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Old 11-22-2010, 11:24 AM   #16
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Avoiding sistering using wire rope?


Thanks everyone, this is a challenging and interesting discussion! Both sides have great points and examples/anecdotes.

I would think that the route to try would be allthread forced into a too-narrow slot, so that each tooth of the thread is biting into the wood above and on each side of it, (typically 16 threads per inch in a 5/16inch allthread) then a PL-P bead on top of a hardwood slat of the thickness of the slot, hammered into place beneath it.

I read in another DIY forum about the military attempting similar in-situ joist remediation, they nailed Simpson continuous steel strapping to the bottom of the joist, but they didn't apply any PL-P to it first. The poster said that each nail would be free to crush the bit of wood beside it, allowing the whole strap to stay in place while the wood stretched almost the same amount as before the steel was added, allowing the joist to bend. He said they should have tried it by also adding a thin layer of PL-P to protect the wood/steel junction.

Thanks for all your input, Jp

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Old 11-22-2010, 12:11 PM   #17
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Avoiding sistering using wire rope?


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Since you are increasing the moment of inertia of the beam, you reduce deflection (mnake the beam stiffer), and you increase the ultimate breaking strength of the beam. What makes the problem somewhat more complex with wood is that wood has a different strength in compression than in tension, so adding steel on the tension side has a more complicated effect than with concrete, however the principle is the same, you increase the moment of inertia I.
I find your arguments convincing and must admit that my intial reaction was overly dramatic. One final question.... for this to work, would it necessary for the cable to be continuously bonded (somehow) to the wood or can it be separate, tied-in only at the ends?

I remain highly skeptical that this can be done effectively as a DIY project without a lot of engineering support. Still, the conversation has been interesting.
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Old 11-22-2010, 03:14 PM   #18
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Avoiding sistering using wire rope?


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Originally Posted by jpwarren View Post
Re Ron6519; I'm not attempting to strengthen the joist, just stiffen it by not allowing the bottom of it to stretch lengthwise, without adding any wood or increasing any dimensions. Threaded rod would likely work better than wire rope, since by making the slot narrower than the threaded rod each thread would be biting into the wood once it is forced into place. It probably wouldn't need PL-P above it, just lay a bead in under it once its in place and then run tape over the whole thing to keep the PL in there till it cures. 5/16 inch rod would probably be plenty beefy. I haven't tried it but want to! I'll try it on a scaffold board to see if it stiffens it any.
As much as I haven't really fully taken the time to comprehend what you're proposing in the initial post, when I got to this point in reading the thread, I started to think "Hmm... this is starting to sound ill-advised"

You may want to think about a real danger that might come with localized stiffening of a structural member. Let me refer you to some background material:

http://www.buildingscience.com/docum...d-but-strange/

The part I'm thinking of in particular is the nature of wood to expand and contract with moisture being absorbed and expelled. Now this is normal and natural. What concerns me is if the bottom of the wood is being constrained by anything that would stiffen the bottom, what could happen is the bottom will be relatively fixed in length, the top will be less constrained and try to expand, and what you will get is a piece of wood that wants to not be straight.

When you sister 2 pieces of wood together, you are joining similar materials so when 1 expands, so does the other and the entire piece doesn't warp. If you reinforce a structural member with a steel plate, you also do not have this problem because you're generally covering the entire cross-section.

This is all hypothetical, but before testing these hypothesis on a house I'd have to investigate whether they are real possibilities... But more likely I'd just stick with accepted standard practices.
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Old 11-22-2010, 03:29 PM   #19
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Avoiding sistering using wire rope?


Post tensioned cables in reinforced concrete construction are usually not bonded at all in the hole. Cables which are not bonded are referred to as debonded cables, as opposed to cables which are bonded (referred to naturally as bonded cables). In prestressed concrete, the cables (typically rods) are stressed BEFORE the concrete is placed. When the concrete cures around them, the prestressed rods become continuously bonded to the concrete, similarly to ordinary reinforced concrete. The only difference is that in ordinary reinforced concrete, the rebars have no initial tension.

The bars are tensioned in both pretensioned and post-tensioned concrete, the only difference is when the tensioning occurs, which makes a difference in terms of technique to tension the rods. However, in answer to the question does the bar have to be continuously bonded to improve strength, the answer is no, typically post-tensioned concrete is mostly, sometimes entirely, debonded. The mechanics of bonded versus non-bonded reinforcing are different, but too complex to discuss here.

In terms of wood, you have probably seen cables used to reinforce wood that are effectively post-tensioned. The cables typically have turnbuckles on either end, which are used to tension the cable after installation. I have never seen a pretensioned rod or cable used in wood construction, you would need a very positive glue to hold the rod in place if you tensioned it first, applied glue, and waited for the glue to harden and then released the tensioning device. Not saying it can't be done, I have never seen it, whereas post-tensioned wood cables and rods are pretty common. And just like post-tensioned concrete, the cables and rods used to post-tension wood are usually not bonded to the wood.

As for using Simpson strapping to field fix a joist, this can certainly be done, provided enough nailing is provided to fix the strapping to the beam. The idea would be to tension the strapping enough so that when the beam begins to deflect, the strapping would immediately pick up load. I have never seen this done, but in theory it could work.

Postscript: This is an amusing thread, which I STRONGLY RECOMMEND be left to professionals to attempt, since the outcome of a DIY project is speculative at best. Use of highly tensioned wire is dangerous, reliance on glue to attach steel to wood is tenuous at best, and use of threaded rods is likely to produce difficult to estimate improvements in strength, which should not be relied upon.
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Old 11-23-2010, 08:32 AM   #20
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Avoiding sistering using wire rope?


I'll try it in the shop using a 12ft 2by4, embedding a 10ft allthread. This approach is contemplated for where sistering isn't a ready possibility, as in a crawlspace or for bolstering attic rafters. No tension will be put on the rod, pre or post. It will be held in place via its own teeth and a liberal dose of PL-P. Cheers,
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Old 11-23-2010, 08:23 PM   #21
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Avoiding sistering using wire rope?


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Originally Posted by oberkc View Post
While I might not have used these words in a post, they reflect the thoughts going through my mind.

The two examples I have seen for post-tensioning are in the Sidney Opera House and in the Frank Loyd Wright structure "Falling Water". As I understand such things, there is also a bit of angles and geometry involved in order to achieve additional strenght (or rigidity) in a structure. There is none in the proposed method.

Before embarking on this, I suggest you perform an experiment. Run a cable between two points equal in distance to your crawlspace. Tighten it up comparible to what you think you can achieve in the final installation. I think you will be surprised at how little force it takes to deflect the middle of the cable a few inches.

In my mind, you are wasting your time at best. Cutting grooves in your floor joists is weakening your structure. I recommend holding off on this project.
I was not trying ot belittle the poster, just wanted to make a strong point that it is a total non practical or functional idea as proposed.
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Old 11-24-2010, 07:06 AM   #22
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Avoiding sistering using wire rope?


For a joist to bend does not the upper layer have to go into compression while the lower layer expands? Therefore any material that expands less than wood does under the same load that might be used to replace the wood in the lower section and can be bonded adequately to the remains of the joist above it, will halt or decrease this expansion/stretching from occurring -no? The way I understand it, even if something as flexible as seatbelt webbing were tightly bonded along the bottom of the joist, under no tension whatsoever, this should keep the lower section of the joist from stretching. Of course other issues such as decreased breathability might become relevant, and blocking might be desired to control any lateral buckling tendencies. You strongly hold that it is totally non practical and non functional -can you expand on your reasoning for this conclusion, as you haven't laid it out for us.
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Old 11-24-2010, 08:17 AM   #23
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Avoiding sistering using wire rope?


When a beam is loaded vertically, either by a continuous load (like floor load) or one or more point loads, the upper part of the beam goes into compression, and the lower part goes into tension. This means that the upper part of the beam shortens (compresses), and the lower part of the beam lengthens. The relationship between the change in length and the stress developed is the fundamental law of mechanics, Hookes law, which states that Stress = Strain x Modulus of Elasticity.

In order to develop stress, you have to have strain. No strain, no gain. If you attach a stiff element to the bottom of a wooden beam, like a steel rod, what this means mathematically is that the stiff element has a greater modulus of elasticity than the wood. In the case of steel, the modulus is about 17 times as great as wood.

The bottom part of the beam still extends, and the top part still compresses, its just that the steel picks up load at a smaller strain than the wood, so the steel ends up picking up most of the load until the steel begins to fail, at which point the wood begins to pick up load. The exact ratio of load picked up by the wood and the steel is somewhat complicated, but the essence of the mechanics is that for working loads the steel picks up the majority of the load at a smaller strain than the wood requires, so the net effect is that the beam extends less than it would without the steel, but not zero. If there were zero extension, the steel would not pick up any load at all.

This analysis does not apply if the steel is pretensioned. That is a different case, with a different analysis required.
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Old 11-24-2010, 08:54 AM   #24
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Avoiding sistering using wire rope?


I think, too, that this is all based upon the ability to attach the rod to the joist. Mr Holzman earlier mentioned two scenerios: "Bonded" and "debonded". (I assume that debonded cable require secure attachments at either end of the joist.) Bonded require, I assume, continuous bonding along the length of the joist. I assume from your posts that you are planning to attempt the bonded method.

If so, then your success or failure will depend in great part on the properties of the adhesive. It is my suspicion that it would take a pretty special adhesive to make this work. I am not sure that loctite PL is this adhesive. My suspiction is that the adhesive will not provide sufficient strength in your case to pass the strain from the board to the rod.

I remain skeptical that this can be made to work without some fancy engineering and materials. But I could be wrong.
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Old 11-24-2010, 07:19 PM   #25
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Avoiding sistering using wire rope?


Sorry, I should have made clear that I was asking for input from Troubleseeker, the post directly above mine, but thanks for the clarity and suggestions Daniel and Oberkc. I assume PL-P would be the strongest bond possible, as it typically is stronger than the wood it clings to, tearing away the wood when its removal is attempted.
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Old 11-24-2010, 07:23 PM   #26
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Avoiding sistering using wire rope?


Daniel when you say working loads I assume you mean in opposition to static loads. Working loads being loads that are dynamically in action as opposed to static loads being loads that are applied and then left in a steadily applied state. Working loads of course being the more dangerous in the short run and more detrimental overall to the structure in the long run.
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Old 11-24-2010, 07:48 PM   #27
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Avoiding sistering using wire rope?


Good luck.
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Old 11-26-2010, 05:36 PM   #28
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Avoiding sistering using wire rope?


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Originally Posted by jpwarren View Post
Anyone try slotting the bottom of the joist and PL Premium-ing into the slot a 1/8 inch wire rope for the length of the exposed joist? You could cut the slot twice as deep as the diameter of the rope and then run a bead of PL into the slot and then lay in the wire rope and then add another bead onto the wire rope to encase it in PL. You'd need to tape over the final bead with wide tape and maybe staple-gun it a few times to hold it in place until the PL cures. I guess a threaded-rod (all-thread) might work too, and you could make the slot just wide enough so you'd have to force the all-thread into it. This wire rope as truss rod approach would mean you don't have to deal with the plumbing and wiring, you don't have to man-handle lumber into a crawlspace, and you don't add weight to the joists/rafters. You might need to flatten the tip of the PL canister to get it to fit into the slot, and work quickly to not have it dripping out while you're trying to get the wire rope installed!
Don't do it! It won't work. Do you have room to install a steel flitch plate on it?
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Old 11-26-2010, 09:46 PM   #29
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Avoiding sistering using wire rope?


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For a joist to bend does not the upper layer have to go into compression while the lower layer expands? Therefore any material that expands less than wood does under the same load that might be used to replace the wood in the lower section and can be bonded adequately to the remains of the joist above it, will halt or decrease this expansion/stretching from occurring -no? The way I understand it, even if something as flexible as seatbelt webbing were tightly bonded along the bottom of the joist, under no tension whatsoever, this should keep the lower section of the joist from stretching. Of course other issues such as decreased breathability might become relevant, and blocking might be desired to control any lateral buckling tendencies. You strongly hold that it is totally non practical and non functional -can you expand on your reasoning for this conclusion, as you haven't laid it out for us.
The facts you mention are theoretically true as individual statements, but in order for the cable to offer any support, it would have to be tensioned and then adequately fastened to the wood, so that it could not flex when the load was transferred to it, and I do not see such a connection by burying it in a groove filled with construction adhesive.
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Old 11-27-2010, 08:25 PM   #30
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Avoiding sistering using wire rope?


Here's how its beginning to make sense at this desk: Its given that a load applied to a joist will create compression in the top layers and expansion in the lower layers, resulting in a bend. Same is true with a steel rod suspended at both ends; the rod will experience compression in its upper layers and tension in its lower, and will easily bend in the middle. However, when the rod is embedded within the bottom-most layer of said joist, it now wholly resides within the expansionist layers of the joist -only expansionist. There are now no or very little forces trying to compress the top region of the rod as all forces it is subjected to within its new home in the bottommost layer of the joist are expansionist; the rod is simply placed under tension by the load being applied to the joist above it, albeit likely having differing layers of tension, the highest being along its bottommost surface. We know that the steel rod is very good at resisting tension compared to the wood fibers it replaced, so that tension job is assumed by the steel rod and the joist/rod combination resists bending.

I understand this but I still resist believing it will work.

The bonding is critical to this transfer of force from wood to steel so the teeth and glue are components integral to its long-term success.
Thanks, Jp


Last edited by jpwarren; 11-27-2010 at 08:32 PM. Reason: clarity
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