handrail wire rope infill - design question 4

[cliff234]

I am designing a handrail that has a 5mm wire rope infill. The wire ropes will be pre-tensioned. The variables are the rope spacing and the span of the rope between the vertical posts.

I have been told that I have to design the wire rope to be sufficiently taut so that a 4" diameter sphere can't be pushed between two ropes. What is the force that I am applying to push this theoretical sphere through the rope? The harder I have to push it, the larger the required rope pretension force will be to prevent the sphere from passing through.

 

[jayrod12]

Wouldn't it make sense for it to be the 200lb concentrated load they make you design the handrail for?

 

[SteelPE]

What direction are your cables running?

Typically, the IBC requires a 50# load over a 1 square foot area.... so a 4" dia ball would have approx 4.4lbs of force behind it. Not much, but it's not an easy problem to solve. I found the information here to be helpful:

http://thecableconnection.com/data-on-cable-flex.html

Good luck

 

[SteelPE]

FYI, the 50# is the load IBC requires for infill, which is what you have here.

 

[SAIL3]

Yes, as stated, it is a theoretically complex problem. I would tend to approach it from a deflection standpoint.
Say, I space the wires 3" apart and design for a max opening of 3.5".......this would be a 1/4" deflection in each wire with the other 1/2" as a safety factor.Depending on the span of the wire this could require a significant pretension in the wire and a resulting moment in the last post in the system.This would be an iterative process to get to a pretension one can live with.
The next step is to take that design deflection and translate it into a point load on the wire and then check that point load with the 50# requirement from the codes....using the 3.0" sp as an example,........P=50x3/12=12.5#...so maybe the required pretension will not be as bad as first assumed.

 

[cliff234]

The cables are 3" apart and they are running horizontally. The vertical posts through which the cables are threaded are about 5' apart.

 

[JStephen]

I interpret the 4" sphere idea as a purely geometrical constraint, not related to strength or stiffness of the handrail.

 

[racookpe1978]

NO!

The 4 inch "rule" is a bureaucratic interpretation of the very real need to prevent a baby's head from being trapped between the bars of stairway railing ... Theory is: If a baby can't get her through the bars, the baby's head can't get stuck, the baby won't choke - nor can it get its body through the bars and fall downstairs. A second part of the requirement "against" horizontal bars (wire in your case) is the lesser-know rule that the horizontals can't be used as a ladder by a baby/infant/toddler/small child. Why 4 inch sphere? Its also an approximation: no real head is shaped like that at any age!

So, you need to make a reasonable estimate of how hard a baby can push. Add a conservative factor to that force, then use it as your force. Keep your notes. The code (by itself) is not smart enough to have figured this out already.

 

[cliff234]

The link that SteelPE provided is excellent. (Thanks Steel PE - and thank you everyone else...)

The force applied to the 4" sphere is about 9#.

The procedure is straightforward but intense.

 

[paddingtongreen]

It is a nonsense problem. There is no starting point. Without a starting force, you cannot select wire spacing, pretension or anchor spacing.

There must be a starting point.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[XR250]

I always show "cable rail by others" on my drawing so the question does not get asked to me.
Don't forget to check deflection in the end posts as they may give enough to be significant.

 

[XR250]

Looking at SteelPE's post, they are assuming 3-400lbs of tension on each cable or about 2400 lbs of constant tension on the end posts if all cables are tightened to that value. A 6x6 will creep over time under that load which will slacken up the cables. Hopefully, tube steel will be used. I have never seen anyone crack the cables down that tight, however.

 

[SteelPE]

Excel, the formulas can be changed to allow for less of a pretension force by reducing the spacing of the cables, or reducing the spacing of the posts, or increasing the size of the cable.

While the method may be intense, it is really simple once you write a MathCAD spreadsheet.

 

[cliff234]

I missed the complexity of this when we gave our fee for designing these cables. Yes. In the future we will say "cable rail by others". Yes, the end posts are steel, and yes, we will make sure that they can resist the significant tension force from the cables.

I wish I could find some sort of chart that would make this easier.

Thanks again everyone.

 

[Ron]

A 5-foot spacing on the posts presents a large moment for the posts (50 plf x 5 ft x 42 inches height....10,500 lb-in). Be careful with that.

As for the cables, I use the approach that Sail3 described.

 

[JStephen]

Interesting post. In the article cited, they mention that the codes do not specify any load associated with the passing of the sphere, and that's why I've interpreted it as a geometrical constraint only (note that I don't deal with cable railings). But I note that in the discussion above, we have suggestions to use 200 lbs, 4.4 lbs, 50 lbs, and 9 lbs as the appropriate load to be applied. My guess is that this was originally written assuming solid railings and thus the issue wasn't addressed at all. And come to think of it, do any of you ever check solid railings for deflection from spheres being forced through at specified loads?

If I remember right, that sphere size changes to 12" for areas not accessible to the public and to 21" for industrial areas. I would assume in those cases that any specified railing loads were applied directly to the rail and not to a frictionless sphere being forced between the rails.

In certain instances, OSHA allows wire rope toprails, and googling around on the OSHA site shows some requirements of a maximum 3" deflection with the design load in those cases.

 

[dik]

SPE... excellent link. Information, galore... a well deserved star...

Dik

 

[dcarr82775]

I am on the side that the Code requires 50 psf loading AND a 4" sphere shall not pass. They are two individual requirements not linked.

 

[XR250]

Around here we are allowed 5 1/2" spacing for horizontal members and 4" for vertical. Has something changed.

 

[XR250]

oh, maybe that is for residential.