Hopu Ruahine bridge destabilises when cable breaks

Vandalism has been ruled out. http://www.nzherald.co.nz/hawkes-bay-today/news/article.cfm?c_id=1503462&objectid=11514981 "'Initial reports indicate that the release of the tension cable was caused by an unexpected failure in a link of heavy duty iron chain,' Department of Conservation general operations deputy director Mike Slater said. "'Further analysis of the chain is under way to understand why it failed but there is no evidence of any suspicious circumstances surrounding the failure. "'We hope that clears up unfounded claims about 'sabotage'.'" Sooooo.... a link gave way and the chain snapped?

Only as strong as the weakest link. Odd thing to happen out of the blue, tho ?.

Yeah. Is this the type of thing that'd typically be expected to be picked up in an inspection, or is it possibly something a fault in the chain's manufacture which couldn't have been detected externally?

Good news that sabotage has been ruled out. Basically steel fails in three possible modes: 1. Fatigue failure; an accumulation of stress cycles leads to micro-cracks that eventually propagate catastrophically. Or more memorably - bendo snapo. 2. Brittle failure caused by a sudden high energy impact. The ability of a steel to absorb impact is called it's toughness. This measure is very dependent on both temperature and the grade of the steel. For instance cast iron is relatively brittle, but spring steel is very tough. 3. Corrosion failure. Generally steel rusts in bulk and it's obvious when the material is loosing strength. However corrosion can be hidden in micro-cracks caused by fatigue or brittle damage. Or it can be hidden under coverings or coatings intended to protect the material, or maybe just buried in the anchor block. We can rule out direct fatigue failure because the very nature of a properly installed chain eliminates bending stresses. Brittle failure is harder to determine, but again the flexible nature of a chain means that it would likely absorb any high impact blow without damage. We've no information about corrosion damage, but this far from the sea makes it less likely this is the proximate cause. This leaves a manufacturing defect or hidden damage during installation as the most likely causes in my mind. Large chain links are often cast or drop-forged. If an inclusion or brittle micro-crack had been present for many decades, combined with hidden corrosion and perhaps low -temperatures on the day of the failure - then you have a plausible scenario. Or maybe the chain was simply installed incorrectly and this placed unintended bending stresses on an individual link. All speculation of course - it will be interesting to see the proper report to get a better understanding of whether this was an unlucky one off or if it's a potential hidden failure mode in other bridges.

I thought it might have been a group playing silly-beggers, but if it's rated for 10 people, then that's not likely.

That book got me out of watching shortland sreeet This is an interesting thing from that "The ideal strength of cable elements shall be the proof load of the element as taken from the above Standards. Where, in these Standards a proof load is not specified, the ideal strength of a cable element shall be taken to be as twice the working load limit" My understanding of this is an element ie chain shackle rope etc typically has a swl or wll of 1/6 of its breaking load Doc are engineering there structures at twice the SWL which is still 1/3 of the capacity of the elements. The structures are then intended to last 25 years. The catch here is that in lifting all ropes and slings must be replaced every 2 years at most. Remember a few years ago in Auckland where a shackle on the main power line broke plunging most of Auckland into darkness. Those shackles were intended to last 25 years but this had corroded half way through then fractured in about 10 years. Those shackles could not be replaced or even properly inspected without depowering the line so the inevitable happened.

I observed the construction of a suspension bridge on a local cycleway. It was all prefabricated and went up surprisingly quickly. At the end, it was tested by filling bladders with water to a prescribed weight. The movement of the cable support towers and the sag of the bridge were measured before, during and after the loading. The bridge was plenty strong enough but such a test does not measure the ability of the bridge to withstand corrosion or metal fatigue. The load limits on the bridge are presumably such as to avoid metal fatigue. I went undetected when I cycled onto the bridge while the engineers were making some adjustment to bracing cables. Pedestrians invariably shake and stress the bridge far more than wheeled vehicles.

The use of the word "destabilise" is curious. If I drop a glass does it "destabilise" when it hits the concrete floor? Or, if a drive my car into a power pole, is there a risk of destabilization? Is "destabilise" less alarming than "fail" or "break"? When your flight is delayed or cancelled it may be because of technical or operational difficulties depending upon whether an engine has fallen off or the pilot is drunk. There must be a better term than "destabilise" for describing a transient structural anomaly.

I had a pack destabilize once. It had been the wifes pack and was quite old. A leaky dry bag on a river trip caused one of the shoulder straps to break. Other shoulder did not like that one little bit. Wallet didnt like buying wife a new pack to prevent more destabilisation.

RadioLive twitter feed reporting that DOC's initial report believes that the chainlink failed due a manufacturing defect. Edit: found DOC press release http://doc.govt.nz/news/media-releases/2015/doc-and-tuhoe-release-findings-of-bridge-investigation/ "Date: 24 September 2015 Deputy Director General Operations Mike Slater says laboratory analysis conducted at the University of Canterbury School of Engineering has revealed manufacturing defects, known as quench cracks and embrittlement, in a section of 24 millimetre diameter high tensile chain used on the bridge. Mike Slater says there were no previous indications of weakness with the chain used on the Hopuruahine Bridge and this type of sudden link failure is highly unusual. Mike Slater says DOC is seeking confirmation of the defects through a second round of specialist testing. Mike Slater says the specific batch of chain found on the Hopurahine Bridge was not used in the construction of the other nine smaller bridges on the Lake Waikaremoana Great Walk and they have all passed detailed engineering inspections. He says checks have not revealed any sections of this batch of chain being used in DOC's other suspension bridges around the country. Mike Slater says DOC has kept Te Urewera Board fully updated on the bridge investigations. He says DOC and Tuhoe are working with the trampers involved in the incident to ensure they are also briefed on the findings. Mike Slater says DOC will be installing new cable strops on the Hopuruahine Bridge over coming weeks and will be working with Tuhoe to recommission the structure after fresh load testing is completed." Quench cracking usually happens when you get rapid uneven cooling (rapid cooling or quenching is used to generate a higher strength steel by forming martensite - a crystalline form of steel where carbon atoms are trapped within the crystal structure, generating an internal strain)