Strengthening Joists

[pauls_workshop]

Even without math, why is it so difficult to see that the 2x4 is helping to prevent the sag of the joist through tension? You can't stretch the attached 2x4, which is the direction the downward force is trying to do. If the 2x4 was laying flat & wasn't attached to the joist, the downward force applied would bend the **** out of it because there wouldn't be anything to create tension.

hi all, just saw this thread today. I am a mechanical engineer of 24 + years practicing and design engines for a living. I'm not a civil, but think about stiffness and deflections almost every day of my life. Looking at this original idea of the bottom 2x4, if it was under the existing beam FROM END TO END and properly fastened to the existing beam with screws or epoxy or something extremely stiff and rigid, then it would indeed add to the vertical stiffness of the beam construction and also reduce the tensile stress. The vertical component of height has a cubed effect on the vertical stiffness, so whatever can add to the height of the beam is a good thing. However, just adding the 2x4 to the MIDDLE section of the beam and not having it below the ends does not come close to the benefit of the above situation and is not the same at all. There would still be a benefit if it is again solidly attached to the existing beam. Nails would not be so good for this. Sistering ideas are also good ones, esp if a little steel could be used. A very thin strip of steel on one or the other or both sides of the beam FROM END to END would be much better than the 2x4 across the MIDDLE section of the bottom. Any and all scenarious could be calculated analytically by an engineer to get exact comparisons (assuming each was properly connected to the existing beam). I don't have the time to do this, unfortunately, but thought I'd chime in.

Also, engineers come in all flavors and different levels of experience, so they are not all the same in level of understanding on any given topic. Always beware! For instance, even among mechanical engiineers, you can have experts in structural design, or just analysis, or validation, or combustion, or fluid flow, or powerplants, or field inspection of things, or engine design, or heat transfer, or plastics, or biomedical, or construction/civil, or aeronautical, or computer science, or environmental engineering, etc. While all "mechanical engineers", any one of these may not be so good at one of the other topics if they never have done much work in those fields. One of the reasons I became a mechanical was because the field is so broad, you can get a job somewhere, doing something, without much risk and then become an expert in that area if you want to. This is sort of useful when many college degrees don't always mean you end up with a job at the end. - Paul

 

[serkan]

This is an old thread with a lot of good information.

Has anyone thought of attaching two equal length 2x4s like rafters to the floor joist.

Upper Mid span of the joist would be the ridge. 2x4 ends will have ridge cut and fit tightly. And the bearing points at both ends would be the seats.

This form would mimic a gable end of a roof.

Obviously, the tension on the floor joist would have to be taken by jacking it up in the center before attaching the reinforcements.

I think this would add more strength than sistering a one piece 2x4 to the joist.

 

[GMCGarage]

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

Just like every other bit of advice, take with a big grain of salt.

 

[m9faye]

I came on to this post trying to find the answer of whether or not I could use screws to mimic something similar to this question. In my case could screws holding a plywood ceiling to stiffen and strengthen the joists be nearly as good as glued and screwed. I think I know the answer is no. However to the question on this feed I do know the answer. I am an engineer but only Civil so I took the same classes in school as a structural engineer but haven't used it professionally as I deal in water/wastewater. I have however done 3 full gut reno's which removed all the interior structural and non-structural walls (on first floors) and replaced with beams and columns with concrete foundation pads in the basement (for the street cred that might be necessary for some on here). I've used Weyerhauser structural beam design software for that, and I've also opened a hole in a brick wall and mortared in a steel lintel for an exterior door. So I'm not exactly green when it comes to solving structural home reno solutions.

To answer the question - yes a 2x4 glued well and screwed (to ensure the glue sticks the 2x4 perfectly to the joist) might be the best solution if you aren't worried of head space. But only if the 2x4 is perfectly adhered to the joist. People saying they think the top and bottom of an engineered I-beam are just to keep the centre from falling over don't know what they are talking about. Basically the further away from the centre of the joist the more value the area at the edges is to resisting bending. Thats why those skinny wood-I beams work better than normal dimension lumber. The thick part that is wider is further away from the centre. You will notice those I-beams are a lot taller than dimension lumber typically. If you glue and screw that 2x4 perfectly - like the bottom of joist is not warped or twisted and neither is the 2x4 - then this solution would actually be stronger than sistering (doubling up) the joist (stronger for sagging). At least for a 2 x 10. I just did the calcuation. Moment of Inertia (to simplify - how much a shape can resist bending) is 1/12 x width x height^3. for a 2 x 10 which is actually 1.5 inches by 9.25 inches, this is 1/12 x 1.5 x 9.25 x 925 x 9.25 = 98.9. The moment of a 2 x 10 with a 2x4 perfectly glued and screwed to bottom (all parts are touching, no warps) is approximately 209 - therefore stronger than sistering a 2x10 with another 2 x 10 which would just be 1/12 x 3 x 9.25 x 9.25 x 9.25 = 197.8. The 2x10 with a 2x4 at the bottom is logically stronger than a 2 x 12 (as a good check on the calculation) which would have a moment of inertia of 1/12 x 1.5 x 11.25^3 = 178. Here is a link for a video to show you how to do the calc.

To answer another question on the feed can this 2x4 reinforcing be applied only in the centre - yes kind of. If it is all the way that is better but if you covered at least half in the middle that would accomplish a significant reduction. I'm going to guess you could reduce the bending at least 2/3 from what doing the whole length would accomplish if you did only the middle half. However this is an offhand mental calc and I'll leave an actual structural engineer to answer that. The reason that the value is much more in the centre is that that is where the max bending stress is. At the edges (if the floor above is loaded equally across), there is little bending stress. Without getting into explaining why, at the edges it is shear stress that matters. Bending stress is zero at the far edges, and shear stress is maximum. Shear stress is resisted by the area of the joist. The extra area of the 2x4 is 1.5 x 3.5 inches = 5.25. The 2 x 10 is 1.5 x 9.25 inches = 13.875. 5.25/13.875 = 37.8%. Therefore the 2x4 only makes the joist 37.8% stronger at the edge. If you had a huge load above - 1 foot from the side wall or beam or whatever, then you wouldn't be worried about the bending strength in the middle as much as you might be worried about the weight ripping the joist apart. If that was the case - sistering (doubling) the joist would be better as that would double the shear strength (because its double the area).

Someone else said something about failure in the glue. I agree with others comments that you don't need to worry about glue failure, it is stronger than the wood in resisting pulling apart. That's part of the reason why plywood is stronger than dimension wood. I've seen mini bridges made from popsicle sticks and glue that weight less than 2 pounds and can hold thousands of pounds. And that was white glue. If you use a good construction adhesive like PL premium and screw it properly so the bottom of joist is all touching the face of the 2 x 4, then it will be extremely strong.

 

[duneslider]

I was part of the JBForum when that post was made and I have used that method a number of times to strengthen floors and it absolutely works well. In my case I was strengthening floors to decrease deflection for installing tile. It made a very noticeable difference to the feel of the floor when done. The intent of that original post was not to increase the weight bearing capacity of the floor but rather it was to decrease deflection to create a better substrate for tile and stone installations. L/360 is the min for tile and L/720 for stone. This was brought up as a way to get slightly questionable floors to an acceptable level.

When I did it, I used a PL construction adhesive and nailed it with my framing nailer (faster than screws). Technically, the mechanical fasters aren't needed after the glue dries.