# strengthening floor joist



## jpaz (Aug 19, 2011)

i need to strengthen my floor joist so i can lay a tile floor in my kitchen. my plan is to glue 6" strips of osb to the side of my 2x8 joist then glue and nail a 12' 2x6 to the osb. the reason for the 2x6 is with wires and other utilities i can in most places fit a 2x6 in tight to the floor board. is this a waste of money for what i might gain in strength? i've read about glueing 2x4 to bottom of joist but dont think its an option due to the condition of my joist. with cut outs of the joist on bottom i dont think i would have a solid enough bond for it. opinions please.


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## Bud Cline (Mar 12, 2006)

And the OSB will accomplish what.........?


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## Joe Carola (Apr 14, 2006)

jpaz said:


> i've read about glueing 2x4 to bottom of joist but dont think its an option due to the condition of my joist. *with cut outs of the joist on bottom *i dont think i would have a solid enough bond for it. opinions please.


Why are there cut outs on the bottom of your joists...and how many joists have cut joists?


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## ddawg16 (Aug 15, 2011)

One of the best ways to stiffen up the floor joist is to rip a 4x8 sheet of 1/2" OSB into 7" wide strips. I'm assuming your floor joists are 2x8 12' long....If that is the case, one strip will be the full 8' long...the second strip will be 4' long. 

Then take those OSB strips and glue and screw them to the floor joist (with no load on the floor) If you have some deflection already...maybe use a jack to level the floor.

Alternate where the 4' sections are...every other joist....

Give the glue time to dry....at least a day....that will stiffen up the floor quite a bit.

Another option....install more floor joists....not that hard...cut to length...insert...use a BFH to knock them into postion.


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## jpaz (Aug 19, 2011)

bud, i thought the osb would add alittle extra strength, not sure if im saying this right but doesnt it add tensle strength? is this not a good option? as far as the cut outs that joe asked about, my house is over a hundred years old and previous owner over time found it not to be an issue to cut out sectins of joist for any reason, mostly for duct work and even gas & water lines. maybe 6 out of 14 have somthing cut out. one even shaved to 1" in a 2' section.


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## Bud Cline (Mar 12, 2006)

I would think plywood would be the product to use if you go that route. OSB isn't that strong under those circumstances.

I guess you know there are prescribed limits as to how much you can cut away from a floor joist and where (within the joist) those cuts can be made. Same goes for drillings in them. Sounds like some of yours are way over the limits.


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## oh'mike (Sep 18, 2009)

Post some pictures----


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## Gary in WA (Mar 11, 2009)

*Good shear strength.* “OSB is stronger than plywood in shear,” says a report by the Building and Construction Technology program at University of Massachusetts--Amherst. “Shear values, through its thickness, are about two times greater than plywood. This is one of the reasons OSB is used for webs of wooden I-joists.” From: http://www.ecohomemagazine.com/gree...-oriented-strand-board-vs-plywood.aspx?page=2

If the joist bottoms are compromised in the middle 1/3, we really need pictures, as Mike said: http://www.mybuildingpermit.com/Constuction%20Tip%20Sheets/TipSheet06_11.pdf

Will any other option work other than #3, here? http://www.finehomebuilding.com/PDF/Free/021184090.pdf

The 2x6’s will add little to future loads is already sized for 2x8’s…

Gary


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## ddawg16 (Aug 15, 2011)

Good find GBR....better than the one I was going to post.

OSB has a significant shear strength....OSB vs plywood.....a discussion for another day....each has advantages....with plywood typically being a slightly better choice for floor and roof sheathing...but plywood tends to have a shear strenght greater in just one direction...whereas OSB has equal shear in all directions.

I know a couple of guys that did the OSB glued to floor joists thing....they said it made a significant difference....in one case it was to some 2x6's in the ceiling joist of his garage. Before the OSB, he had about a 1" drop in the middle. After OSB, maybe an 1/8" drop at most. And adding weight up there (storing his junk) caused maybe another 1/4" drop.

Let me stress...for it to work properly....it must be attached properly. The glue is what helps keep the deflection down...and the nails help prevent the deflection from breaking the bond on the glue.


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## Bud Cline (Mar 12, 2006)

What does the "shear-strength" of OSB have to do with using it for scabs to improve deflecting floor joists? Shear doesn't enter into it.

I can't argue with OSB manufacturers or organizations hyping their product but common sense tells me that using OSB slabs for joist-scabs doesn't involve shear stresses of the OSB. The shear-factors in this case have to do with the adhesive and the fasteners only. Is that not true? What am I missing?


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## ddawg16 (Aug 15, 2011)

Bud Cline said:


> What does the "shear-strength" of OSB have to do with using it for scabs to improve deflecting floor joists? Shear doesn't enter into it.
> ?


A LOT.....

OSB is not the best thing to use for say shelves....it's strength pependicular to the plane of the sheet is not that great.....that is where plywood has the advantage...

But its deflection along the same plane (shear) is excelent when compared to sawn lumber. That is why you see it used in engineered joists...they basically take a 10" strip x number of feet long and cap each edge with a 2x2 to give you the equivilant of a 2x12 but with much better deflection. In cases where you would be required to use 2x12's on 12" OC, you could use engineered joists 16" OC.

One important point....adding OSB strips to your existing floor joists is not the way to increase your load bearing strength. Your existing floor joists need to be sized for the live and dead load. Yes, it does increase the load...but you can't factor that into the load calculations.

Sawn lumber tends to have some flex in it...so floors tend to be bouncy with it....the OSB does not flex...hence, it helps the joists by reducing the flex.

A good analogy for any of you mechanical guys would be comparing a composite structure to an aluminum one....for a given part, both have the same strength...but the al one will bend quite a bit before it breaks...where as the composite one...not much....


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## Daniel Holzman (Mar 10, 2009)

I am not interested in starting a flame war, however it is reasonable to expect that posts have some relevance, and be factually correct. So let's set the record straight on the significance of shear strength AS IT PERTAINS TO DEFLECTION of a structural element, which is what we are talking about here.

The deflection of a structural element under bending load (we are talking about bending loads here, because the element we are discussing is a horizontal beam or joist loaded along the length of the element) depends on the following factors:

1. The moment of inertia of the element. The greater the moment of inertia, the lower the deflection. Moment of inertia increases as the cube of the depth, and is linear with the width of the element.

2. The modulus of elasticity of the element. The greater the modulus, the lower the deflection. Steel has a modulus about 20 times that of wood, therefore a steel beam of the SAME SHAPE as a wood beam will deflect approximately 20 times less than the equivalent shaped wood beam. This is a discussion about deflection, NOT strength.

3. The load. The greater the load, the greater the deflection. In this case, we are comparing structural elements under identical load, so this factor does not come into play.

So let's see what this tells us. If you take a sawn wood element like a 2x10 and uniformly load it, you will get maximum deflection at the center. If you strengthen this element by sistering another 2x10 to the first one, you will have doubled the moment of inertia, and the deflection will be half that of a single element, under constant loading. If you attach another 2x10 to the bottom of the first 2x10, making effectively a 2x20, the moment of inertia is now 8 times as large, and the deflection is 1/8 that of a single 2x10.

The kicker is that you will have a much deeper beam, and you need to glue and/or nail the second 2x10 onto the first adequately so that you carry the full amount of horizontal shear generated at the interface. This is HORIZONTAL shear, not vertical shear. If you do this, you get a composite beam with 8 times the moment of inertia, and 1/8 the deflection. Again, this is NOT a discussion about strength, merely deflection.

If you nail on a piece of OSB which is 1/2 inch thick and 9.5 inches deep to the side of the 2x10, you will increase the moment of inertia of the composite section by approximately 1/3 (1/2 inch is 1/3 of 1.5 inches, the presumed width of the 2x10). Assuming the modulus of the OSB is approximately the same as that of the sawn lumber, which it usually is, the deflection will be approximately 1.5/2 = .75, so there will be a 25% decrease in deflection. You get exactly the same result if you nail on a 1/2 inch thick x 9.5 inch thick piece of plywood or sawn lumber, assuming the modulus of elasticity of the nailed on piece is the same as the lumber, and it is usually close. This assumes you use an adequate number of nails and/or glue to resist the horizontal shear induced by the composite action of the beam under load.

Note that in this discussion of deflection, the issue of vertical shear has NOT COME UP. Vertical shear almost never controls in residential construction, and it is therefore irrelevant in the case of deflection how strong plywood or OSB is in vertical shear (vertical shear is normally just called shear), therefore Bud is exactly correct, it makes no difference for this discussion how strong in shear OSB is vs. plywood. We also had no need to discuss strength to determine deflection. In the real world, the engineer designing your beam always checks the structural element for strength and deflection, and sizes the beam based on the more restrictive condition.

Conclusion: It is perfectly reasonable to stiffen a beam or joist by nailing/gluing on plywood, OSB or sawn lumber. It is more effective to attach the new wood to the bottom of the existing beam, but this is rarely done because of the difficulty in getting adequate attachment to the bottom to resist horizontal shear. The most effective choice would be the product with the greatest modulus of elasticity, however since they are all about the same, the grand conclusion is it makes little difference which type you use.

Note: You need to do some deflection calculations before you go nailing or gluing to your existing joists, since you need to know how thick an addition you need to make.


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## AndyGump (Sep 26, 2010)

I read everything Daniel writes with great interest and he never fails to impress.

Andy.


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## Bud Cline (Mar 12, 2006)

Great explanations guys. Thank you.:thumbup:

My concern is more about how the OSB will handle the screw heads over time (1). The use of adhesive is a must but what type of application would a guy use is my concern (2). A few squiggles squeezed from a tube wouldn't be the answer for me. And if squiggles were the method of choice how would the waffered OSB handle each squiggle at each squiggle site?

I guess it is a matter of preference and my preference would be plywood way over OSB for this application.

I don't argue the strength of OSB when used as a web dado-ed between two wood flanges as with I-joists. But this use is a totally different application.

I have work benches in my garage that began shedding wafers after a few years. I have to keep them sealed to control the shedding. They have never once been wet, never. The only moisture they have seen is atmospheric moisture on a regular basis. I also have portable work benches that I have waterproofed and even then they separate from the inside out after a time.

I just wouldn't trust OSB in the above application.

Excellent information tho.:thumbup:


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## jpaz (Aug 19, 2011)

thank you all for the info. i would love to post pictures, i'm sure it would help but at this point i am not able to. so from what i gather is osb vs plywood doesn't make much difference, but with moisture in the basement the osb may swell and de-laminate. but what i wasn't aware of is how very little it may help. i'll be honest i dont know alot about deflection, but i was reading a earlier post about joist and deflection, i was surprised at how the numbers went up very little with sistering as opposed to almost triple with a support beam. my joist still need work for sure, but maybe i should consider a support beam also. yes its in a area where we walk for washer & dryer but maybe i could use somthing like steel 4" or 6" wide c-channel?


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## BigJim (Sep 2, 2008)

I am way old school and can't say much when it comes to the tech end of OSB vs plywood so I will just go with what I know. Plywood would also be my first choice but that is just MHO. If that were my floor I would put a drop girder under the floor joists and forget it. I know a drop girder will work and take bounce out if installed properly.


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## SteelToes (Oct 5, 2010)

Let me throw something new on the fire here...

What about using OSB rim board inserted in to lets say 2x8 or 2x10 steel track all glued and nailed together?


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## Gary in WA (Mar 11, 2009)

Check with the steel manufacture or a structural engineer for the I-beam. 

I was wrong on the OSB for shear strength, it has been discussed here before; http://www.diychatroom.com/f19/floor-bounce-joist-sistering-34846/

http://www.diychatroom.com/f5/osb-stronger-than-plywood-23523/

Let us know your end solution.....

Gary


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## cortell (Nov 27, 2010)

Daniel Holzman said:


> Assuming the modulus of the OSB is approximately the same as that of the sawn lumber, which it usually is, the deflection will be approximately 1.5/2 = .75, so there will be a 25% decrease in deflection. You get exactly the same result if you nail on a 1/2 inch thick x 9.5 inch thick piece of plywood or sawn lumber, assuming the modulus of elasticity of the nailed on piece is the same as the lumber, and it is usually close.




I came across this old thread and found it very educational. It leaves me scratching my head, though, on how engineered I joists (with an OSB web so much thinner than 2x sawn lumber joists) resist deflection so well, given the statement here that the modulus of elasticity of OSB and sawn lumber is the same. I would expect the 2x4 flanges to contribute hardly anything to resisting deflection.


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## Daniel Holzman (Mar 10, 2009)

Ancient thread, new question. Deflection is controlled by the modulus of elasticity of the material and the moment of inertia of the section. An I joist works well because the moment of inertia is high, due to the location of the flanges far from the centroid of the section. That is why steel I beams have the large flanges. The web carries vertical shear, and prevents buckling. The web is sized as small as possible to save material, and the web contributes essentially nothing to the moment of inertia, hence the flanges do all the work resisting bending, and the web does virtually nothing. In a sense, the web is only there to hold the flanges apart.


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## cortell (Nov 27, 2010)

Daniel Holzman said:


> Ancient thread, new question. Deflection is controlled by the modulus of elasticity of the material and the moment of inertia of the section. An I joist works well because the moment of inertia is high, due to the location of the flanges far from the centroid of the section. That is why steel I beams have the large flanges. The web carries vertical shear, and prevents buckling. The web is sized as small as possible to save material, and the web contributes essentially nothing to the moment of inertia, hence the flanges do all the work resisting bending, and the web does virtually nothing. In a sense, the web is only there to hold the flanges apart.


Hm. I'm going to have to study this bugger known as _moment of inertia_. It's not intuitive to me at all. The following presentation looks promising, but I'm going to need about three reads and two cups of coffee before it sinks in. Thanks, Daniel. 

http://paws.wcu.edu/radams/Intro_to_Beam_Theory.pdf


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## cortell (Nov 27, 2010)

Daniel Holzman said:


> Ancient thread, new question. Deflection is controlled by the modulus of elasticity of the material and the moment of inertia of the section. An I joist works well because the moment of inertia is high, due to the location of the flanges far from the centroid of the section. That is why steel I beams have the large flanges. The web carries vertical shear, and prevents buckling. The web is sized as small as possible to save material, and the web contributes essentially nothing to the moment of inertia, hence the flanges do all the work resisting bending, and the web does virtually nothing. In a sense, the web is only there to hold the flanges apart.


Sorry; follow up question. So would a 4x12 and an I-joist of the same width and height have comparable _deflection_ for a load that does not exceed the design strength of both?


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## M3 Pete (May 10, 2011)

Wow, this reads like a post from eng-tips.com Thanks to Daniel for the detailed info, even if this is an old post. 

As long as we are on the subject, maybe Daniel can comment on a completed project. 

I did the unthinkable, I strengthened the bottom chord of a truss for an attic floor, and to raise the floor to allow for insulation. 

The chord is 2x4, and I used a 2x6 stacked vertically on top of the chord. To secure the 2x6 I used 5/8 plywood, 8" tall and 4' long, one one side of the 2x4 and 2x6 stack. 

The plywood was secured to the stack with PL Premium (polyurethane glue) and heavy duty 2" screws. 

I also glued the 2x4 and 2x6 edges with the PL Premium, and pulled them together using an 8" Timberlok screw (all the way through the top of the 2x6 and into the 2x4) about every 18".

Ignoring the truss issue for the moment (pun intended), have I strengthened the 2x4 to any significant degree? Would I have been better to sister a 2x6 or 2x8 alongside the 2x4?

The floor feels quite solid (for light attic storage), so I know that it did stiffen things up. But if you have a few minutes to give your expert opinion, I'd appreciate it. We can talk about the truss later, but for now let's discuss the strength of my "sandwich."

BTW, I tried to ask this on the truss forum on eng-tips.com before I did it, but the question alone got me thrown off the site, without any explanation from the administrator, who would not return my emails. Seems that they don't look kindly on people trying to DIY solutions with engineering input.


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## Daniel Holzman (Mar 10, 2009)

Moment of inertia is a bit complicated to wrap your arms around, but it is central to the flexural design of all beams. Without getting into the derivation, the moment of inertia for a rectangular beam is bd^3/12, where b is the width, d is the depth, ^3 means you cube the depth. So for example a beam 2 inches wide, 8 inches deep, has a moment of inertia I = 2*8*8*8/12 = 85.3 in^4.

Note that doubling the depth of the beam increases I by a factor of 8. Sistering a beam of identical size doubles I. So if possible, you get much greater increase in I by increasing the depth of the beam. The limitation is that of course the beam gets deeper, so you may have headroom issues.

The other issue is more complex. When you connect two beams together along the bottom, they need to act as a unit to work. One beam wants to slide past the other when the beams are loaded. The force acting parallel to each beam at the point where they touch is called horizontal shear, and it is critical that the two beams be properly joined so they don't pull apart when the beam is loaded. This can be done with glue, nails, screws, bolts, or pegs, but in EVERY case you have to check the shear force across the connection to make sure the beam does not fail.

By adding a joist to the bottom of the truss chord, you substantially increased the moment of inertia of the chord member, and therefore increased the bending strength of the chord member. However, simple truss members do not act in bending, they are always either in tension or compression, so increasing the bending strength probably has little effect on the overall stiffness of the structure, unless you have a truss with fixed joints (not pinned). In that case, you have a space frame, which is far more complex to analyze than a pinned truss, and way beyond a DIY forum to get into the complex mathematics of space frame trusses.


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## M3 Pete (May 10, 2011)

Daniel Holzman said:


> By adding a joist to the bottom of the truss chord, you substantially increased the moment of inertia of the chord member, and therefore increased the bending strength of the chord member.
> 
> However, simple truss members do not act in bending, they are always either in tension or compression, so increasing the bending strength probably has little effect on the overall stiffness of the structure, unless you have a truss with fixed joints (not pinned).
> 
> In that case, you have a space frame, which is far more complex to analyze than a pinned truss, and way beyond a DIY forum to get into the complex mathematics of space frame trusses.


I need to find a truss primer to remember what pinned means. 

I understand that a truss is designed to keep all members in compression or tension. I believe bottom chords are always in tension. Adding tensile strength along the length was not my intent, as I was trying to effectively make the truss chord into a beam for the purposes of avoiding too much floor flex. I also supported the "beam" on two interior walls, and not make the truss try to support itself over the entire span. 

But the top chord of a truss does require some bending strength in order to hold up the roof sheathing, right? I know how it's supposed to work, but the bending strength has to be in there somewhere, right? I have a concrete tile roof, so it weighs a fair amount. Bottom chords usually just hold up drywall and insulation, so their bending strength does not need to be as significant. 

This is probably too much for this forum. Come on over and I'll buy you a beer and we can talk.


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## JohnKruk (Dec 8, 2012)

*Moment of inertia*

Daniel, great info thanks.
I have a very similar issue as jpaz right now and was not sure if should start a new post or not (moderator?). I need to get my floor deflection number up by about 15% to meet L/720. So need to strengthen the joist. Would putting a 2x4 on the bottom of existing 2x8 be sufficient. If so how much glue/screws would be needed to resist the shear?
Thanks again.


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## oh'mike (Sep 18, 2009)

I suggest you start a new thread--------Moderator---------


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