Oxy-Acetylene brazing of 270 ksi 7-wire steel strand in butt joint

[Ingenuity]

I have to butt-join two different diameter 7-wire high-tensile steel strands (270 ksi tensile) to each other. The smaller diameter strand - 3/8" dia - is a 'fishing' strand to assist installing the larger - 1/2" dia - to be installed into a 1/2" dia greased-and-sheathed HDPE conduit embedded in concrete (aka: repair of unbonded post-tensioning tendons).

I have done this many times - works well about 95% of the time - with the 5% failure typically due to me rushing on the prep of the strand and not getting good bond. There are no life-safety issues with this short-term temporary application, but it is a pain to have to fish out the strands and re-do the braze, and start the install application again.

I have a project with multiple 300 foot long strands to be repaired and 'kinda-don't-want-to' have the braze joint fail part way into the install.

What I do is un-ravel the outer wires of the 1/2" strand to expose the king (center) wire, and cut it back about 2" from the outer wires. On the 3/8" strand I un-ravel the outer wires and cut them back 2" from the king wire. Acetone clean all wires, then insert the 3/8" male king wire into the 1/2" king wire female, butt-end all the outer wires, then braze the joint. See following graphic:

I figure maybe I can get 50 ksi tensile capacity from a good braze, and if only assuming butt-joint capacity of the outer wires of the 3/8" strand I can maybe 4 kips of tensile capacity.

Is there a better way to make this joint stronger to reduce my 5% failure rate?

Worth much additional capacity in increasing the male-female insert length greater than 2"? The interstice space of the two varying diameter king wires may not be ideal for brazing - I am not really sure.

I may well do a few sample brazing tests and load test them in a center-hole ram to see how they compare.

 

[racookpe1978]

Will the wire splice EVER go over a pully or be twisted or bent?

I'd really question any "splice" not approved for crane or lift rigging. Or a suspension-type cable splice.

 

[Ingenuity]

Thansk for the reply, racookpe1978.

The application does not have any crane, lifting or suspension loads, nor will it be ever go over a pulley.

It is for installing (using an external hydraulic jack) a new strand to a tendon in a concrete building - the smaller strand is simply a 'fishing' line to pull through the larger strand diameter. After successful installation the splice is cut and removed.

Forces and displacements are monitored at the hydraulic jack end, and displacements only, at the opposite end.

 

[berkshire]

I have done the same thing by welding rather than brazing , but only where the heat treated area is cut off and discarded afterwards.
B.E.

You are judged not by what you know, but by what you can do.

 

[Tmoose]

If I understand the joint correctly, I don't think the king wires butting together count for much.
I think the 2" over overlap is way more effective, especially if I was able to clamp the very ends of the hollow outer wires to the naked king, and twist the fishing strand so the outer wires cling tight to the extended king wire before finish brazing.
Clamps -

https://www.grainger.com/category/ear-clamps/hose-clamps/pneumatics/ecatalog/N-afwZ1z0qh6h

Twist to tighten-

https://encrypted-tbn0.gstatic.com/...NJ6y81aoVWAOPFVpEoz7pIZvC-b71qJ4warnLEKJcEv8A

In fact I'd choose to extend whichever king had the most unruly or stiff outer wires, unless that combo is more likely to create a brazed joint with wires oriented to more easily snag during the pulling process. .

What brazing rod do you use ?
Is it flux coated ?
I'd be tempted to tin the naked king before assembling the joint, clamp and twist, and use a liquid flux to creep into the tightly wound bunch of outer wires before brazing.

 

[EdStainless]

TM makes some great points.
You will improve the quality of the braze if you pre-wet the surfaces with braze before you try to join them.
When you lay the strands over the larger king for the joint you need to use some wire to wrap the free ends so that you can assure that they lay tightly along the king and not splayed out. This will give you more surface area to braze.

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P.E. Metallurgy, Plymouth Tube

 

[racookpe1978]

If you are fishing the wire through piping/conduit/"holes in poured concrete" the problem changes from "ensuring you have sufficient strength not to separate" but "how to get enough strength WITHOUT making a bundle or rough surface that will get stuck".

A very smooth, low-friction outer surface of the final joint becomes most important.
Unraveling the strands to join the center cable is very low priority.

 

[dhengr]

Ingenuity:
I’ll bet that there is a fairly light and inexpensive swaging system that might serve your purpose without all the wire prep. and welding. The electrical people use these on transmission line splicing, without too great an increase in wire dia. Also, I’ll bet that there is a wire pulling system like the Chinese finger-locking puzzle device. The HDD people use these for pulling wire underground.

 

[kingnero]

^^^ Chinese finger-locking device exists for electrical cables, I am sure there work on steel cables if you use the proper insert (the things you see on the attached pic are made of two pieces: one piece you wrap around the cable, the second part (with the eye) you wrap around the first part.

 

[Ingenuity]

Thanks for the replies.

We often use Chinese-type finger-locks - aka cable pulling-socks - but typically for a group of strands in a bundle, like say 12 x 1/2" strands. The largest I have done is 3 each @ 10 strands over a distance of 1,000 feet for a bridge. Lots of friction - pulled with a D8 Cat dozer!

But pulling-socks don't work in this monostrand application - the HDPE sheath is tight-fit to the strand, except for a thin layer of corrosion-protection grease, so max dia that can be pulled has to NOT exceed the 1/2" strand diameter.

The strand wires have significant stiffness with a defined 'lay' such that it does not require clamps to re-secure the strand pattern. Easily un-raveled and re-raveled without destroying or deforming the lay pattern.

I did a trial braze and cleaned up the joint:

Did a quick tensile test with a 10 ton center-hole ram:

As expected, failure occurred through the braze joint where the outer-wires butt, and interestingly the king-wire of the 3/8" strand fractured, so effectively the 2" king-wire over-lap onto the 1/2" strand resulted in sufficient strength to result in fracture of the king-wire.

The failure load was approximately 4.5 kips. There was a small inclusion in the braze, but probably not significant in the capacity.

Ultimate tensile strength of the 3/8" strand is about 23 kips.

The capacity of the splice has to greater than the accumulated friction resistance over the pull length. I can work with a splice capacity of about 4 kips or less - I think!

So I am going to use 2" king-wire overlap and see how is goes in the field.

Thanks for the assistance everyone.

 

[Compositepro]

If you want to increase the strength of the joint use the king wire from the 1/2" cable to penetrate the 3/8" cable. It is a larger and stronger wire. This will increase the diameter of the 3/8" cable at the joint, but this diameter is still smaller than the 1/2" cable.

For yet more strength, stagger the lengths of of the six outer wires (cut-off 2" of every other wire)on both cables. That way you get way more shear transfer between overlapped wires. It does not seem much more complicated than what you are doing now.

 

[Ingenuity]

Compositepro, interestingly, last night, (before I saw your post) I actually brazed the larger king-wire from the 1/2" strand into the 3/8" dia strand and tested it this morning and achieved a tensile capacity of approx 10,000 lb.f. - about 2x more than I need.

The capacity of the larger dia king wire in only about 6.5 kips for 1/2" dia strands - so the braze of the butt-jointed outer wires provided about a 50% additional capacity across the joints failure plane.

With varying diameter strands the 'lay' of the strand outer wires differ by more than 2" over approx 6", so makes staggered-laps of the outer wires tough, and also some strands are RHS lay, and others are LHS lay. Addiitonally, the braze is done in the field, in exterior conditions, often at elevation, so the easier the joint connection, the better. However, I agree, a superior and stronger splice would be achieved via staggered-laps of the outer wires. Thanks for the idea.

With 10 kips splice capacity I am good to go.

Thanks all.

 

[Compositepro]

Good point. But even 1/4" of overlap would be a significant improvement. However, the problem now appears to be fixed.

 

[EdStainless]

It does sound like you have this sorted.
If the braze becomes problematic you could consider using a crimp connection joining the full 3/8" cable to the king wire of the 1/2".
I have used connectors that have an abrasive inside of them and crimp with a hydraulic ram and die.

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P.E. Metallurgy, Plymouth Tube