PEDESTRIAN SUSPENSION BRIDGE

[LUISGUTIERREZ]

The attachment, labeled SOUTH ANCHORAGE, shows a pedestrian suspension bridge built in a resort area in Central America. The span is about 90 meters. The main cable (on either) side is composed of three cables of about 1 inch in diameter. The bridge collapsed recently due to overload that broke the "eye bars" of the south anchorage. The eye bars of the NORTH ANCHORAGE did not break. The cable did not break either. If you examine the pictures carefully you will notice a difference that may explain the difference in behavior of the anchorages.

I am not a bridge engineer but I am getting the idea that designing a suspension bridge in this scale may be a task accessible to non-specialized structural engineers. Would the participants in this forum provide technical references with guide lines for design and construction of this kind of bridge. I will appreciate information. Also, do you notice the two small wires bracing the columns in the picture of the south tower? It seems they were needed during construction, but do they serve any purpose after completion of the structure? Will appreciate your insight.

Thanks

 

[LittleInch]

your attachment says north anchorage and looks better than the one in this post which is I think the end that failed.

http://www.eng-tips.com/viewthread.cfm?qid=385689

That and the fact 52! people were apparently jumping up and down on it a the time presumably because it bounced nicely. they clearly hadn't heard of the tacoma narrows bridge....

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[hokie66]

The three cables and the reinforcing bars to which they are attached look about the same size. So in the absence of some stress riser like a notch in the cables, you would expect the bars to fail first, as the steel in the cables is much higher strength than the bars. In addition to the tensile stress, the bars have to cope with bending at the eye. But you would have to see the failed end to know for sure. From what we see in the photos, you couldn't rule out the concrete anchor block being pulled out of the ground.

 

[LittleInch]

The "design" of this bridge is absolutely horrendous and has stress concentrations and work hardening all over the place. This end looks a lot better than the south end which is the end which failed - see the other post.

as for design - try this - I haven't looked at it all, but it looks fairly comprehensive. Of course always get someone that actually knows what they're doing to do or check the design....

http://www.ilo.org/public/english/employment/recon/eiip/download/footbridges.pdf

or

https://mcedc.colorado.edu/sites/de...ridge-Design-for-Rural-Construction_FINAL.pdf

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[Lomarandil]

LittleInch

Those references are good, but are the predecessors to the Bridges to Prosperity guide (available for free through that organization). Volume 4 of their guide goes specifically into suspension bridges (rather than their typical "suspended" type bridge), and is very thorough -- going through calculations, adaptable drawings with constructable details, and with instruction on construction management issues.

B2P actually has a field office in Nicaragua. Their in-country program manager -- Brandon, is a pretty sharp guy, and was quick to jump in and help another organization when I was there by giving advice for our design of a pedestrian bridge. I suspect he's already aware of this.

As to this being a task "accessible" by non-structural engineers, tread with caution. I'm a bridge engineer myself, and a suspension design like this would be challenging on several levels. I'm going to be pragmatic here -- not every pedestrian bridge in developing countries needs a PE/SE stamped design. But I'd certainly take a good go-by design like the B2P manual provides and either follow that closely or really understand any changes made to the design.

 

[LittleInch]

Great - It's amazing what crops up here and I'm no bridge builder myself, but you can just see when things are not great.

How you stop 52 people apparently purposefully bouncing up and down or include them in the calcs is a different matter. Just as well they didn't have far to fall into the water and that the treads were made from wood by the look of it. Now safe in my useful documents folder - thanks

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[jayrod12]

a bridge like that should be designed for much more than 50 people jumping up and down on it. I can't imagine a designer that wouldn't account for at least that.

The design loads I would use account for so many people they wouldn't physically be able to jump up and down.

 

[LittleInch]

Depends if they hit resonant frequency. Those cables looked fairly puny to me and using bent re-bar as the anchors...

also may not have been "designed"

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[jayrod12]

I bet it wasn't, because regardless of frequency, had this been designed at all, then 50 people jumping should have had a negligible impact on the structure at all.

And judging by the photos, I don't think the failure necessarily has to do with the bridge structure design but most likely anchorage. The designer of the anchorage should not really care about resonant frequency, but rather about fatigue, stress concentrations and work hardening which they did not. By the time you provided a robust enough anchorage detail to mitigate strictly the fatigue and stress concentrations issues the actual structural capacity would be far in excess of the design loading.

 

[rb1957]

I think the "jumping" was a key part of the failure; I think it could handle the static load (what, 50 people, 100000 lbs, 5 tons, distributed over 90m (or concentrated over the middle 25m) but they would have excited the natural frequency (afterall, that's why they were bouncing the bridge).

you can't design out "operator stupidity". maybe it would've been better to design as fail-safe ... let part of the foundation pull-up at some "stupid" load, and then with the bridge hanging on by a thread, maybe sanity will take over ?

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

 

[jayrod12]

50 people is more like 10,000 lbs. I'm assuming your finger got stuck on the zero key.

I don't feel as though the jumping should of contributed to the failure if it was anchorage as suspected.

10,000 lbs is peanuts.

 

[rb1957]

I'm sure the static load on the Tacoma bridge was "peanuts" too. particularly if instead of acting down (as gravity intended) you've managed to get the bridge deck whipping up and down, adding in-plane tension to the decking support loads ...

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

 

[btrueblood]

Jayrod,

What is the dynamic load factor for a structure at its resonance? Bridge collapses due to live loads exciting resonance have been known and studied for over a century at least (I'll bet the Romans knew it too), which is why all soldiers are taught to march in "break step" when crossing bridges.

http://www.livescience.com/34608-break-stride-frequency-of-vibration.html

http://en.wikipedia.org/wiki/Angers_Bridge

http://en.wikipedia.org/wiki/Broughton_Suspension_Bridge

 

[jayrod12]

My point is the bridge is 300 ft long and call it 10 ft wide. That should mean the total design gravitational live load should be in the range of 300,000 lbs for live load alone. Is your dynamic load factor 30? Didn't think so.

 

[IRstuff]

"I am not a bridge engineer but I am getting the idea that designing a suspension bridge in this scale may be a task accessible to non-specialized structural engineers. "

I don't see what could lead one to conclude that a non-specialized engineer is necessarily capable of handling this job. Whoever designed this bridge, whatever their qualifications were, failed.

TTFN
faq731-376

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[hokie66]

Luis,
I think you should get your hands on the "Bridges to Prosperity" guide which Lomarandil suggested.

As to the purpose of the two small stay wires, in this instance they may have prevented the towers from falling toward the water, which would have made the failure much worse.

 

[LUISGUTIERREZ]

Thanks hokie66 and Lomarandil,

I have been looking at the manuals of "Bridges to Prosperity" (B2P) and similar publications that a Google search returns for this topic. So far, the B2P bridges I found are of a different configuration, they make a distinction between "suspension bridges" (Golden Gate type) and "suspended bridges" in which they don't use high towers, run the suspension cables directly under the deck, and work with another set of suspension cables that are also used as hand rails. I am going to take a second look at the manual an see if I can find Volume 4, thanks Lomarandil. Hokie66 is right about the function of the guy cables, they are a last line of protection when a catastrophic failure happens.

About the "accessible task" idea. I was not referring to non-structural engineers, I was referring to engineers trained in structural engineering but not specialized in suspension bridges.

I posted another thread with a more detailed explanation about the failure of the anchors, but now I cannot find it. I am still not familiar with the use of this page. If you find it, I would like to learn about your insight.

Thanks

 

[IRstuff]

I understood that; I'm just saying that even if digital design and analog design were both EE subdisciplines, that does not mean that a digital EE could design an analog circuit well.

Likewise, just because someone is an SE does not mean that they could simply read a book and go and design a bridge.

TTFN
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There is a homework forum hosted by engineering.com:

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[LittleInch]

The third thread is here

http://www.eng-tips.com/viewthread.cfm?qid=385515

If you click on your own name it shows which forum you have posted stuff in and on the LHS shows which forums you visit.

It is a pity we don't have actual shots of the failure to see whether it is the re-bar which has failed or the cables either due to crimping and gradual failure at the connection to the re-bar or due to crimping of the cable by the clamps. The system at the top of the towers looked poor and given it is a long way in the air could have been fraying and snapping cable cores for a long time without anyone noticing.

I suspect cable failure as otherwise the whole south tower would have fallen over which it hasn't as it is held up by the thinner wires.



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Also: If you get a response it's polite to respond to it.

 

[hokie66]

LittleInch,
The towers do have a stiffened base, anchored with quite a few bolts. Without base fixity, they would be prone to fall toward the land.