Cable Guardrail Design 4

[dik]

Does anyone have a reference for designing guardrails using 3/16 stainless steel aircraft cable as noted below. Properties of the cable will be determined.

The total length of cable is 50'-60' horizontal. The cable is supported by posts at 4' o/c vertical and the cable passes through the posts without hinderance (or friction). The cables are anchored at the ends by posts. The cables are located at 3" o/c horizontal. The cable spacing must prevent a 4" dia sphere from passing. I'm looking at determining the tension required to prevent passing of the sphere. Can anyone suggest a paper?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Celt83]

PTI has Technote 14: Link

 

[dik]

thanks Celt...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

In order to calculate the tension required, you need to know the force pushing on the sphere you're trying to fit though.

 

[dik]

Yes the sphere acts as a 'wedge'. I was going to treat the cable as an element with the stipulated point load, as if from the sphere.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[XR250]

With 60 ft of length I would be surprised if you can make this work due to the amount of elongation.
I think in practice most contractors just use the "looks good to me" approach for tightening and will ignore any unrealistic crap the engineer asks for.

 

[dik]

The 60' was representative of a length. I'll plug in lengths that will work and limit the total length between anchors to whatever value works. Thanks...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Deker]

Try this: Link.

 

[BridgeSmith]

That's great, Deker! I was just realizing that calculating the force was going to get very complicated, and require us to know the coefficient of friction between the cable and the sphere, as well as the force on the sphere, to get the cable tension. As long as the inspector's 4" ball isn't super slippery, that ought to work.

 

[XR250]

Hmm, their tests do not indicate Lct - total cable length - which I imagine would affect the results.
Funny story. I used that UltraTec cable rail in my house (the one in the link). I had to rent their special $400/week swaging machine for the ends which was basically a modified hydraulic nut splitter. Was installing the cables in the basement and one of the hydraulic lines burst. Made a complete mess of the basement and me.

 

[BridgeSmith]

Hmm, their tests do not indicate Lct - total cable length - which I imagine would affect the results.

The tension in the cable was held constant for the tests, which would be analogous to a situation where you had a cable of infinite length. In a real-world installation, the tension in the cables would increase as the sphere was pushed through. For a long cable length, the increase would be small, and likely wouldn't affect the outcome substantially, since as sphere start to pull through, the angle of the force on the sphere approaches zero, so when the stretch of the cable (and its tension) is at it's max, the pull force goes to zero. If the cable length was short, the tension could increase, which would increase the pull force required. IOW, the test results are conservative; very slightly conservative for a long cable length, and a little more conservative for a short length.

 

[dik]

I agree... like a wedge, as noted

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[XR250]

mmm, IDK. The cable tension was not held constant. The recorded tension appears to be the tension in the cable before the pass thru of the sphere which undoubtably would have increased the tension.

"which would be analogous to a situation where you had a cable of infinite length"

An infinitely long cable cannot be tensioned.

 

[BridgeSmith]

An infinitely long cable cannot be tensioned.

An infinitely long cable can't be manufactured, either. No need to be argumentative over semantics. Ok, how about the longer the cable, the less the tension in the cable changes when the cable deflects at one point for the sphere to pass through? Does that make you happy? Anyway, the length of the cable is one of the parameters in the equation that was ultimately derived and calibrated to the test results.

Of course, friction between the cable and sleeves where it passes through the intermediate posts isn't accounted for, either, so it's not going to be perfect. No engineering model is ever perfect.

I still think using the results of the testing provides a good approximation of the tension the OP will need to design for in order to satisfy the criteria.

 

[dik]

Well, if you could live forever?

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[BridgeSmith]

If the OP is so inclined, the equations necessary to calculate the force applied to the steel sphere for a given tension are on pages 12 and 15. If someone really wanted to be an overachiever and had too much time on their hands, the equations could be manipulated into one horrendous equation to get the required tension for a given force on the sphere.

 

[dik]

Thanks, Bridge...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Craig_H]

I'm still surprised that these are allowed. Not sure about in the US of A, but in Canada there are clear requirements about guards not being "climbable" (the intent being that children should not be able to easily climb up the guard). If the cables are tensioned enough to resist the 4" sphere, these cabes form a pretty good ladder.

For us Canucks, the code references are NBCC 2020 3.3.1.18.4, 3.4.6.6.7, 3.4.7.6.5, and 9.8.8.6.

Edit: had a closer read and the "no climbable features" detailing is only required where the drop is >4.2m. I guess it's OK for kids to fall ~13'-9"? That seems like a pretty fatal fall distance to me...

 

[TLHS]

dik,

Don't know what Manitoba has, but I know the provinces often lean on references from OHS orgs in the other provinces. WorksafeBC has special requirements from anything using rope or cable that would apply to things being used in a place of work.

https://www.worksafebc.com/en/law-p...ulation/part-04-general-conditions#schedule4A

Mostly stuff I'm pretty sure you'd cover, but not a bad reference for the standard of care regarding deliverables and directions.

It kind of implies that you can't use them for permanent installations in a workplace since they need to be tension checked each work shift. I'm not sure if that was actually their intent with regards to the type of system you're looking at. If it's fully designed based on the building code requirements or CSA A500 it seems like it should be functionally suitable regardless and there's likely a justifiable path you could find through the codes. The interface between OHS/Worksafe and building code has always been weird with regards to guards and resolving that might be hard if you feel the need to do so. At least they're resolved the major dimensional conflicts and loading stuff between the two here at least.

If it's primarily a residence or something, this doesn't come into play at all.

edit: for anyone that is interested, this is the practice guidelines for guards that EGBC put out. There was a run of discipline findings a while back about guards that led to this.

https://www.egbc.ca/getmedia/b73c48...-Designing-Guards-for-Buildings-V2-0.pdf.aspx

 

[Deker]

The ICC CTC has performed a study on the issue of climbable railing and determined the risk to be low: Link.