Steel Suspension Bridge

[dcarr82775]

Somebody posted a picture of this suspension bridge over in the Earth retention Forum. I was looking at the anchorage in horror. Went looking on the internet, and found the following

http://delsurnewsonline.com/san-juan-del-sur-foot-bridge-collapses/

Sadly, not too surprising

 

[Lomarandil]

I'm curious what the actual failure looked like. I'm not a big fan of the embedded rebar or the alternating Crosby clips, but I'm not sure that either of those are obvious mechanisms.

 

[Canuck67]

The majority of the Crosby Clips were installed improperly. I have been taught to "never saddle a dead horse", i.e. the saddle is not to be installed on the dead end of the wire as that results in a pinch of the live end of the wire at the first clip. I wonder if it was the rebar or the actual wire that let go, perhaps even the wire at the anchorage clip locations.

 

[LittleInch]

Is it me or in the photo does the top loop of re-bar spliced with wire to the "thinner?" single bar coming out from the back.??

Who actually thought this was a good idea? It has stress concentration and work hardening /fatigue written all over it. Not to mention what seems like a very small radius at the top of the main tower - Do the wires go over a flat plate???

Seems to be a miracle it lasted at all.

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[Wallache]

I'm not sure I would have even walked on that, let alone jumped up and down with 55 other people.

 

[JStephen]

You all are assuming that somebody designed this, and that is not always the case.

 

[LittleInch]

well it is Nicaragua and was apparently built over 5 years ago so did well to last that long. I suppose "design" is probably a bit strong in this context, but that is a decent tower.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[dcarr82775]

Given the cables are visible each end I would wager the cables broke somewhere along the span? Never done one, but I would think if the anchorage failed the cables wouldn't still be over the top of the towers.

 

[Lomarandil]

dcarr, I'm not sure if the cables on the left side of the photo are the suspension cables, or guy cables for the tower.

 

[Sparweb]

Avoid making comments about the country.

This one is in Canada:

http://mw2.google.com/mw-panoramio/photos/medium/46705313.jpg

If you intend to walk across it, I recommend you do NOT look at the anchors when you get to the other side. Ignorance is bliss. It's a lonnnnnng walk back to town from the opposite side of the river.
You would think that in Alberta, they'd know not to "saddle a dead horse".

STF

 

[chamokinawan]

I don't do bridges, but can someone explain the "Don't saddle a dead horse", please.

 

[Lomarandil]

In a wire rope termination using clips, as shown above, the cut end is doubled back onto the wire rope to form an end loop.

The cut end is referred to as the "dead end", while the remaining portion would be the "live" end that carries load.

A wire rope clip is composed of a U-bolt, a saddle, and the nuts. The piece of wire rope up against the bare U-bolt will tend to crimp/bend when the clip is tightened, while the saddle is shaped with radii to prevent crimping. This crimping would locally reduce the strength of the wire rope.

Therefore, never saddle a dead horse -- because the saddle should be on the live end of the rope to prevent losing strength.

 

[LittleInch]

lomarandil,

so basically you're saying the clip in the photo sparweb attached is the wrong way around? Sparwebs photo makes it look like a splice though rather than an end connection.

Does more than one clip actually make a difference or do people do it just to make it look good / provide a bit of redundancy?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Lomarandil]

Right, in Sparweb's photo, the saddle is on the dead end.

You're right, wire rope clips can be used to make a splice, and that may be what's going on in that photo. It's typically preferred to connect two end loops (either threading one through the other or using a shackle, depending on permanence) rather than a straight splice though. In the case of a straight splice, you'd switch the orientation of the clips halfway through the connection to keep the saddles on the live ends (assuming all load transfer at the midpoint of the splice).

Typically it requires several wire rope clips to make a full strength connection -- varying based on wire rope diameter. Crosby at least has a table specifying the quantity and spacing of clips required. On the topic of straight splices, I believe Crosby also advises you to double the number of clips (part of why the double end loop connection is preferred).

 

[Sparweb]

I should dig up my old photos of the bridge and see if I took any from the side of this joint. This linked picture was just a stock photo that came up on a google search. I knew to look for it because I noticed stuff like this when I vacationed there a few years ago.

STF