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

 

[PSEPK]

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

Link

A good structural engineer is often a blessing for others.

 

[BUGGAR]

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".

 

[dhengr]

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.

 

[MikeHalloran]

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

 

[rb1957]

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 ?

 

[LUISGUTIERREZ]

To PSEPK,

Thanks for the link to the manuals.

 

[LUISGUTIERREZ]

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.

 

[MikeHalloran]

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

 

[IRstuff]

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
faq731-376

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[LUISGUTIERREZ]

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.

 

[LUISGUTIERREZ]

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.

 

[IRstuff]

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
faq731-376

Need help writing a question or understanding a reply? forum1529


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