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Old 01-21-2011, 07:41 PM   #1
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joist reinforcement


I have read on some blogs about adding a 2x4 to the bottom of a joist it is said it is as effective ia sistering anyone tried this?

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Old 01-21-2011, 10:42 PM   #2
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I found this thread: http://www.johnbridge.com/vbulletin/...ad.php?t=54371

Very interesting. You're sacrificing 1.5" of headroom if you have living space underneath, but not everyone has a 7' basement like I do. You are perhaps reluctant to remove cross bracing or plumbing to sister?


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Old 01-23-2011, 09:47 AM   #3
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Originally Posted by benjamincall View Post
I found this thread: http://www.johnbridge.com/vbulletin/...ad.php?t=54371

Very interesting. You're sacrificing 1.5" of headroom if you have living space underneath, but not everyone has a 7' basement like I do. You are perhaps reluctant to remove cross bracing or plumbing to sister?

Thanks for posting that link on the joist stiffening, it will be very handy for yet another project I have to take care of.

2 x 8's were used in my home for all the joists, & between the plumber & the electrician, they drilled the crap out of them near the bottom 3 inches. I bet at least 80% of the ones in my basement ceiling are drilled with 1-1/4 to 1-1/2 inch holes to feed copper & romex through. Every room shakes & moves when you walk through, & when my kid has friends over, I swear they will end up going through the floor when they jump around.
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Old 01-23-2011, 09:55 AM   #4
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joist reinforcement


sorry and I could be wrong but adding that 2x4 doubles stiffness? I dont buy it, people would have been using that method in remodel a long time ago
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Old 01-23-2011, 05:23 PM   #5
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joist reinforcement


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sorry and I could be wrong but adding that 2x4 doubles stiffness? I dont buy it, people would have been using that method in remodel a long time ago

I tend to agree, I don't see it doubling the load capacity, but I do think it will be a viable way to stiffen up an existing joist. Especially when sistering a new joist to an existing one isn't an option.
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Old 01-23-2011, 05:57 PM   #6
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joist reinforcement


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Originally Posted by tpolk View Post
sorry and I could be wrong but adding that 2x4 doubles stiffness? I dont buy it, people would have been using that method in remodel a long time ago
I don't know about double, but it does make a significant difference in the deflection of a joist. When load is applied to a joist, it wants to bend. When it bends, it wants to move out of plane. By out of plane, I mean as it deflects downwards, the joist is no longer truly plumb, the bottom is no longer aligned with the top. When adding the 2x to the bottom of the joist, it resists those forces, helping to reduce the defection. Applying a drywall ceiling also improves the floor performance for the same reason. It braces the bottom of the joist.
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Old 01-23-2011, 06:57 PM   #7
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I think whoever you heard that from must have meant to nail the 2x4 to the side of the joists flush at the bottom. I can't imagine that just adding an inch and a half to the bottom would add any notable strength at all, just more weight.
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Old 01-23-2011, 09:57 PM   #8
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joist reinforcement


In fact adding a 2x4 to the bottom of a joist substantially increases the strength of the joist. This is because the 2x4 at the bottom adds to the moment of inertia of the beam, and the further from the centroid of the beam, the more effective the additional wood is. This type of construction is commonly used on steel beams, where you often see steel plate welded to the bottom of the beam. This type of construction is known as a plate girder in steel world.

The same principle applies to wood, and you actually see this type of construction commonly used in new construction, specifically you see the use of I joists. With the I joist, the 2x4 is attached to the bottom of a plywood or OSB web, however you can do the same thing with a 2x10 joist by adding a 2x4 on either side of the joist flush with the bottom, which does not increase the depth of the joist, or nailing the 2x4 to the bottom of the joist and increasing depth by 1-1/2 inches. The key to making it all work is to use sufficient nails and glue to make sure the horizontal shear between the joist and the 2x4 is carried by the nails.

Adding a 2x4 to the bottom of a 2x8 joist practically doubles the moment of inertia, and increases both the strength and the stiffness. You can do the same thing with metal strapping, although it is a little more complex to get the strapping to bond effectively to the wood. I saw this technique fully described in Fine Homebuilding, December 2006.
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Old 01-23-2011, 10:56 PM   #9
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joist reinforcement


I think this is the article.

Andy.

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