Analysis of cable suspension structure supporting a mass when subjected to wind load.

[Skellen7]

Hi All,

I was hoping for some assistance/discussion on how to tackle the analysis of a cable supporting structure.

Our practice has been asked to undertake the design of a new suspension structure for a cross to the front facade of a church (see picture).

The cross is reasonably lightweight, made from timber (final details tbc) and was originally suspended from the roof structure using a series of steel cables, triangulated in each orthogonal direction to provide stability.

The structure unfortunately failed in recent high winds, hence why we’ve been asked to design a new supporting structure.

I’ve no issue determining forces in the triangulated cables due to weight of the object, but become slightly unstuck when applying lateral loading (I.e wind!), aside from just triangulating the support structures - the last thing I’d want is to turn this into a wrecking ball!

Also if anyone is aware of any design codes or relevant texts which might assist, then that would be great!

Thank you for your time!

 

[rb1957]

it looks as though it was well stayed, 4 cables on three ends.

What failed ? did the cables part ? did the anchors let go ? did the cross itself fail ?

How much tension in the cables ? turnbuckles, to add tension ?

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

 

[Skellen7]

From what I understand / have seen so far, the cable connections back to the roof structure failed.

I’ve yet to do an inspection to be sure, but it looks like the connections weren’t particularly good and degraded over time anyway.

The cross is absolutely fine too, it looks like one of the connections failed and that un-stitched everything.

 

[azcats]

Something just looks fundamentally 'wonky' to me here.

 

[JStephen]

I think the appearance is pretty cool.
I would think failure would be dynamic/vibration issues, not just a steady pull.
And, that might be hard to come up with a good analysis for.
I would be tempted to estimate the strength of the deteriorated anchors, versus the estimated wind speed, and see what kind of upgrade would be required to avoid it.
Which reminds me of the Calvin and Hobbes comic about establishing weight limits by driving bigger and bigger trucks over the bridge.

 

[Skellen7]

Haha so true..

Well it’s apparently been suspended from the church since it was built in the 1960’s so I’m sure there will be the usual resistance if we say it doesn’t work!

To be honest, I want to see something rigid in there… almost like an inverted guyed mast.

 

[ThomasH]

Hi
From what I see, you have a fairly lightweight cross hanging in a number of cables. An the only stabilizing load counteracting the wind is actually gravity.
So a wind component acting upwards could cause all sorts of interesting things .
I suspect there can also be fatigue involved.

Thomas

 

[ThomasH]

Hi again
I thought of a simple solution that might help you.

If I understood correct your support consists only of cables, they can only support tension. If you can replace a select few of them with rods that can support both tension and compression that might do the trick. If some of the "cables" can support compression they can "lock" the others ad support the lateral load.

Thomas

 

[RangeRock]

rb1957 is onto the right idea... Add turn-buckles so you can pre-tension the cables to create all tension members. When winded area is loaded from any direction, design cables to resist in tension opposite to winded surface. Would also ensure the connections can handle sustained live-loads and attach to something solid that wont degrade over time (ie. welded plate to (ex.) steel or through bolts)

 

[BAretired]

I don't see any cables angling downward. That would help to stop the cross bouncing around in the wind, but not sure what they could be anchored to.

BA

 

[dhengr]

Skellen7:
You don’t seem to have any guy wires to the lower tip of the vert. leg of the cross. And, I suspect what is happening is..., although it took a lot of years to show up, since the bot. is unsupported, it tends to act a bit like a pendulum swinging N-S (in-out of the photo) or E-W (rt.-left in photo), depending on the wind direction. N-S would have (might have) a pivot point about at the level of the of the cross cross-arm ends, and yank on the four guy wires at the top of the cross; E-W might have a pivot point at the top and yank on the cross arm wires. I would think someone might have heard the cross bouncing around a bit in a strong wind. I’m not suggesting the cross was dancing in the wind, but it might have been moving enough to put a slight impacting load on one of the support connections. What does the cross weigh and what are the guy wire and connection details and materials? What are the main bldg. frames made of, wood or steel? After 60 years, you don’t have to improve things much, you just have to settle down the pendulum motion and tighten/adjust the guy wires a bit.

Study the failed support connection, why did it fail, corrosion, wear on the cable, wood rot, need for longer eye bolt into wood, proper flashing and caulking, etc., can it be made a little stronger? I would add four guy wires near the bot. of the vert. leg of the cross, two to the north frame and two to the south frame, not to carry much weight, so inclined up only slightly, but to control the pendulum motions. There should be some means of tightening the individual guy wires slightly, maybe turnbuckles or couplings of some sort. Cable tension structures work because you have several cables working against each other (or in unison) to hold things in place. Then you tighten them to control deflection or movements. Also, for example, the top four cables are well positioned and triangulated to carry the weight of the cross, but are considerable less efficient at controlling lateral movement of the top, because of their steep slope. Tighten the top four cables first, to plumb the cross and take its weight. Then, in a systematic fashion snug groups of opposing cables without moving the cross out of position.

 

[Skellen7]

Thank you all so much for your replies - really insightful & helpful stuff!

I’ve got a site visit to inspect the roof & cross and will report back out of interest!

Thank you all again.