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Unread 10-11-2014, 06:42 AM   #76
bradesp
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I've been following this thread and want to confirm/clarify something... Is it true that if I apply the 2x4 cording to the bottom of a joist that I don't have to run this cording the entire length of the joist? If so, what would be the minimum length? I realize I need to be centered on the length of the joist, but is a 2/3 length on the cording enough? how about 1/2?
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Unread 10-11-2014, 07:18 AM   #77
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The answer to your question is related to the degree of 'curvature' through the length of the joist. (The curvature of the joist is directly 'responsible' for stress on the tile which will cause it to fracture due to floor loads).

Note that the stiffness requirements for tile flooring focus on the weakest area of the joist - the middle which has the highest curvature...

The downward load-induced curvature of the joist would be maximum in the mid-span decreasing to zero at the ends. Therefore, in reality, the stiffness of a joist needs to be greatest at the mid-span and does not generally need to be especially high at the ends.

This simplified explanation illustrates why re-enforcing the 'middle' of the joist is especially important to ensure ceramic tile floor integrity.

Because I do not know the extent to which your present joist does not meet tile stiffness requirements, I can not speculate the area of necessary joist re-enforcement. You should compare the stiffness of a completely un-reinforced joist with a completely re-enforced joist (see Table provided earlier) to answer the question of how much re-enforcement is required. That is, this information should be referenced to stiffness requirements for your tile floor.

I personally would attempt to re-enforce at least the middle 2/3 of the joist...
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Unread 12-06-2014, 03:57 PM   #78
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When using this technique, should you stay off those joists for 24 hours to allow the adhesive to setup?
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Unread 12-06-2014, 04:09 PM   #79
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I'll avoid loading up the floor for a few hours depending on the complexity of the structural additions. For glued and screwed standard sisters joists no wait is necessary. If I'm building up L or T beams with 3" screws and PL3, I'll let them set up overnight before I release the screw jacks.
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Unread 12-06-2014, 04:50 PM   #80
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You should wait until the adhesive has effectively stabilized. The adhesive manufacturer's data should state this minimum period. I would expect that a wait of 24 hours would generally be adequate for most adhesives...

Now, the maximum curvature due to floor loads would be at the middle if the joist, if it is supported at the joist ends only. Accordingly, only the 'middle' section of the joists is required to be re-enforced. If there is a relatively small difference between joist stiffness requirements and the present stiffness situation for your joists, then only a 'small section' in the middle of the joists needs to be re-enforced. If there is a larger discrepancy, then a larger section along the middle of the joists must be re-enforced.

I realize that this 'recommendation' may not be as definitive as you would want, but without 'quantitative' data, it is difficult to be more precise...
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Unread 12-06-2014, 04:59 PM   #81
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I'll second the waiting, but I'd recommend at least 48 hours. I think that's what PL recommended for their Premium when I researched it for a more critical project.

I recommend the wait time even for sistered joists before removing supports if used.

My opinion; worth price charged.
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Unread 06-11-2019, 11:02 AM   #82
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Deep in the weeds...

Hi all. First post, but I have read through this thread and multiple others on sistering techniques (and joist bracing options), multiple times over the past couple of years. I once sistered a 2x10 joist spanning 20 ft (a TRUE 2 inch wide board, termite damage, 1940's home) with three boards, cut with overlapping 2 foot L's, bonded together with epoxy and vertical lag screws, and then glued and fastened (using TrussLoks) to the weak joist - thus creating a full length joist. When the civil engineer came out, he looked at it and laughed - ended up being way over kill. But it did is job... =)

There is a no one size fits every job rec. The option that is chosen depends on WHY you are modifying your joists (reduce bounce, decrease deflection, increase max load strength, increase sheer strength).

Anywho, on this topic, Ive seen the following options (other than full sistering), for lets say, a 2x10x16 at 16oc --- right at the max of how long a 2x10 should run:

- Placing a 2x4 or 2x6 along the entirety of the bottom of the joist creating a bottom T.

- Placing a 2x4 or 2x6 along the top edge of the joist. While going the full length is best, you can do it in sections as long as the smaller sections bump up against each other firmly and are fastened well to the main joist (crap tone of glue and fasteners).

- Sistering JUST the center of the joist with the same width board (in this case), but shooting for at least 2/3 of the distance (so about a 10 ft board on a 16ft span)

- And, similarly, instead of a 2x on one side, doing 1x on both sides of the beam, including the use of ripped plywood sandwiched between the 1x and joist (if there is room), as plywood has little vertical flex.


BUT!!! In reading about the use of plywood as being stiff on end, I noticed another comment earlier in the thread on the use of a 2x4 as an I-beam. It was something about a 20ft 2x4 being like a wet noodle. And this is true, but it is particularly true if you are holding the board with the grain facing up, right? A 2x4 has much less flex if stress is placed on the board going across the grain vs. with the grain. I mean this is why joists are placed on edge instead of laying flat... (duh).

So my while the 2x4 I-beam increases the depth of the board and increases the surface area, it seems that it is being placed with the grain in a direction that has more flex (and thus less resistant to deflection). Would it be better/worse/any different to rip the 2x4 into 2x2s (very carefully - noodles and all) and then join the 2x2s to the bottom of the joist so that the grain runs parallel to the joist instead of running perpendicular?

I guess, simplified, my question is whether the 2x4 I-beam (continuous, full length) is as strong, less strong, or any different than adhering a 2x2 strip with the grain running in the same direction?

Ill hang up and listen...
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Unread 06-11-2019, 01:06 PM   #83
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Bill:
While others may be able to understand (and hopefully respond) to your question, I can not envision the orientations of 'perpendicular' etc. If possible, could you provide a simple diagram illustrating these terms?
Michael
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Unread 06-11-2019, 03:19 PM   #84
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Welcome, Bill.

Gotta agree with Michael on this one. The longest span you can get out of a nominal 2x4 perpendicular to the grain would be about 3 1/2 inches, so I'm afraid I can't see where you're going with any of that.
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Unread 06-11-2019, 07:26 PM   #85
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Bill,

In my opinion, your questions falls in the category of unachievable outside of a controlled production environment. The glue joints as typically the most error prone part of field adaptions like this thread discusses. Introducing another set of of joints would reduce the odds of a sound end product.

I know for certain production environments can produce glued up joints that are strong that the original wood. Unfortunately that’s just not where we live.
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Unread 06-11-2019, 09:01 PM   #86
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What PC said. I think there is some discussion about that aspect earlier in this thread. Good in theory, lacking in practical application.
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