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Old 10-25-2011, 08:43 AM  
joecaption
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We have worked on 100 plus year old houses for the last 15 years.
One sure way to fix this is to add footings 24" X 24" by 6 to 8" thick with rebar in them every 8' running along the middle of the floor joist and add a doubled up 2 X 8 beam the full length of the area your trying to fix sitting on concrete block piers. We use constrution adhesive when forming the beam and through bolt with 1/2" carrage bolts.
We have been able to use bottle jacks to lift the floor so there's no more sag and stop the bounce with this one beam.
If the old floor joist are soft, fungus eatten or broken we sister new ones and just tack the two ends, install the beam them them attach the new joist to the old one.



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Old 10-26-2011, 04:54 AM  
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papakevin,

Yes, in the sophisticated sense, when massive increases in strength are required, a steel plate is often installed at the bottom of any member in bending. In your case, however, that would be definite overkill, and not at all necessary (unless your joists are made of bamboo, or balsa wood!). But seriously, for your situation, simply gluing and screwing a 2 x 4 or 2 x 6 (the flitch plate) to the bottom of the joist (bigger, the better, but often not necessary for shorter span, residential framing) in the center 2/3 or so, where bending stresses are greatest, can often double the strength (and resistance to deflection) of each joist. With the 2 x in the horizontal direction, and relocated away from the joist's neutral axis, it contributes significantly more to the moment of inertia and subsequently, section modulus, of the built-up joist. I won't try to confuse you by showing the formula for computing a member's overall strength, but trust me, it's true.

Think of it as acting like a steel girder, whose flanges are always oriented perpendicular to the web, and whose bottom flange is often larger and thicker than the top flange, to contribute to its resistance to bending stresses, and subsequently, deflection. As a test to convince yourself, you can take a standard 2 x 4, 8' long, and nail it, 3-1/2" dimension vertical, on some raised supports at each end (to prevent it from tipping). Then step on it in the center, and note how much it deflects. Then screw (skip the glue, it's just an experiment) a flat 2 x 4, 5' or 6' long, to the underside of the long 2 x 4 in the middle, and repeat the step test--you will notice a significant difference in the amount it deflects with the same amount of downward force--probably only half as much movement with the same downward force.



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Old 10-26-2011, 10:24 AM  
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Bridgeman; I have never heard that before and it makes sense I think. So if I have it right you would just strap the floor joist with a flat 2x4 on the bottom of each. I kinda hope papakevin tries this so he can tell us the results. It would be the quick fix.

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Old 10-27-2011, 06:15 AM  
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Ok, I like the 2x4 idea and will give it a try. Not sure I get the concept exactly, but then again, all I need is for it to work. I may come back and post a drawing of how to install to verify I understand the installation process.

Prior to reading the last few posts, I had my options narrowed down to bridging with 2x8s twice on each span (there's an existing 1x cross bridging in the middle I didn't want to remove - or - cutting 8" strips of 1/2" plywood, glue and screw into the existing joists, then repeat a second time staggering the seam. Since there is insulation in place and I have a limited amount of height in the crawl space (it is a crawl space after all) the 2x4 option seems like a much easier and quicker solution.

Taking some vacation time next week, so planning to give it a shot then.

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Old 10-28-2011, 08:14 PM  
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OK, BridgeMan, I want to make sure I understand your 2x4 suggestion before I try it.

My assumption would be you attach the 2X4 to the bottom of the floor joist, with it laying flat, so it makes an upside down T, attaching with lots of Liquid Nails and screws.
The other option I can see is to sister it to the side of the joist, at the bottom side and not to the bottom of the joist, but this doesn't seem to be what you are saying.

Please let me know if either option above is what you are recommending or if there's something else. I plan on tyring this next week and I will take plenty of photos to document the process for future posts.

Again, appreciate any and all information.

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Old 10-29-2011, 09:10 AM  
joecaption
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A real simple and 100% sure way to correct this problum is to add a row of footings 8' apart running down the middle of the room your trying to stop from bouncing. Add a doubled up beam of 2 X 8's and have it sitting on concrete block piers.
No form of 2 X 4 is going to do anything to stop the sagging or bouncing.
By installing this beam you can also lift the floor in the center and take out the sag that's most likly there now.

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Old 10-30-2011, 09:50 AM  
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Quote:
Originally Posted by joecaption
A real simple and 100% sure way to correct this problum is to add a row of footings 8' apart running down the middle of the room your trying to stop from bouncing. Add a doubled up beam of 2 X 8's and have it sitting on concrete block piers.
No form of 2 X 4 is going to do anything to stop the sagging or bouncing.
By installing this beam you can also lift the floor in the center and take out the sag that's most likly there now.
Well, that was my first plan, but thought there might be a simpler and easier way to correct the problem. In my primary residence, that's exactly what I did to fix my bounce issue, in my 13 year old home with composite iBeams. There, I had an unfinished basement, so I was able to fix it when I finished it out.

In this instance, it's a small crawl space, maybe 3 feet of headroom between the floor and bottom of the joist. With the tight space, I wanted to look at different options. The floor doesn't have any noticeable sag now, but I haven't stuck a 8 foot straight edge across the middle either. The bounce isn't terrible, but it is definitely there.

Curious, have you tried the 2x4 method and found it to be ineffective or are you simply pointing out the best option which is guaranteed to work? I figured I'd give the 2x4 approach a try first on four or five beams to test it since that solutions seems to be the easiest. If it didn't work well enough, then I could look at adding cross bracing (bridging) or metal straps.
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Old 10-31-2011, 04:21 AM  
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Quote:
Originally Posted by joecaption View Post
No form of 2 X 4 is going to do anything to stop the sagging or bouncing.
Sorry, joecaption, but the following numbers I've crunched clearly prove that a 2 x 4 flitch plate installed flat on the underside of a floor joist can help considerably to reduce unwanted deflections. I know it's a concept that some (many?) may find hard to believe, but perhaps the following will change a few minds.

For this purely hypothetical example, I used a modulus of elasticity (E) for both the joist and flat 2 x 4 of 1,800,000 psi, and assumed a simple (not continuous) span length of 10' = 120". I've used a 2 x 8 joist section having a depth of 7.25", a width of 1.5", an area of 10.880 sq. in., and having a moment of inertia (I) of 47.64" (to the 4th power). The flat 2 x 4 has a depth of 1.5", a width of 3.5", an area of 5.25 sq. in. and an I of 0.984" (again to the 4th power--it's so small because of the flat orientation).
The total I of the built-up section is computed by adding the individual I amounts to the value of A x d squared, where d is the distance from the revised neutral axis of the built-up section (computed by moment-area method) to the centroid of the flat 2 x 4.

Applying a concentrated 10,000 lb. load at midspan of the plain 2 x 8 results in a maximum deflection at midspan of 4.198". Now, if we glue and screw a flat 2 x 4 (only has to be 7' long, centered, to resist the maximum bending moment occurring at the point of load) to the bottom of the 2 x 8, and apply the same 10,000 lb. load, the deflection of the built-up member computes at only 2.112". The numbers speak for themselves--4.198" vs. 2.112".

To summarize, the difference in deflections is very close to a factor of 2 if the flat 2 x 4 is not attached to the bottom of the 2 x 8. Or in other words, the joist will deflect only half as much with the 2 x 4 attached as it would without it. If anyone would like to check my arithmetic, the formula for maximum midspan deflection is /_\ (delta, total deflection) = P (load) x L (span length, in inches) cubed, divided by the product of 48 x E x I. The formula for moment of inertia of any rectangular section is I = b x d cubed, divided by 12. The foregoing formulas have been used and accepted in the engineering world for close to 100 years, should anyone think or suggest I'm making things up.
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Old 10-31-2011, 06:37 AM  
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No form of cross bracing will do anything to stop the bounce. Cross bracing is only to stop the joist from twisting.
A 2 X4 is not good for any type of support because there only rated for a very short span and will do 0 good if just attached to the bottom.
Take even a 4' long 2 X 4 and set it between two cement blocks and stand on it and see what happens, it bends, if stood on end it's stronger but even an 8' one would stap if stood on.
A 3' High crawl space is more then enough room to work in, not fun but can be done, we've done dozens of them.
We use a metal concrete mixing pan with holes drilled in two ends and attach a rope to each end. Ones for pulling the material in and ones for pulling it back out.
Do not try and preform the pads then try to pull them in place, it takes over 3, bags at 80 lb. each to form a proper pad, 240 lb.

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Old 10-31-2011, 06:51 AM  
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Maximum Span Calculator for Joists & Rafters


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