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Old 11-19-2010, 11:12 PM   #1
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Avoiding sistering using wire rope?


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!

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Old 11-19-2010, 11:19 PM   #2
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Avoiding sistering using wire rope?


Wouldn't add size either. -Maybe there's a thread or forum for people like me who aren't carpenters !

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Old 11-20-2010, 07:09 AM   #3
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Avoiding sistering using wire rope?


You know,that might work--but probably not,without the ability to put tension on the wire--

The bottom line ,however is it would cost more in materials and time than the conventional method of sistering in the new lumber--With known results---Mike---
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Old 11-20-2010, 09:07 AM   #4
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Avoiding sistering using wire rope?


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Originally Posted by jpwarren View Post
Wouldn't add size either. -Maybe there's a thread or forum for people like me who aren't carpenters !
Maybe the Magic forum would help, because it will take a lot more then a wire rope to strengthen a joist.
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Old 11-20-2010, 01:24 PM   #5
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Avoiding sistering using wire rope?


I have heard about using a strip of metal "wrapped around" a joist.
Its fastened on one end roughly parralel to the joist and then going down the length and also around the joist simultaneously to the other side continuing to the other end. Like a stirrup.
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Old 11-20-2010, 01:33 PM   #6
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Avoiding sistering using wire rope?


Quote:
Originally Posted by rodeo View Post
I have heard about using a strip of metal "wrapped around" a joist.
Its fastened on one end roughly parralel to the joist and then going down the length and also around the joist simultaneously to the other side continuing to the other end. Like a stirrup.
I'm curious as to the source of this information.
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Old 11-20-2010, 02:45 PM   #7
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Avoiding sistering using wire rope?


I've never referred anyone to that page 5: http://www.finehomebuilding.com/PDF/Free/021184090.pdf

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Old 11-20-2010, 08:02 PM   #8
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Avoiding sistering using wire rope?


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.
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Old 11-21-2010, 10:50 PM   #9
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Avoiding sistering using wire rope?


Quite honestly, that is just about the most absurred thing I have ever heard of. Sounds as accurate as some of the advice I read in the "Ask the Handyman" column in the Saturday real estate section of the paper.
I guess someone is basing this thinking on the principle of post stressed concrete construction, but the tendons used for that are tensioned to tens of thousands of pounds.
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Old 11-21-2010, 11:19 PM   #10
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Avoiding sistering using wire rope?


Say if your going to test it on scaffolding get it on Video , might be good for a laugh.
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Old 11-22-2010, 07:06 AM   #11
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Avoiding sistering using wire rope?


Quote:
Quite honestly, that is just about the most absurred thing I have ever heard of.
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.
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Old 11-22-2010, 09:32 AM   #12
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Avoiding sistering using wire rope?


You know, the execution part of the proposal is weak, but the idea is sound. It is not uncommon to strengthen large timber beams using post tensioned cables run through grooves in the wood. This is sometimes done for seismic reinforcing, or occasionally to strengthen an undersized beam where there is no way to sister another beam in place.

Because steel is so much stronger than wood, it is possible to add considerable strength using relatively small diameter cable. Of course, there are significant structural issues that have to be addressed, like buckling of the wood, crushing, and how to install the groove, but these issues can be addressed.

By the way, I am not suggesting this is a practical DIY idea in this case, it isn't, but the use of cable to strengthen wood is a well proven concept, and can be advantageous under the right conditions.
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Old 11-22-2010, 09:58 AM   #13
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Avoiding sistering using wire rope?


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This is sometimes done for seismic reinforcing, or occasionally to strengthen an undersized beam where there is no way to sister another beam in place.
This is interesting.

In the case such as this where the desire is to add "stiffness" in bending, however, would not one require the wire to have some vertical element (a truss-like structure) in the path of the wire? I am having trouble believing that a wire, tensioned straight across horizontally, can provide a practical benefit in the verical direction. All the examples I have seen of this, include an angle in the path of the wire to resist the vertical forces.

Is not the purpose of seismic reinforcement more to keep structural member tied together (walls to joists, for example) rather than increasing load-bearing capacity of beams?

If I were really itching to try cable re-inforcement of a structure such as this, I think I would tie a cable at each end of the span, with a bit of slack, then add tension and angles by adding a vertical structural member below the center point of the joist and attached to the cable, creating a bit of a triangle structure much like a truss.

Last edited by oberkc; 11-22-2010 at 10:13 AM.
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Old 11-22-2010, 10:02 AM   #14
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Avoiding sistering using wire rope?


A cable run horizontally in a groove or hole in a horizontal beam requires a firm anchor at each end. The cable will exert a horizontal pull at both ends when weight is placed at the middle of the cable. There would be a tendency of the calbe to tear loose from its end mountings, or to pull the side walls or foundation walls inward if attached directly to those. Still, the cable will not prevent sagging of the beam.

Whereas a beam itself puts no horizontal pull on the foundation walls or house walls but rather puts downward pressure at both ends upon said walls.

Using a sister makes it more difficult for the beam to sag in the middle because the sister must sag together with the original beam. The sister must be quite long so it together with the compromised beam does not merely result in a pivoting linkage (beam to left of compromise, sister, beam to right of compromise) that offers little or no sag resistance.

Starting with a horizontal cable, downward force in the middle starts by putting tremendous inward pull with small lenghtwise movement on each end; as the cable starts to sag due to stretching or due to intentional give at the anchors, the force on each end decreases.

When a cable is used as part of a truss, that part has a tendency to stretch lengthwise (say, horizontally) by a large amount as a load (say vertical) is put on the truss and the pull is not that tremendous. Here, the cable functions well at resisting such stretching.

Another use for a cable running horizontally in or around joists is to keep the side walls from separating as a result of an arched or peaked roof sagging, say, under snow load. For a typical home, properly installed joists will hold the walls the proper distance apart without the need for cables.
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Last edited by AllanJ; 11-22-2010 at 10:27 AM.
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Old 11-22-2010, 10:41 AM   #15
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Avoiding sistering using wire rope?


I think there is a lot of confusion about the mechanics of post tensioned cables. Even if the cable is not tensioned, as in ordinary reinforced concrete, the cable adds to the moment of inertia of the element. Since the cable is typically installed near the edge of the beam, not typically in the center, a small amount of steel adds substantially to the moment of inertia of the beam, whether the beam is wood, concrete, or plastic.

Since steel has a modulus of elasticity of about 29 million psi, versus about 1.8 million psi for wood, each square inch of steel is equivalent to about 16 square inches of wood (see transformed section concept in a mechanics of materials text). By placing the steel near the edge on the tension side, you maximize the strength of the steel (the Ay concept discussed in mechanics text books).

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.

Post tensioning and draping (non-horizontal steel strands) are done for reasons other than introducing a vertical component to the strand. This is way too complicated for this forum, but suffice it to say you can use perfectly horizontal steel strands and still increase the moment of inertia of a beam. You can either prestress the steel, post-tension the steel, or simply embed it non-tensioned. Different techniques are used to achieve different specific design goals.

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