Floor Joist Blocking

barbie13607

My brother just had to take over construction of a home where the contractor folded. He's got one problem and I told him I'd ask here.

The prior contractor installed the floor joists - 2 x 10 on approx 11' spans without any blocking. This is in a 30" crawl. Some of the joist ends are starting to twist.

What's the best way to block at this point?

thanks

mighty anvil

Lateral restraint at supports is required to be by solid blocking or by nailing to a band joist.

Mid-span bridging (or blocking) in this instance is not required by any code I know of or by accepted engineering practice, nor would this kind of bridging/blocking contribute anything meaningful to the deflection resistance of the design which already has a maximum deflection of over L/1000.

If the ends are twisting they either haven't been nailed or blocked properly depending on the condition.

reveivl

Go under with a bunch of 2x10 blocks 14-3/8" long (for 16" centers) and pound them in and nail them off. Here, you would also need cross bracing at the center point of the joists as well, but if the floors on you won't be able to put that in so I would solid block at the center line. Stagger the blocks either side of the line for ease of nailing.

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Cole

Scroll down to post #13 and see the blocking images:
http://www.diychatroom.com/showthrea...light=blocking

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Kent Whitten

I really haven't heard that it MUST be mid span blocked (especially your span)for code, but ends definitely, just for the fact you are talking about. You need to get under there and block where the bearing ends are or very close to it. If you run into a pipe or wire, you can usually drill a hole, rip the piece through the hole, then place the two pieces together.

How twisted are these? You have any pictures of this?

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mighty anvil

The IRC requires mid-span blocking for joists with a width to depth ratio greater than 1 to 6 which means that even 2x12's don't need mid-span blocking.

Since the span is short enough that even 2x8's would have met the code, it would be pointless to put blocking at the mid-span of 2x10's since they are already more than stiff enough. If the ends are severely twisted you probably need to add blocking or additional nailing at that location.

Don't add blocking if it serves no structural purpose.

neolitic

Building to code is building to the minimums. Surely you must have noticed that blocking/bracing makes a difference in the real world? Can't tell you the number of customers with "bouncy" floors have been happy after I've put in blocking, or nailed off the western braces left hanging---framer to lazy to nail off the bottoms. Guess it could be placebo effect.
Sorry barbie, this isn't on point for you question.

mighty anvil

I didn't recommend that the floor be built to code; it's already built. I am saying that, in this case, because there is no code requirement for blocking 2x10's, the original poster can do whatever he wanted.

In this case, when the deflection is only .13 in. (L/1040) at the full design load of 40 PSF, the floor is super stiff so adding blocking is not necessary and would not even be noticeable in normal use. Joists have to be weak enough to twist when they deflect in normal use for blocking to make a difference and these joist are not going to twist at all.

I might add that, in general, if blocking makes a floor feel noticeably stiffer, a deeper joist should probably have been used in first place.

In this case, someone used 2x10's when 2x8's would have exceeded the code requirement by quite a bit so there is little point in trying to make an unusually stiff floor any stiffer.

But I agree that poorly designed floors that just meet the code deflection limit of L/360 at spans greater than 14 ft. (.46 and greater deflection) would feel bouncy to most people and would greatly benefit from blocking. But a good designer would set the limit at L/500 for 14 ft and increase it as the span increases so blocking would be entirely optional.

Kent Whitten

I haven't looked at any of the IRC codes and I'm curious anvil about the ratio you are giving.

From my recollection of how they figure this stuff out is they are trying to prevent lateral movement which can also be accomplished by a piece of strapping or continuous restraint, like sheetrock or similar.

What I've been told by an engineer is that if the floor has a 3/4" layer of plywood, then blocking serves no structural purpose except in the case of needing it for lateral sway of mid span. It doesn't really do anything for structural bouncing. Not really pointing to anyone, just kind of speaking out loud...so to speak.

I'm trying to figure how that ratio works though. 1 to 6, meaning 2x12's need blocking at the middle of a span of 11'-3"?

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mighty anvil

When an improperly designed floor joist is loaded it tries to take the path of least resistance which is to twist to the side. Blocking, bridging or strapping at 8 ft on center forces the joists to take the path of maximum resistance (down) making them slightly stiffer.

Unfortunately, building codes are traditionally concerned with resistance to bending and only require enough deflection resistance to keep plaster ceilings from cracking.

Blocking to reduce floor bounce is generally too little, too late. Joists must be properly designed to avoid excessive deflection regardless of whatever might be done to reinforce them.

neolitic

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