood is as strong or stronger than the joint would be if it were made of solid wood, then

the modified joist is stronger than the original joist by the ratio of their moments of inertia, and will deflect proportionately less under any given load.

And, just as you calculated the effective I of the composite beam, it would also be possible to calculate the effective EI of a composite joist reinforced with steel plates on the sides, or with a square cross section steel channel glued and screwed to the underside of the joist.

It's reassuring to have someone else in this forum that understands that the strength of a joist can be increased OTHER than by sistering it. Adding material to the bottom is a far more cost effective method of strengthening the joist.

And yet, I am at a loss to explain why architects and engineering firms don't suggest doing this as a quick and cheap method for strengthening joists.


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## ijs12fly (Jan 3, 2009)

Try this link:

http://darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture28/461_lecture28.html

the address after the http:// is 

darkwing.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture28/461_lecture28.html

This would be such an easy test to setup for someone who is in the business. Take two 2x10's, seperate by 16", attach some blocking to prevent joists from rotating, add 300 lb load to the middle of it. Measure deflection. Glue and Screw two 2x4's and measure again. 

If this test shows that D is not reduced by around a factor of 2, then post the results and edjucate us math type people.

I don't have the room for a T beam config, one thing I am considering is using a steel plate rather then a wood beem fixed to the bottom of the joist. I have been told that adding a 3/16" x 1.5" steel plate would take my "I" from 99 to 216, or a 2.1 multiplier. I need to find the math to prove this so I can show it to my town inspector. 

Any ideas on the math?


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## Nestor_Kelebay (Jun 17, 2008)

I graduated from Mechanical Engineering in 1974, so it's been a long time.

I did, however, find that your US military has already investigated the effectiveness of using steel strapping to strengthen floor joists in the following report entitled: "*Evaluation of Expedient Techniques for Strengthening Floor Joist Systems in Residential Dwellings*. By: Michael S Black."

The web location of that report is supposedly at:

http://stinet.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA013987

but, that link doesn't work for me. Maybe contact the Army Corps of Engineers and see if they'll send you a photocopy of that report.

It'll be depressing, it found only a 10 % improvement in the deflection of 2X10 joists after NAILING 1/8 inch thick steel strapping to the bottoms of the joists.

The effectiveness of this method was found to be SMALL. That is, it wasn't very effective in strenthening the joists. However, my understanding is that the steel strapping was NAILED to the floor joists rather than being glued and screwed. If it wuz me, I'd use a strong construction adhesive like LePage's PL Premium moisture cure polyurethane adhesive to glue the steel strapping to the joists. Using nails allows the joists to deflect without the benefit of the steel strapping reinforcement until the tolerance between the hole drilled in the steel strapping and the nail is accomodated by deflection of the joist.

The other thing to consider is that joists are made of SOFT wood, like spruce, fir and pine. The result is that deflection of the nails holding the strapping to the joist would result simply because the wood around the nails would compress on one side. Wood is a very accomodating material.
Thus, SOME nails would compress the wood while others wouldn't even be carrying a load.

This study needs to be redone with the steel strapping bonded to the bottom of the joist, not just nailed to it.

If one were to GLUE the steel strapping to the joists, then the joist and steel strapping would behave as a single composite beam with the steel carrying it's share of the tensile force deflecting the beam right from the start. That is, I'd like to see your Army Corps of Engineers redo this study with the steel strapping glued and screwed to the bottoms of the joists with PL Premium construction adhesive.

http://ths.gardenweb.com/forums/load/repair/msg0517002328620.html

http://ths.gardenweb.com/forums/load/repair/msg0915531723856.html

THE MOST IMPORTANT aspect of composite beams is that there can be NO relative movement between the different sections of the beam. That is, if you simply screw steel strapping to the bottom of a floor joist, there is no bond between the wood and the steel, and there is the possibility of the two acting as two seperate beams, independant of one another. In order to ensure that they act together as one beam, it is necessary to GLUE (with construction adhesive like LePage's PL Premium) the steel strapping to the bottom of the joist. Without that, any clearance between the screws holding the strapping to the joists and the holes drilled to accomodate those screws is going to allow the steel and wood to act independant of one another. Thus, until the clearance between hole and screw is taken up, there will be virtually no benefit from the steel strapping.

However, by gluing the steel strapping to the bottom of the joist, then the steel strapping begins to stretch as soon as the joist begins to bend, and that means that you get more resistance to deflection right from the get go.

The only concern I'd have is to ensure that the glue layer was as thin as possible to minimize the "slack" attributable to deformation of the glue layer thickness as the joist bends.

I'd check that study by the Corps of Engineers to see if they actually glued and screwed the steel straping to the bottoms of the joists, or only screwed it to the bottoms of the joists. The former would be far more effective than the latter.


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## buletbob (May 9, 2008)

Nestor_Kelebay said:


> Steve:
> 
> And yet, I am at a loss to explain why architects and engineering firms don't suggest doing this as a quick and cheap method for strengthening joists.


Most engineers would see it as, You will be Loosing Head Room by adding wood to the bottom of the beam. but you have my attention on gluing and screwing strapping to the bottom of the beam. BOB


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## spiral (Jan 14, 2009)

*Sistering/strengthening TGIs*

Hi

Any one have any information on how to strengthen TGIs?

Thanks Gary


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## jogr (Jul 24, 2007)

You can also create a similar effect by glueing and nailing/screwing 1/2" or 3/4" plywood across the entire ceiling. It creates a nice box beam configuration and significantly reduces bounce. 

I'd suggest adding blocking (use glue to prevent squeeks), and see if that's enough before going further.

By the way, if you have no windows down there you are likely violated code egress requirements for finished space.


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## tmiles2246 (May 10, 2009)

With this theory I came up with the following:
Moment of Inertia (I) : Support
99 : 2x10
198 : Two 2x10's
159 : 2x10 with two 2x4's
153 : 2x10 with one 2x8
167 : 2x10 with two 2x6's
210 : 2x10 with one T 2x4
250 : 2x10 with one T 2x6

So you can see that using a 2x4 T will give you a 2.1 benefit and a 2x6 will give you a 2.5 benefit if you start with a 2x10.

Regards

Steve[/quote]

Very interesting Steve,

I'm wondering if you'd calculate for my situation in order to install tile.

My kitchen = 
Thank you for using the John Bridge Forums Deflect-O-Lator 
For joists that are Unknown wood, but in good condition, 7.25 inches tall, 1.5 inches wide, 12 inches on center, and 12 feet long between supports, the deflection calculated is 0.468 inches.
This translates to a deflection of L / 307.
Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.


My bath = 

For joists that are Unknown wood, but in good condition, 7.25 inches tall, 1.5 inches wide, 16 inches on center, and 11 feet long between supports, the deflection calculated is 0.457 inches.
This translates to a deflection of L / 289.
Since the maximum deflection for tile is L / 360, and for natural stone is L / 720, your floor is rated for Sheet Vinyl or wood.

My current flooring is 3/4" thick. 

Making the "T" is a matter of installing the 2X4 flat against the bottom of my existing joist with glue and screws?

Thanks, Tom


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## Daniel Holzman (Mar 10, 2009)

The issue of excessive floor flexing is quite different from the issue of beam strength. The 2x10 joists are sufficiently strong by code for 15 foot spans, however strength refers to the ability to carry load without breaking. It is not the same as deflection, which usually controls, since excessive deflection can occur on a beam (joist) which is considered adequately strong.

The purpose of X bracing is to increase the lateral stiffness of a beam, to prevent sideways buckling under unusual loading conditions. The X bracing has the additional benefit of transferring a small portion of the load from one joist to adjacent joists, however this is not the main purpose of the bracing. The bracing adds virtually no moment capacity to the joists, so it does not make the joists "stronger", however your joists appear to be sufficiently strong by code anyway.

So, on to the deflection problem. I don't think additional X-bracing is going to do much to solve your problem, although it certainly will not hurt, just cost you time and money. Your original idea about stiffening the joists by sistering lumber onto the side of them is good, and will certainly work. Since you do not have a strength issue, there is no reason you need to run full length sistered joist. The post recommending centering the sistered joist was correct, you should have a continuous beam across the center of the span, where the deflection will be the greatest.

The important thing is to make sure the sistered joist is adequately secured to the existing joist. This can be done using an adequate number of nails or screws. The idea is to make the two pieces of wood act as a single unit by transmitting the shear from one joist to the other (hence the nails or screws). I don't have the tables for your particular application, but in similar circumstances I have driven three inch structural screws every six inches, installed about 1 inch from the top and bottom, staggered. In other words, when you are done, there will be a row of screws along the top, about 1 inch from the top of the joist, and a row at the bottom, about 1 inch above the bottom of the joist, with the screws six inches on center. The two lines of screws are offset three inches from each other, that way the screws do not line up vertically, reducing the potential for splitting the wood.

The additional stiffness of a sistered beam would reduce your floor flex substantially. By the way, you could also add wood to the bottom of each joist, however this is rarely done because of headroom clearance issues.


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## tmiles2246 (May 10, 2009)

I'm not so much concerned about the head clearance as it is in the garage side of a walkout basement and there is the additional duct work that falls much lower. The good news to my situation is that the joists run parallel with the length of the upstairs bathroom floor. Being a small bathroom, I should only require sistering a couple of the joists. There are also double joists already under each wall of the bathroom.


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