The floor is probably spongy because joist sizing has increased over the years.

 

Old building codes required all joists to be blocked or braced every 8 feet, but that requirement is no longer in place for 2x8 joists* because the subfloor by itself prevents the joists from rotating (not twisting) under loads. (Joists still need to be blocked at their supports.) That means that you don't need to bother replacing the missing old braces or adding new ones.
* d/b for 2x8 joists = 7.5/1.5 = 5. See NDS 4.4.1.2

 

* d/b for 2x8 joists = 7.5/1.5 = 5. See NDS 4.4.1.2

That means that you don't need to bother replacing the missing old braces or adding new ones.

Bridging (either solid, wood/metal cross, or continuous 1x3 across bottom) is not code-required for a joist smaller than a 2x12 given proper attachment of the subfloor. The IRC is what is adopted most anywhere, so inspectors will not consult AWC's depth/breadth ratios to determine bridging, but they would probably ask for it on a 2x10 just as habit.

However, bridging is still installed by most, especially when remodeling older houses where the subfloor has been hacked over the years and may not have been fastened correctly originally. It will stiffen a floor, and solid blocking can spread the load out more and lower deflection, even if by just a hair. So it is good practice to install/replace it, especially when having tiled floors.

 

Note: I'm talking vertical blocking (as in a block from top to bottom chord, all the way across the bay), not the horizontal blocking at the bottom like Neal has in his pic.

 

Which is?

 

I can see it with my eyes and imagination, but for an explanation you'll believe ask a real structural engineer.:smile:

 

We install bridging from above and there are lots of places with odd sizes and they get a 2x6 block or what is handy from above. After plumbers and HVAC guys get done taking out the bridging and blocks, we go back and just put flat 2x4s from below. evenly spaced no more than 7 ft from support or each other.

 

That's because a panel of flat steel costs more than crossed bars. Also, a flat panel isn't as aerodynamic as bars. You don't need to worry about aero for your joists, and if blocking instead of X-bracing breaks your budget, well...

 

I'm only a self taught enjiner but I'm thinking you're missing the point.

 

When possible I prefer X bracing. After all the Eiffel Tower, trestle bridges, elect. power transmission towers ect. didn't use blocking to prevent compression.:wink2:

 

The theory is that the joist is held firmly by the subflooring. Applying the 2x4 to the bottom negates twisting, and as Neal said, either position is fine.

 

Replace missing bracing with a 2x4 block flush with the bottom of the joist

 

Oriented with the face of the 2x4 up and down, you mean?

 

Thanks for the reply. I get that the bridging assists in stopping twisting and that is fine however, I am adding in a new joist that will not be sistered, so I will be bridging that one for that exact reason as well as the added load displacment that comes along with it. The floor is still above in this instance. That being said, your question about subfloor did get me thinking..I will actually be replacing most of the subfloor with a 3/4" plywood as well as a new 1/2" underlayment. I think I can do most of the bridging after o pull the subfloor up. That will make those easier for sure. However, there will still be a few areas on a couple of the joists where I'll have to bridge where the floor won't be removed as it isn't in the bathroom.

There is no way to fasten the top of the bridge at a downward angle into the top face, can I go from the back of the joist the bridge is attached to at an upward toenail into the bottom face of the bridge board? Honestly seems like I don't really have a choice really.

 

Bracing helps keep the joisting from twisting. Since it was built, it has twisted all it will. Make sure your plumber(s) haven't cut joists to install toilets, and things like that. Adhering sisters is a good idea along with structural screws, not drywall or brittle ones. Are you adding subflooring layer to what you have? Incidentally, what do you have for a subflooring? Thickness. 2x8's can span 11 1/2', but it depends on their age, condition and by sistering them, you are adding to that strength.