Help with bouncy floors

Thanks, joecaption, for furnishing the link to the span tables and mentioning how ineffective cross-bracing is for reducing joist deflections. But you haven't presented anything showing the example numbers I presented in Post 18 are incorrect.

To summarize those numbers--for a typical floor joist, the amount of vertical deflection caused by applied loads can be cut in half by simply adding a flat 2 x 4 to the bottom of that joist. For the example I used (a 10', 2 x 8 joist with a 10,000 lb. concentrated load in the center), the actual deflections are 4.198" (without the 2 x 4) and 2.112" (with a 2 x 4). In summary, the difference is a factor of 4.198 divided by 2.112, or 1.99. Pretty close to 2 when rounded (or twice as much deflection without the 2 x 4 flitch plate on the bottom).

Let me try to explain the concept in a different way--the next time you are out and about, driving your vehicle on public highways, take a look at the shape of each girder on steel bridges you drive under. You'll notice the girders are almost always shaped like an "I". This particular shape has been used for more than 90 years because of its ability to economically resist applied loads, while minimizing vertical deflections caused by those loads. And the lower flange of each girder (the flat, horizontal part on the bottom) is what enables each girder to resist heavy loads applied to the span. By itself, the flange would be useless in resisting loads (it would bend and deflect, just like the flat 2 x 4 you mentioned). But because it's attached to the rest of the girder, it becomes integral with the overall member, enabling the member to withstand loads. And if we take a closer look at the lower half of a typical girder, you'll notice how it resembles an inverted "T".

Which is the same shape as a floor joist with a flat 2 x 4 on its bottom. Acting just like the bottom half of a highway bridge girder, carrying its applied loads safely and without abnormal deflections.

Ok, started installing the 2x4's on the bottom of the joists today. Only got 3 installed, so won't know fully it is works until the rest are installed.

Ended up installing a new vapor barrier first because I had to pull up the old one after the crawl space flooded (bad plumbing leak) few months back. Made it easier and more pleasant to move around in the cramped space.

Predrilled the 2x4s before taking them under the house so the center screw holes were there and aligned. Used about a 1/2 tube of Heavy Duty liquid nails on board to secure it to bottom of joist. Cheated and got 10' 2x4s which allowed me to put one end on top of the block foundation, which helped with my one man install.

I will post results and photos once this project is complete.

Disappointed to hear that cross bridging will not help bounce. My plan B was to install solid block 2x8 bridging, but it sounds like that isn't a good option, right?

Solid blocking only helps a little when one person is on the floor, when six people are on the floor, it bends down, open up the bottom of the blocking and starts squeeking. If Bridgeman's 2x4 don't work, you are either doubling the joists or installing a beam as Joe suggested.

I am fascinated and eager to know how this experiment turns out. Please keep us posted papakevin! And BridgeMan, thanks for the mathematical rundown. I've never had the time to try to puzzle out the engineering stuff on my own.

OK, I'm trying to post some photos, so I hope it works. Here's an update on where I am as of tonight:

(1) - I'm sore as hell and feel like I've been beat with a bat. Prior to putting down the new vapor barrier there was one down when the crawl space flooded. When I crawled around under the house, I created a ton of small imprints in the mud below. It dried hard when I removed the old vapor barrier, so it's like crawling / laying on large rocks and it sucks. Plus it's a tight crawl space, so I used some muscles I haven't used in a while.

(2) - Smart thing I did do was buy a couple clamp lights to use to light up the crawl space. Prior to doing this I was working with a flashlight which sucked. Being able to see what you are doing without holding a light is a big help.

(3) - A +1 vote for NASA. As my coveralls, I wore an old Challenger flightsuit / jumpsuit that I purchased at Kennedy Space center years ago. It's not a real one, but the cheap ones they sell to visitors. Anyway, after the Challenger accident, they removed the shuttle patch and sold them at discount. The placement of the zippered pockets on the arms, chest, made really great places to hide a small flashlight, camera, screws, etc.

(4) - Used longer 2x4's than necessary so I could rest them on the block wall. Because the house is old construction (40's) the sill plate is just a bit thicker than the 2x4, which allowed me to prop it up while I attached the other end. Installation was easy once it was in place, which was good because I was working solo.

(5) - Strong drill with a fully charged battery is key. With the number of screws you install per board, you need to do a lot of screwing (with the drill that is), and you really need to push up hard to verify you are getting a good bite. Used a Craftsman 19.2 volt drill and it worked well.

Will check it out tomorrow to see how successful the process is thus far. I didn't want to bounce on the floor or walk on it until the liquid nails has dried. Hopefully more updates will follow this weekend.

Trying to post photos... we'll see if this works.

Just had a look at your plumbing thread. Can you have a look at the beam running the center of the house that we see in those pictures. Is that one or two plys 2x8 on top of the flat 2 x ?.
Bridgeman will be pleased with you work so far, that old lumber can be a bear to screw into.
Looking forward to the results and hopeful that it works, we all have things to learn.

OK, here's a couple updates. First, the ruling on the 2x4's, then an attempt to answer the questions on the beams.

So, the verdict is....... it worked! BRIDGEMAN for President! Yes, there still is some bounce in the floor, however the improvements are noticeable and it should be good enough that no additional floor support is needed. My only regret is not completing the entire job all at once, because now I need to go back under and do the 3-4 joists that I didn't do in the living room. And yes, I'm still sore and walking with a slight limp from my last trip in the crawl space....

On a related note, I may go ahead and install underneath the beams that support the floor that supports a couple of the interior walls. (Let me explain...). Apparently the house was built so that the entire hardwood floor was installed first, then the walls were built on top of the wood floor. The walls themselves are not necessarily aligned with the beams, so in some instances the walls are actually resting on the wood floor in between two beams. Seems a little odd to me, but maybe that's the way they did it back in the 40's.

Now the beam question Nealtw asked about. Sad to say I didn't give the center beam a close look, but I did take some random photos while I was under the house. From what I can see, I believe the center beam is actually two 2x10's nailed together, with some 2x 2 block strips installed along the bottom to hold the 2x8's in place. Guessing they did this to rest the 2x8's on the rails, then toe nailed the 2x8's into the 2x10's. Maybe they didn't have joist hangers back then, or maybe this was just the cheap way of doing it. I'm adding some photos for additional review / comments. Don't know if adding anything to the bottom of these joists (like with the 2x4's) would help add structural support or hurt, but open to feedback and discussions.

Bridgeman, thanks again for the suggestion. I really didn't know if it would work, but I did have some faith and it worked for me well enough to become a believer. Thanks again!

You're totally welcome, papakevin, and just glad I could help. I suspect there still may be a few "unbelievers" out there, but all I can say is that what I've suggested is based on proven engineering principles. And in reference to my earlier posting explaining the action of a flat member, acting integrally in the tension zone of a bending member, more than 2.5 million* bridge girders can't be wrong.

* (My estimate, probably very conservative, for the total number of steel girders in bridges in this country--double or triple that for the rest of the world)

So BridgeMan, would the same theory apply when trying to strengthen ceiling joists in an old house? And since the load is on the bottom, would the 2xs need to go on top?

I am asking because my 1920's bungalow had some storm damage and we had to take down the lathe and plaster ceiling in one room. The 2x4 ceiling joists span about 10 feet and all but 3 of them are spliced. Without the lathe, I don't think they will hold up drywall, and there is no room under the bottom of the roof slope to put up 2x6s.

Ceiling joists experience what's known as positive bending moment (just like floor joists), a downward force due to the weight of the members themselves, ceiling surfacing/covering, attic insulation, etc. Floor joists often don't have drywall and insulation loads, but have heavier live loads in the form of people and furniture.

Think of positive bending moment as bowing downward like a bowl, and holding water. Such bending moments place the bottom fibers of the loaded member in tension, while the tops are in compression. Any additional stiffening (flat) members would still best be added to the bottoms, primarily because most wood species are typically 50% stronger in tension than compression, parallel-to-grain. In the case of ceiling joists, you'd want to install the flat 2 x 4s full-length on the joist bottoms in a given room to make the drywall application surfaces uniform, even though the additional members are only needed at the center 2/3 of any span for structural reasons. An added advantage of flat 2 x 4s on the bottom of each joist is gaining the additional nailing/screwing area (3-1/2" vs. 1-1/2" wide) they provide.

An exception to the foregoing is, if instead of being oriented in simple-span configuration, load-carrying joists are continuous over one or more interior supports. Not normally seen in residential construction, as the members would be abnormally long and not economically practical. In such a scenario, however, there will be negative bending moments (tendency to bow upward) at the interior supports, and in such instances the 2 x 4s could be installed on the tops of the joists at support locations, and at the bottoms in mid-span locations, to do the most good.

Dionysia said:

. . . . and there is no room under the bottom of the roof slope to put up 2x6s.

Click to expand...

Actually, 2 x 6s could be sistered onto each 2 x 4 ceiling joist by taper-cutting the ends where the roof slope interferes. But a stiffer ceiling will result if each 2 x 4 joist would have a 2 x 4 flat member glued/screwed to its bottom. Crunching the (approximate) numbers for the section modulus of each configuration, it works out to 13.62 cu. in. vs. 10.63 cu. in., or a difference of approximately 28% stiffer with the flat 2 x 4s on the bottom.

Thanks BridgeMan - I will pass this info onto the Mr. As always, I appreciate your number-crunching!

In your case I'd just use at least 2 X 6's. That's the way it's always done with a new home with a low slope roof.
Those 2X 4's are not even close to being wide enough to support the load or plaster. I'd remove the plaster and use light weight 5/8 sheetrock.

This will likely be my last post on this thread unless something else comes up.

Ended up getting a few more 2x4's to finish up the floor under the living room. It might have been a mental thing, but it seemed like I could tell where I stopped putting the 2x4's in the middle, so I went ahead and finished off the room. Execpt for the insulation hanging down, I think it turned out alright.

Thanks again Bridgeman for the idea. Love it when a plan comes together.