## Analysis of Cable Balustrade

## Analysis of Cable Balustrade

(OP)

Hello, has anyone had to calculate the tensile stress on a cable balustrade which has UDL applied along its length?

Looking from guidance online most calculations seem to be based on an initial sag, which we don't know and for horizontal loads it will probably be 0mm.

We know the tensile capacity of the cable from the manufacturer but that's all, we are back checking something which has already been constructed on site to keen to keep this as a cable balustrade.

Thanks

Looking from guidance online most calculations seem to be based on an initial sag, which we don't know and for horizontal loads it will probably be 0mm.

We know the tensile capacity of the cable from the manufacturer but that's all, we are back checking something which has already been constructed on site to keen to keep this as a cable balustrade.

Thanks

## RE: Analysis of Cable Balustrade

You must specify the initial sag as part of your design. Alternatively, you can specify the initial tension. Your calculation starts with either assuming an initial sag and then calculating the initial tension, or vice versa. Note that your initial sag cannot be 0mm - as your initial sag is reduced, the initial tension increases...in order to get 0mm you would technically need an infinite tension (for short spans and light cable you can get to very near zero sag, but there is still some).

I don't have time write the full procedure, but the basics are this:

- Use your install conditions to define initial sag/tension (you have an existing condition, measure the sag)

- Use this to determine your initial cable length

- With a UDL, you can use the same equations you've used to calculate initial tension/sag but this time use the UDL instead of the cable-weight. In this calculation, use your initial cable length/sag.

- This will give you a larger tension than you had in the self-weight condition, thus a larger strain and an increase in cable length. Calculate the increase in cable length.

- With the slightly longer cable, you'll have more sag and less tension - and less strain. This is where you need to start iterating for a solution

That is the basis of the procedure. If you have a long stretch of cable you also need to consider large temperature swings as that can have a big impact on the sag/tension.

## RE: Analysis of Cable Balustrade

For a cable guardrail/fence structure, the allowable deflection with the applied point live loading on the cable typically dominates the stress and force calculations. The selfweight of the cable can typically be ignored in situations where a live load is applied.

If it doesn't have to support any loading other than the initial applied, then the solution becomes incredibly simple - just pull the cable to an initial tension within the allowable stress. Unless it's spanning an incredibly long distance, the sag will be virtually imperceptible.

## RE: Analysis of Cable Balustrade

## RE: Analysis of Cable Balustrade

Don't know what building control will think of it but we are trying to prove structurally the cables are fine for people leaning against them etc.

## RE: Analysis of Cable Balustrade

Is the spacing (300mm) connected with the raised height (600mm) ? Is this meant to look like a boxing ring ? (I guess so, since you say that !?)

If I had 3 cables spaced 300mm apart, I'd get 900mm height (you wouldn't have one cable on the floor). The problem I see with this is that it isn't very high, and easy for people to overbalance … 900mm is only about 36". I'd expect at least 5' to have people leaning against it.

Spacing between cables is probably limited by code. 300mm (12") sounds a little large ?

Your original post says "UDL" … does someone leaning on your cable model well as a UDL ? (and not as a point load ?)

another day in paradise, or is paradise one day closer ?

## RE: Analysis of Cable Balustrade

The 300mm spacing was an estimate as I dont have the project infront of me right now. Normally barriers in UK are designed to a height of 1100mm.

I am trying to design for both a UDL and a point load. When designing balustrades according to eurocodes and document pd6688 there are two load values given for this occupancy class, a PL(0.5kN in this instance) and a UDl (0.74kN/m in this instance) so I would be checking both and designing to the worst case as per code.

## RE: Analysis of Cable Balustrade

You better make sure that your lateral load on the one or two top cables is at least equivalent to two heavy guys running across the ring and crashing into the cable. Sooner or later, you can bet that some jokers will make that happen in a gym setting. I’ll bet the wire rope spacing is more like 200, 200 & 200mm. And, as soon as you start the problem, you will see that the biggest issue is going to be anchoring the four corner posts (as cantilevers), so that they don’t just buckle over under the cable loading. How are the wire ropes fixed at the corner posts? You will see that a 100lb. lateral load on the wire rope leads to a wire rope tension of many hundreds of pounds, and thus, a mighty high lateral load on the corner posts. Look for some ref. books on wire rope tension structures for how to approach the wire rope problem.

## RE: Analysis of Cable Balustrade

Yes that's true, we should have a rethink of the load we are looking to apply, bit of engineering judgement.

The posts for the wire rope is 100x100 SHS's. I will need to have a look at the baseplate connections depending on what comes out of this analysis, as some are only fixed into timber.

Product is similar to link at bottomw of this post, same end connections. I will need to check how these are currently being fixed onto the SHS with my colleague tomorrow.

Could you recommend any books? I have been looking everywhere but they all seem to assume an inital sag or have more information before starting the analysis.

Link - https://www.ironmongerydirect.co.uk/product/balust...

## RE: Analysis of Cable Balustrade

## RE: Analysis of Cable Balustrade

Did you not see my first reply? First reply to your post. All of the solutions use initial sag because it is a key parameter. And since this is an existing condition, initial sag should be easy to determine.

## RE: Analysis of Cable Balustrade

How would there be sag in the horizontal direction, would there not only be sag in the vertical? Or do you mean the sag after the horizontal imposed load has been applied?

## RE: Analysis of Cable Balustrade

The initial tension and initial sag are linked together, if you know one, you can calculate the other. Once you know the initial sag, you can calculate the initial cable length.

Once you have the initial cable length, you can start looking at your loaded condition. If it is a UDL, as I mentioned above, use the same equations you have for cable under self weight, but replace the self weight with your UDL. If you have a point load, you can assume your loaded cable forms a triangle between the supports and applied load (from there its easy to calculate the cable tension, basic statics). You can now calculate the tension in the cable, the strain in the cable, and the increase in length. With the increase in length, you now have a new loaded geometry and a new tension/strain/cable length - you need to iterate until this converges.

A few other things to looks out for:

- cable properties - they have an effective area and an effective elastic modulus.

- support movements - small movement in the supports will effect your cable tension

## RE: Analysis of Cable Balustrade

## RE: Analysis of Cable Balustrade

CANPRO - very good description that made it all alot clearer to me now, initial sag is only really to find your initial tension.

HOTROD - Yeah I have seen various handrails with rigid top rail throughout, although never had to design on. I can understand why now maximum deflection if the cable has such a big influence on the design.

Is there a percentage sag you would normally specify for the initial condition, I don't really know what value seems reasonable. 1 percent of 1500mm span is 15mm deflection (initial sag), I thought that might be slightly high.

## RE: Analysis of Cable Balustrade

Whether the amount of initial sag that keeps the loads on the posts and cable acceptably low, is acceptable to the client is up to you to find out.