Strengthening Joists

[Old Moparz]

I ran across this topic on another forum & thought some of you might want to read it over for your own projects. One place it looks like it would help out in, is the garage ceiling. It might increase the ceiling joist strength so you could get an increased load capacity in attic storage.

Attaching a 2x4 to the bottom of an existing joist doesn't lose much headroom. It's also a great idea for the house too, one that is much easier & cheaper to install than sistering joists or using some other method.

http://www.johnbridge.com/vbulletin/showthread.php?t=54371

 

[SpeedCoach]

From a strength standpoint, I wonder how that set-up (basically a poor mans I-beam) compares to sandwiching the joist in ripped down strips of plywood???? I'm comparing options for doing this myself to improve my attic storage.

 

[scott37300]

I've done a lot of framing and this goes against everything I"ve ever been taught or read before. You have a source for this information? This is the first time I have ever heard or seen anyone recomend this. In theory the bottom 2x4 is only keeping the joist from twisting laterally. A 2x4 on flat like that has almost zero strength. The reason metal I beams and wood TGIs have a bottom and top flat is to keep the relatively thin verticle peice from twisting, stiffins the side to side movement, not the up and down strength. It might give a little tension strength but not enough to be stronger than sitering two joists together like your picture states.

I would be interested in any proof or facts you might be able to provide on this to back your picture up.

 

[Old Moparz]

The guy who posted it in the original forum is supposed to be an engineer. I read through the entire thread & what is covered makes a lot of sense. Spend some time reading it because the same question is covered, but it's a bit lengthy to copy & paste here. (The link is in my first post)

I like that it's much simpler to do in the basement ceiling of my home because there are lots of wire & pipe passing through 80% of my joists. Putting up a 2x4 on the easy to access, bottom side of a joist is much more appealing.

 

[rcayot]

It would help only in that it would provide resistance in TENSION for the bottom chord.

How well a 2by works when nailed onto a joist is another question.

Roger

 

[ishiboo]

The 2x10 is in tension, a 2x4 added to the bottom would somewhat improve the tensile strength leading to a bit less bounce. Then again, so would adding OSB to your ceiling with a proper nailing/gluing schedule

I don't think stud-grade or similar lumber offers much tensile strength, in floors built from OSB i-beams, I think the lumber top/bottom is primarily for nailing and keeping the OSB "in plane" so it can bear the load, I don't think it offers substantial stiffing by itself.

There are ways to improve his method as well - a quality 1x4 would likely have better tensile strength than the 2x4 and leave more headroom.

There are definitely some engineering errors from the original poster, ie:

"P.S. If you add the 2x3s on the sides near the top of the joist say, if you have access to this area from above the floor), you get the same increase in joist stiffness (i.e. 60%) as addding the 2x3s at the bottom of the joist"

If the benefit is increasing the tensile strength, that matters only at the BOTTOM of the joist as it's under tension, while the top is under little forces or compression. So the above statement is definitely not true if the main benefit was from tension. I think the calculated gains are far less than he states for all methods.

 

[Old Moparz]

It seems that the method you choose to use will depend on the final results you're looking for. The sister joist method isn't practical if you have utilities passing through joists. The metal stock isn't practical if you plan to attach drywall to the bottom side of the joist. The plywood on the sides of the joist will be time consuming, & also not easy with utilities in the joists.

The 2x4 on the bottom, as well as a 2x6 isn't a perfect method, but just another option. It's also noted that you don't have to cover the entire span to strengthen the joist. This is also appealing because getting something on top of the foundation plate & the main girder to span the same place as the joist isn't going to be easy or possible in some cases.

I'm not an engineer, so I can't be 100% sure of this method, but there are explanations & photos of a completed project.

 

[scott37300]

I'm not an engineer but I do not agree with the engineer on the other forum linked to. His posts have many errors and contridictions in them, I am not going to believe one guy on the web stating he is an engineer that disagrees with everything else I have seen and read and been taught. He states that doing this will be "MORE THAN TWICE THE STIFFNESS OF SISTERING A SECOND JOIST", I have a very very hard time believing this. If you can provide some real engineering proof that this is true I would be very interested but I'm having a hard time believing this one guy who states he used to be an engineer going against everything else.

It would be an easy fix and great if it would work but I do not believe it provides much of a benefit. It will add SOME tensile strength but deffinately not twice the benefit of sistering, it wouldn't even add the same benefit of sistering.

Just because one guy says and does something does not mean it is correct. I'm one of those guys that doesn't believe everything I read.

 

[Old Moparz]

I'll be the guinea pig & post back in the near future.

I have an area in my house where the floor moves quite a bit that I need to address. The 2x8 joists span only about 11 feet, but each joist has two, 1-1/4 to 1-1/2 inch holes drilled through for the copper pipe & romex. If you're in the room & someone walks through, you can see furniture shake.

There is no way I am going to get sister joists against the existing ones, & to cut plywood to laminate it to the joists is another nightmare. The basement ceiling is insulated with fiberglass bats, & it'll take quite a bit of plywood to cover each one.

All I need to do is cut about a dozen 12 foot 2x4's, apply some construction adhesive, with screws & I'll be done. It has to be better than doing nothing.

**edit... I agree, I seriously doubt this will double the strength.

 

[Mike14k]

I'm a physicist.... and an engineer, both by degree But that's in audio and acoustics.

Which means just because I tell people I'm an "engineer" dosen't mean I know anything about construction.

 

[scott37300]

Bob, I wish you the best of luck. I'm sure you will gain something by doing it. I will be taking the hard way and be sistering all th joists, mine are also 2x8s and span 14 feet and are sagging 3/4-1" in the middle, the house is 50 years old. I will be jacking it up and sistering each 2x8 joist with a 2x10 joist.

One thing to consider on your floor, is your sagging at all? I would say if your whole floor is shaking and if your span is 12' and only 2x8s then your floor has some sag to it. If so adding a 2x4 to the bottom is not the right way to do it. You need to straighten the original joist before adding anything to it to get the benefit.

Like I said I'm not an engineer but in my opinion this one guys idea is a quick fix that gives you little benefit and not the correct way to do it.

 

[nehog]

I guess a mechanical engineer could jump in and explain why an I beam is an I beam and not a flat piece of steel. Just nailing a 2x4 on the bottom of a joist (IMHO, not worth much on the open market) won't add any appreciable strength at all. Maybe a tiny bit if it were well glued as well, but just nailed won't do much.

Generally, the flanges add stiffness in the other direction and prevent twisting. I don't think a joist will twist much prior to catastrophic failure. Maybe someone can correct me?

 

[NUTTSGT]

I can see it helping some but not to the effect of doubling the strength of the 2x10.

I see he also states the the screws do nothing more than hold it in place till the adhesive dries. Did I read that correctly? If so, all strength is being on the adhesive?

Another thing I don't understand in the part about only needing to be in the center where the deflection is, and that it does not need to be at the end of the 2x10.

 

[pcmeiners]

Also not an Engineer but unless the 2x is surface planed, as is the joist mating surface, and glued I can't see much gain in compression, only in deflection. As far as nailing only, minuscule compression strength gain, still considerable added deflection strength.
Have a house going up next door, newer engineered beams being used, basically stacked 2x, all glued surfaces are planed. Without the surface planing, irregularities in the mating surfaces would cause major strength loss, also figure in with planing the surfaces provide consistent glue strength (tight mating and cleanliness), not something you could possible get with unpreped wood.
In theory the guy is basically correct, but only if done correctly; how is a typical homeowner/contractor going to plane the bottom of the joist correctly after it is installed ?

 

[thrifty bill]

I posted this on another thread. I had a structural engineer do the design on this one. We dug out our crawl space and put in a basement. Unfortunately around here, houses are built on concrete block piers, spaced out about every 7 feet. Leaving in those piers would have made a basement useless.

Floor beams are doubled up 2x10s (floor joists are mounted above these beams). In order to remove the piers, we sandwiched each main floor beam with steel C channels, bolted through. Did not need to extend under the existing piers that we left (only dug out about a third of the crawl space), as the purpose of the C channels is to keep the beams from deflecting. They deflect in the center, the furthest away from the posts.

If you look at the third picture, you can see how the C channel stops just short of the outside pier.

What ever you decide to do, I would not base your final decision on any internet advice (including mine). Every situation is unique. If you ever want to sell the house later, having a documented engineer's review and recommendation will make things go smoother, IMHO.

 

[thrifty bill]

Also, FWIW, I did reinforce an area under the kitchen of my 1934 mountain house (different house than above). I doubled up the existing 2x8 floor joists that were 13 feet long. It was a pain to install these joists, as all the existing mechanical stuff had to be removed (HVAC, electrical and plumbing), I used pressure treated lumber that was on the heavy side, and I did it myself.

Next time, I would make it a two man job, and I would use a pneumatic nailer. Hammering in all those 16P nails, with only about 12 inches of clearance to swing a hammer, was tough. I would have to find a pneumatic framing hammer that could fit in a tight area. I don't know what is out there.

The pressure treated stuff was also not very true, I had to use a variety of methods to wedge them in square against the existing floor joists. I plan to keep this house for the duration, so the lack of an engineer's seal of approval was less important on that project.

 

[WNYflyer]

Having only read the initial parts of the attached link, the engineer has his numbers correct. Adding the 2x4 the bottom doubles the stiffness of a single 2x10 as long as the material is of proper strength and the nail/screw size and spacing is correct.

The 2x4 would need to be a single piece of material but it need not extend all the way to the ends. How long the 2x4 would need to be can be calculated for any situation.

OSB ceiling as already mentioned would certainly stiffen the floor but you would have to worry about joint locations.

Just another way of stiffening the floor.

 

[Zeke]

I've done a lot of framing and this goes against everything I"ve ever been taught or read before. You have a source for this information? This is the first time I have ever heard or seen anyone recomend this. In theory the bottom 2x4 is only keeping the joist from twisting laterally. A 2x4 on flat like that has almost zero strength. The reason metal I beams and wood TGIs have a bottom and top flat is to keep the relatively thin verticle peice from twisting, stiffins the side to side movement, not the up and down strength. It might give a little tension strength but not enough to be stronger than sitering two joists together like your picture states.

I would be interested in any proof or facts you might be able to provide on this to back your picture up.

I'm not an engineer but I do not agree with the engineer on the other forum linked to. His posts have many errors and contridictions in them, I am not going to believe one guy on the web stating he is an engineer that disagrees with everything else I have seen and read and been taught. He states that doing this will be "MORE THAN TWICE THE STIFFNESS OF SISTERING A SECOND JOIST", I have a very very hard time believing this. If you can provide some real engineering proof that this is true I would be very interested but I'm having a hard time believing this one guy who states he used to be an engineer going against everything else.

It would be an easy fix and great if it would work but I do not believe it provides much of a benefit. It will add SOME tensile strength but deffinately not twice the benefit of sistering, it wouldn't even add the same benefit of sistering.

Just because one guy says and does something does not mean it is correct. I'm one of those guys that doesn't believe everything I read.

I gotta go with Scott on this. I've built my share of structures and done enough framing to offer at least an opinion. The flat 2x4 is indeed a chord, but does little to increase the load for any given span.

 

[Zeke]

Also, FWIW, I did reinforce an area under the kitchen of my 1934 mountain house (different house than above). I doubled up the existing 2x8 floor joists that were 13 feet long. It was a pain to install these joists, as all the existing mechanical stuff had to be removed (HVAC, electrical and plumbing), I used pressure treated lumber that was on the heavy side, and I did it myself.

Next time, I would make it a two man job, and I would use a pneumatic nailer. Hammering in all those 16P nails, with only about 12 inches of clearance to swing a hammer, was tough. I would have to find a pneumatic framing hammer that could fit in a tight area. I don't know what is out there.

The pressure treated stuff was also not very true, I had to use a variety of methods to wedge them in square against the existing floor joists. I plan to keep this house for the duration, so the lack of an engineer's seal of approval was less important on that project.

Palm nailer.

 

[little d]

I gotta go with scott on this, it just goes against every thing i have learned or seen in framing. If it is true, then i have learned an invaluable trick but, i'm from Missouri, ya gotta show me.

 

[SpeedCoach]

Somewhere on the web in the last couple of years I read about something similar to this. The difference was that they used a length of metal flat-stock to do the same thing. If I recall correctly, it was 1 1/2"x3/16" flat, drilled about every foot or so. The floor joist in question was pushed up a tiny bit past level, then the flat-stock was screwed to the underside with #10x 3" screws. When the lifting devise was removed from the bottom of the joist..... the joist settled a bit but was in a more level plane than before. Don't know if it works, just telling what I did read.

I want to say that "This Old House" demonstrated this as an option for addressing bounce in a floor.

 

[Dead Skunk]

I'm a mechanical engineer, and this makes perfect sense to me. It is, however, dependent on how well the 2x4 is fastened to the existing floor joist. You can buy engineered joists that are just an upper and lower 2x4 with an OSB web. This does much the same thing, just without the ultimate strength you would have by adding 2x4 to the upper chord. Not the best way to do it in new construction, but for stiffening an existing floor I'd give it a go.

 

[creativecars]

I'll be the guinea pig & post back in the near future.

I have an area in my house where the floor moves quite a bit that I need to address. The 2x8 joists span only about 11 feet, but each joist has two, 1-1/4 to 1-1/2 inch holes drilled through for the copper pipe & romex. If you're in the room & someone walks through, you can see furniture shake.

There is no way I am going to get sister joists against the existing ones, & to cut plywood to laminate it to the joists is another nightmare. The basement ceiling is insulated with fiberglass bats, & it'll take quite a bit of plywood to cover each one.

All I need to do is cut about a dozen 12 foot 2x4's, apply some construction adhesive, with screws & I'll be done. It has to be better than doing nothing.

**edit... I agree, I seriously doubt this will double the strength.

Bob,
This will be interesting to see in a real world situation. Not an engineer either, just experience with wood and metal construction. A good quality construction adhesive (subfloor adhesive) will hold strong enough to split wood.
How wide is the room you are working on? I see it is 12’long. I would get set up to work quickly, by cutting the 2X4s to length and put a screw in one side. Apply adhesive to the length, screw into place on one end. Support both ends of all 2X4s and jack up in center about 1/8” beyond flat, install screws as directed and allow to dry about a week before removing jack.

 

[Dead Skunk]

creativecars is spot on with how to do this.

 

[jkeyser14]

I'm a mechanical engineer. I'm not going to double check the original poster's calculations, but he has the right idea. You are creating an I-beam. In an I-beam the top and bottom sections are highly stressed, the middle sees very little stress in comparison.

 

[Old Moparz]

I'm glad to see a discussion built around this subject because debate can be a benefit to anyone in deciding whether or not this method has flaws. One thing though, I do wish more people would "read" the info on the link so they wouldn't assume that the 2x4 was simply nailed to the bottom of the joist. I read through all 7 or so pages of the topic, but just skimmed the posts about ceramic tile. There are more factors to his idea.

I would definitely like to know that original posters credentials myself, but there seems to be enough info provided to believe that the info is somewhat valid. He talks about the way the flat 2x4 is used for it's tensile strength & not used to carry a direct, downward load like a joist does. Any 2x lumber laying flat has almost no strength to carry a load. It works in unison with the joist to increase the joist's ability to carry a heavier load.

 

[Old Moparz]

By the way, I recall an episode of "This Old House" many years ago when Bob Vila was on it. There was an older home with inadequate joists that needed to be braced better. They reinforced the bottom side of the floor joist system with 3/4" plywood. All they did was apply adhesive to the bottom of all the joists, then raise a 4x8 sheet of plywood up to it & screw it in place. The whole basement ceiling was done like this.

The 2x4 method is not exactly the same, but it has the same concept behind it. I wouldn't consider the plywood since you'd never be able to get into the spaces between the joists to access the utilities. Again, I suppose it really depends on the circumstances for each project. I would never consider adding 2x4's to new construction, but I do see the benefits behind this.

 

[creativecars]

This is the same way they design and build bridges. Compression forces on top, tension on bottom, a spacer in between. Just make sure they cannot slip on each other. If cost was not a factor the 3/16 metal strap idea is a good one, it has more tensile strength than wood and takes up less space.

 

[Printer Mike]

It sure seems to me that if the 2x4 is glued and there is a little upward stress kept on the "beam" while the glue/adhesive sets, that the result would be much stronger to resist bowing.

 

[scott37300]

I just did a bit more research on this and an important thing to remember....... the size holes drilled in the metal flat-stock must be NO larger that the diameter of the screw.... otherwise there will be slip or creep. They also advise securing the ends first while stretching the metal as tightly as possible, then work your way towards center from both ends.

What metal? I thought this discussion was about adding a 2x4 to the bottom of a joist?

 

[Flange]

Right, first a disclaimer, I am not a Chartered Engineer but my first degree was in Civil Engineering.

In basic terms, a simple beam can support a load until its deflection gets to the point that it fails. Easy so far.

The strength of a beam depends on certain properties, mainly its Youngs Modulus and its Second Moment of Area. The Youngs Modulus is a property of the material and the Second Moment of Area is a property of the cross sectional shape and size.

Google these terms and you will see what I mean.

Different materials and different beam types have different properties and of course each situation where you are using a beam is usually unique so the key is to make sure that you know that what you are doing is safe. If you are not sure consult a professional.

I would be very doubtful of the claims made by this person. Obviously one would have to do the maths to prove it but if it were so simple to increase the strength of a beam by the amount claimed then would you not think that all beams would be like this already?

Just my opinion though.

 

[walrus]

I would be very doubtful of the claims made by this person. Obviously one would have to do the maths to prove it but if it were so simple to increase the strength of a beam by the amount claimed then would you not think that all beams would be like this already?

Just my opinion though.

What it would do is keep(help) the joist from failing to do twisting, I wouldn't waste my time doing it, bite the bullet and laminate, sister something to the existing joists. Glued and screwed

 

[MBeaty]

I found this idea really interesting, so I decided to compute the numbers for it to see if it was true. As it turns out, the "beam" with the 2x4 attached to the bottom would in fact nearly double the moment of inertia of the joist. This translates into essentially double the stiffness.

This "math" does come with many assumptions though. It is assuming that the strength of the wood is completely uniform and the structure is one piece, such as an I beam would be. It is beyond my current abilities to calculate what the actual results of two pieces of wood would be, but I would assume that it would end up being considerably less effective than the simple math shows that it would be.

Just think about it like this. Two boards nailed together would not be nearly as strong as a single board the size of the two.

All said, I don't think this method to stiffen the joist would be very effective unless there was a very good bond between the two boards. To add much stiffness, they essentially need to be a single piece.

 

[WNYflyer]

For those that are contemplating reinforcing their floors, if your conditions will allow it reinforce two joist with 2x4's and then jump up and down between them. Then jump up and down in a un-reinforced area. There will be a big difference.

The key is the connection of the 2x4 to the 2x10. Though I have not done it in quite awhile I am pretty sure you can calc the nail/screw size and spacing (assuming no adhesive) such that the wood would fail before the steel nail. Not familiar with the strength of the wood adhesives but if they can truly give you a bond that will force the wood to fail before the adhesive then you might get away with some nominal nails/screws for erection purpose only.

For those engineers taking a look at this thread, break out that strength of materials book and look up "shear flow" as it relates to built-up sections........."shear flow" is the key !!

 

[Busted_Knuckles]

Related to this, does it matter if you using nails, or screws ? For instance, I love using torx drive deck screws, but is there a difference between nails and screws in their application in light construction ? For instance, if you where to attach the 2x4s to the bottom of the joists, with glue, could you use a 3" deck screw, or would it need to be a nail. And if it was a nail, would a pneumatic framing nailer ring shank nail work ? Like the Paslode .130 x 3".

Ive got all the lath and plaster out of a house I'm in the middle of reconstructing and thought I would give this a whirl. The joists are 2x10s and bouncy.

Screwing would seem to better to me, but nailing it would be faster... but I would try it either way, but Ive got a sense that the nail and screw are not interchangeable, but don't know any better.

Any thoughts...

 

[little d]

Moparz, the difference between the plywood and 2 by 4's is that with the plywood you are spreading out the load over a larger area. in esence, the joists on eather side of any joist is also carryng some of the load, hence stiffining it up. you dont have any of that sharing the load with a 2 by 4.
like i said before, ill be watching this one, will be a neat little trick if it works, D.

 

[Jazz]

I'm a mechanical engineer. I didn't do any calculations on this, but it would make sense because the most stress is placed on the joist at the outside edge. Adding strength at that point makes a big impact because that's where the bending moment is highest. Adding glue to the 2x4 and then screwing it to the joist would add a noticeable increase in strength.

 

[Falcon67]

I'd go with it, but I would treat it like a subframe connector on a unibody race car. I'd use construction glue + deck screw (like weld in a car) and support the assembly at several points until the glue set up. I'd think you get maximum stiffness from making the assembly as much of a single unit as possible and to keep the 2x4 from sliding on the edge of the 2x6 when under load.

To do a whole floor, I see many jacks and temp crossbeams to hold everything in plane - I wouldn't try a one at a time thing.

 

[snorky18]

Finally the numbers behind it:

Bending Stress = (Moment*C) / (I)
Moment is a function of your weight loading
where C = distance to the neutral axis
and I= moment of inertia=(b) * (h^3) / 12 where b = base width and h = beam height.

For the sake of simplicity let's assume your beam is a rectangle, not an I-beam.

Since Bending Stress = (M*C) / (I), the bigger you make I, the less stress in the beam.

Since I=bh3^3/12, the height of the beam is taken to the power of 3, so small increases in beam height can make significant differences in the stiffness of the beam (I), which in turn can reduce the stress significantly.

The two word summary:

It Works.

 

[claymont]

Finally the numbers behind it:

Bending Stress = (Moment*C) / (I)
Moment is a function of your weight loading
where C = distance to the neutral axis
and I= moment of inertia=(b) * (h^3) / 12 where b = base width and h = beam height.

For the sake of simplicity let's assume your beam is a rectangle, not an I-beam.

Since Bending Stress = (M*C) / (I), the bigger you make I, the less stress in the beam.

Since I=bh3^3/12, the height of the beam is taken to the power of 3, so small increases in beam height can make significant differences in the stiffness of the beam (I), which in turn can reduce the stress significantly.

The two word summary:

It Works.

Since you have an understanding of the math involved, it was also stated in that thread, that using a 2x6 would stiffen the joist even more. Would it be possible for you to explain this?