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PEDESTRIAN SUSPENSION BRIDGE

PEDESTRIAN SUSPENSION BRIDGE

PEDESTRIAN SUSPENSION BRIDGE

(OP)
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

RE: PEDESTRIAN SUSPENSION BRIDGE

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....

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

RE: PEDESTRIAN SUSPENSION BRIDGE

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.

RE: PEDESTRIAN SUSPENSION BRIDGE

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... or
https://mcedc.colorado.edu/sites/default/files/Ban...

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

RE: PEDESTRIAN SUSPENSION BRIDGE

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.

RE: PEDESTRIAN SUSPENSION BRIDGE

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

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

RE: PEDESTRIAN SUSPENSION BRIDGE

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.

RE: PEDESTRIAN SUSPENSION BRIDGE

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"

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

RE: PEDESTRIAN SUSPENSION BRIDGE

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.

RE: PEDESTRIAN SUSPENSION BRIDGE

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 ?

RE: PEDESTRIAN SUSPENSION BRIDGE

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.

RE: PEDESTRIAN SUSPENSION BRIDGE

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 ?

RE: PEDESTRIAN SUSPENSION BRIDGE

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-freq...
http://en.wikipedia.org/wiki/Angers_Bridge
http://en.wikipedia.org/wiki/Broughton_Suspension_...

RE: PEDESTRIAN SUSPENSION BRIDGE

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.

RE: PEDESTRIAN SUSPENSION BRIDGE

"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
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Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com: http://www.engineering.com/AskForum/aff/32.aspx

RE: PEDESTRIAN SUSPENSION BRIDGE

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.

RE: PEDESTRIAN SUSPENSION BRIDGE

(OP)
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

RE: PEDESTRIAN SUSPENSION BRIDGE

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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Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com: http://www.engineering.com/AskForum/aff/32.aspx

RE: PEDESTRIAN SUSPENSION BRIDGE

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.

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

RE: PEDESTRIAN SUSPENSION BRIDGE

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.

RE: PEDESTRIAN SUSPENSION BRIDGE

Here is the link for downloading Bridges to Prosperity (B2P) manuals;

Link

A good structural engineer is often a blessing for others.

RE: PEDESTRIAN SUSPENSION BRIDGE

When we design bridges for Caltrans, we generally design to a specific design lifetime. Perhaps this bridge successfully fulfilled its lifetime duty and just "retired".

RE: PEDESTRIAN SUSPENSION BRIDGE

LUISGUTIERREZ:
They have been building suspension bridges, in one form or another, for hundreds of years. In their crudest form, they were a number of ropes made up of braidded vines or hand made rope to make up the needed cables. These ropes were then strung over dry laid field stone piers/columns/towers at the four corners of the bridge. Then a walking surface, or maybe only a large single central bottom rope for stepping on, were hung from the main ropes. These bridges worked and still do, so the general concept is nothing new. The bridge we are talking about appears to be a vast improvement over those rope/vine bridges. But, the failure also would seem to show the importance of having experienced, knowledgeable people involved in such design and construction. These developing countries need to be able to use local labor, industry (small steel/blacksmith shops, etc.) and materials to make such a project practical, but this can make the design and construction all the more difficult to do right, because of the limitations of those local abilities and materials. The local bldg. codes and needs will likely not meet our stds., but then those people put themselves at some risk for the sake of expediency.

I would need to see more detailed photos, before and after, to go into much detail. Several potential problem conditions have already been mentioned, but we don’t know enough of the facts of the matter. What I see of those anchorages so far, is that they are likely deficient as designed and built. Your free body diagrams in another thread are a correct start on analyzing one of the problems. I would sum up my concerns by saying that, be it the rebar/eyebars or the cables which failed, the sum of either of these grouped load carrying members may have been sufficient for the intended load, which may not have been overly conservative, but the way they are arranged, there is a strong likelihood that one of the group will take more than its exactly equal share of the load. That member may actually start to yield/fail before the others really pick up their share of the load. And, on this type of structure that is not redundancy, that is not equal load sharing of a total load by a multi-part member, that is a progressive failure mechanism, designed and built into the structure.

I think that a good, knowledgeable, experienced Structural Engineer should be able to take a reasonable first-cut approach to a problem like this. But, I don’t think I would tackle it without some guidance and review by someone more knowledgeable than I am, on this particular type of construction. There are just too many little tricks-of-the-trade to this specialized construction, and missing one can kill yu. Remember, you should only be practicing in engineering areas that you have a good foundation/background. At the same time there is nothing wrong with learning new things, as long as you do it under the tutelage of an experienced practitioner, particularly when the health, safety and well being of the public is involved. Wanting to understand and delve into a structural failure is a good means of education. That’s how those braidded vine guys learned that they needed six vines per rope rather than just four.

RE: PEDESTRIAN SUSPENSION BRIDGE

Even if you knew how to design bridges, you wouldn't ordinarily include the load case of 56 people jumping up and down on a footbridge in unison, because that would require a very sturdy bridge, and that would impact your budget.

>>>Witnesses say that a municipal security officer stationed at the sentry post on the town side failed to warn them of the danger of jumping on the bridge.<<<

On the other hand, if you did design the new bridge to withstand such an abusive condition, you could fire the municipal security officer and save the ongoing cost of his salary and benefits.


Mike Halloran
Pembroke Pines, FL, USA

RE: PEDESTRIAN SUSPENSION BRIDGE

replace ineffective security with (an equally ineffective ?) sign "You, and X friends, bounce on this bridge at your own risk"

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

RE: PEDESTRIAN SUSPENSION BRIDGE

(OP)
To PSEPK,

Thanks for the link to the manuals.

RE: PEDESTRIAN SUSPENSION BRIDGE

(OP)
To: dhengr,

Thanks for your comments. We are examining the failure of this bridge based on pictures, some of them with very poor resolution. Never the less I find the exercise very interesting. In my other post, that you mention, I believe I reached some conclusions that appear to be valid.

1) The failure happened in the eye bars not in the main cables. Why? The main cables and the guy wires were hooked to the same eye bar. Even though, in the pictures (from the digital editions of the local papers) we don't see the end of the cables, we can see that the main cables and the guy wires are hanging loose, the thin guy wires hanging vertically and the heavy main cables hanging in a steep parabola (or catenary); the two sets separated some distance apart. That could not happen unless all the cables were released after the loop of the three eye bars broke open. The main cables were strong enough to break the three bars. Also, if the eye bars didn't break, the guy wires would have remained tied taught between the towers and the eye bars because the wires were not taking load from the suspension system.

These considerations also rule out failure at the shaft of the bars; pull out from the concrete or failure of the main cables at the connection.

2) The shafts of the tree eye bars were in the same plane at the north anchorage. However, at the south anchorage, one of the bars was out of the plane of the other two (see my pictures). This may indicate a construction error. There is no reason why the anchorages would be different. They must have been installed per the same detail in the drawings.

3) My free body diagrams show, in a qualitative way, what happens when the cable tension is not aligned with the axial forces in the shaft of the anchors. The triangle of forces shows that the shaft out of line takes a force that can be equal or higher than the tension in the cable. If that is the case, a condition of progressive failure was built into the system by a construction error if not by design.

4) General conclusions: The scheme of eye bar anchors improvised from reinforced bars is very fragile and highly vulnerable to relatively minor construction errors; it should not be used to reconstruct the bridge. For the same reasons, the eye bars at the north anchorage should not be recycled for reconstruction even if they did not fail.

RE: PEDESTRIAN SUSPENSION BRIDGE

One other alternative is to rebuild the bridge exactly as it was before, as it demonstrably was able to carry 56 people who were not jumping on it.

... and additionally, post a sign limiting the load to, say, 40 people at a time,

... and additionally hire some new municipal safety officers to stand guard at each end of the bridge.

... with the further qualification that each of those officers should be equipped with a bad attitude, an assault rifle, a crapload of ammunition, and standing orders to shoot to kill to prevent overloading the bridge.


I.e., it's not an engineering problem,
its a surplus of idiots and scofflaws problem.

Mike Halloran
Pembroke Pines, FL, USA

RE: PEDESTRIAN SUSPENSION BRIDGE

I would not be so quick to point to the eye bars. Even in the best base with coplanar eye bars, the outer two eye bars are not in line with the cables.

TTFN
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Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers


Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com: http://www.engineering.com/AskForum/aff/32.aspx

RE: PEDESTRIAN SUSPENSION BRIDGE

(OP)
Thanks IRstuff,

May be I should have explained that I was not referring to alignment of the physical shafts. I was referring to the representation of the force vectors projected on a vertical plane. In that case, the alignment I was referring to is the alignment of the vectors in that projection. Consider those projected vectors, if only one of them is in line with the projection of the vector representing the tension in the cable, the other vectors would have to have a magnitude of zero, to maintain equilibrium. That would explain how the bar out of the "other plane" (the plane containing the other two bars) was overloaded: taking the full tension of the cable, without the cooperation of the other bars. As you just found, the detailed explanation in words becomes cumbersome. It would be better to look at my stress diagram. I think you can find it if you follow this link: http://www.eng-tips.com/viewthread.cfm?qid=385515

Sorry for the inconvenience.

RE: PEDESTRIAN SUSPENSION BRIDGE

(OP)
Jayrod12, LitleInch, rb1957,

Thanks for your comments,

Fatigue failure involves a large number of cycles of fluctuating load, and can happen even at stresses below the yield point. The failure in this case is different, it may have been caused by a few bounces at the natural frequency of the bridge. Keep in mind that the bridge was very flexible. Just for reference: the Golden Gate has a period of 11 seconds for vertical displacements and 18 seconds for lateral displacements (Wikipedia wisdom). I figure it was easy to hit the resonance frequency bouncing at the compass of the Blue Danube of Johann Strauss (the folks on the bridge may have been able to enjoy only 15 to 20 bounces, before they happily and irresponsibly collapsed the bridge). If to that you add the residual stresses induced at the sharp bend at the neck of eye bars plus the possibility of a micro cracks at that location, then you have a plausible explanation of the failure: High stresses induced by force input at resonance frequency, at weakened spots of defective eye bars.

RE: PEDESTRIAN SUSPENSION BRIDGE

I did; my point is that in the one that you claim to be correctly installed, the outer two bars are only in line with the force vectors in one plane, since they are clearly not parallel to the single bar that is aligned.

TTFN
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