Custom Prestressing strand coupler 1

[precast123]

Hello everyone

I have designed a coupler to join 2 prestressing strands to avoid wastage ( see attachment )
1. For 9.3 mm dia strand
a. Load Capacity = 100kN ( strand A= 52mm2, fpu = 1770 N/mm2, 70% tensioning only)
b. Base thickness of plate = 15mm (limited to 15mm to maintain strand cover)
c. wall thickness = 20mm
d. Plate S355 with min yield strength = 355 N/mm2
e. Fillet weld on both sides all around with strength 390 N/mm2 (SG2)
f. From SAP2000 model, max stresses are in range of 190N/mm2

2. For 12.7 mm dia strand ( strand A= 97 mm2, fpu = 1770 N/mm2, 70% tensioning only)
a. Load Capacity = 200kN
b. Base thickness of plate = 15mm (limited to 15mm to maintain strand cover)
c. side wall thickness = 20mm and front wall where the barrel will fix = 30mm
d. Plate S355 with min yield strength = 355 N/mm2
e. Fillet weld on both sides all around with strength 390 N/mm2 (SG2)
f. From SAP2000 model, max stresses are in range of 190N/mm2

My questions are:
1. Did I choose the correct plate for this purpose? Any suggestions? which plate can perform better in welding ect
2. Which rod to choose for welding the plate?
3. Are the stresses in plate and weld are OK?
4. Will this coupler system will work or i am missing something?

 

[Ingenuity]

Hi Khaleel045

As an alternative, have you considered the use of a strand splice reducer, which is a multi-use strand splice chuck where one end fits 12.7 mm dia strand and the other fits 9.3mm dia strand. Also called a "double ended joint" outside the US.

The cost less than US$100 and common in the pretensioning industry.

Link

 

[precast123]

Thanks for reply. Production manager is suggesting me to make this because he thinks that the coupler your are talking about is not safe. There is not enough projection of strand beyond the anchor after inserting the strand. He want the strand to be projected at least 250mm beyond the anchor to avoid accident. So for that reason I made the box length to 300mm. I want to join 9.3 strand with 9.3 and 12.7 with 12.7.

 

[Ingenuity]

I personally feel that your production manager should re-think his position on the use of tried-and-tested multi-use splice chucks.

They are used daily, the world-over, without incident. All have a spring lock, and some use safety pegs to ensure full strand biting. They require cleaning after each use, but so does everything in such applications.

Compare that with your proposed custom coupler that is UNtested and still depends on barrel chucks to grip the strand.

 

[precast123]

What if I give him option to choose between this two with the points you mentioned and ask him to test the box in safe conditions if he choose to use the box? I will submit him a design report mentioning that the box should be tested before using. From the calculations its appearing to be safe. The stresses in Plate and weld. I am just afraid in practical it will work or no.

 

[Ingenuity]

khaleel045,

If the production manager chooses to go with the custom splice box, you are going to have to test it, possibly to a proof load of about 2 x maximum expected applied load, and depending on what part of the world this shall be used, it may require independent laboratory and/or prototype load testing too.

Your also need to consider the maintenance of this custom splice box, and procedures to be taken by experienced personnel to check daily for wear, distortion etc.

 

[precast123]

Thanks for your suggestions. It is his head ache how to test it. I will give him the load 2 times max expected load to test the box.

Its in Bahrain. Extremely humid place. Box must be galvanized. And the maintenance note I will put for him.
Thanks!

 

[rapt]

2 points

The coupler must be designed for a load of at least 110% of the strand strength plus material factors etc.

How are the wedges held in place and set?

 

[jonkirk31]

I would be seriously hesitant to use some sort of custom designed coupler that you are looking at as opposed to tried and true proprietary devices used successfully all over the world. I'm sure your production manager has good reasoning for his feelings about the lack of safety about proprietary devices, but with all due respect he is just plain wrong. I would try to reason with him about the liability of having his own device versus that of something that has extensive history of safe use.

I'm not trying to say your design is unsafe or not logical, by all accounts it appears to make sense. But the concept of basically reinventing something that already exists and has extensive history of safe use carries a high level of risk just seems like an unnecessary liability concern.

 

[Brad805]

A basic risk vs. reward evaluation should lead your supervisor to the realization this is a bad idea. One failure, and any profits made by this venture will vanish and your company will be digging far into your pockets to pay lawyers or settlements. I would suggest a review of typical product development. It is not a simple case of an engineer doing a design, some simple tests and off to the races. Companies in this market spend years developing products because of risk.

 

[precast123]

After reading all your comments.. I am dropping the idea of submitting this box to him. Instead will convince him for couplers already in market. We are using Paul wedges and barells. They are also selling this couplers with small holes to check strands for safety.

 

[Ingenuity]

Good decision.

PAUL (from Germany) are a well recognised and long-time manufacturer of pretensioning components and hardware.

With competition from SURELOCK (US manufacturer) and CCL (UK manufacturer) the prices for 12.7mm couplers are about US$80 each - just took delivery of some last week.

Engineering, reliability and safety trumps production!

 

[precast123]

This one

 

[Ingenuity]

Yep. And make sure you purchase additional wedge sets to suit, along with cleaning kits (wire brushes for the wedge teeth and barrel) and PAUL can probably recommend a suitable grease to use with their barrel to increase it service life.

The splice chucks (and any re-usable chuck or barrel) needs to be cleaned and maintained after each use.

Best of luck.

 

[precast123]

Yes.. Thanks.