{"id":3334,"date":"2015-03-27T13:33:47","date_gmt":"2015-03-27T13:33:47","guid":{"rendered":"http:\/\/www.ericjorden.com\/blog\/?p=3334"},"modified":"2015-03-27T13:33:47","modified_gmt":"2015-03-27T13:33:47","slug":"wind-construction-crane-and-inadequate-cross-bracing-caused-edmonton-bridge-failure-an-initial-hypothesis","status":"publish","type":"post","link":"http:\/\/www.ericjorden.com\/blog\/2015\/03\/27\/wind-construction-crane-and-inadequate-cross-bracing-caused-edmonton-bridge-failure-an-initial-hypothesis\/","title":{"rendered":"Wind, construction crane and inadequate cross-bracing caused Edmonton bridge failure: An initial hypothesis"},"content":{"rendered":"

(Forensic investigations are carried out by hypothesizing the cause of a failure or accident and\u00a0revising the hypothesis as the evidence comes in.\u00a0\u00a0This revising might be done several times during an investigation<\/em>.\u00a0 <\/em>The following is an example of how an initial hypothesis can be made of the cause of a failure based on information from the early stages of a forensic investigation – after reading documents or carrying out a visual “site” assessment)<\/em><\/p>\n

Simple visual\u00a0“site” examination\u00a0– one of several initial steps in forensic work, identifies cause of failure<\/strong><\/span><\/p>\n

It was fairly easy to figure out the probable cause of this unusual\u00a0bridge failure based on a simple visual\u00a0“site”\u00a0examination and\u00a0knowing the weather at the time.\u00a0 This type of examination is\u00a0one\u00a0of the first steps in a forensic investigation.\u00a0 Also noting the fairly conventional design of the bridge and how such bridges are constructed.\u00a0 I\u00a0did this in about an hour last Monday after some discussion with a structural engineer.<\/p>\n

My “site” examination was of photographs and video in the\u00a0Globe and Mail\u00a0and on-line.\u00a0(Ref. 1)\u00a0 Photographs\u00a0can be thought of as\u00a0documents in\u00a0a forensic case the examination\u00a0of which is another preliminary step in a forensic investigation.<\/p>\n

Another basic step in a forensic investigation is\u00a0comparing the failed structure to its condition before it failed.\u00a0 Parts of the bridge failed and some parts didn’t, as evident in the photographs, providing this before-after comparison.<\/p>\n

The\u00a0procedure followed in constructing such a bridge is also important information.<\/p>\n

Bridge failure<\/strong><\/span><\/p>\n

The bridge on 102 Avenue in Edmonton reportedly failed on Monday, March 16th when the middle section of three of the seven long, deep, parallel\u00a0beams\u00a0spanning the area below bent sideways into a curve.\u00a0 The beams support a concrete bridge deck (not in place at the time of the failure).<\/p>\n

The bridge is constructed a little like the floor in your home<\/strong><\/span><\/p>\n

The view from\u00a0above\u00a0in a low altitude\u00a0aerial photograph taken\u00a0from a drone would\u00a0be similar to that of the\u00a0floor\u00a0joists in your home supporting the floor you walk on.\u00a0 The beams are cross-braced horizontally and diagonally\u00a0by pieces of steel.\u00a0 The bracing keeps the deep beams upright, apart, and parallel.\u00a0 Floor joists are also braced like this for the same reason.<\/p>\n

The large beams – several 10s of feet long and several feet deep,\u00a0are constructed away from the bridge site in two end sections and one middle section.\u00a0 The end sections are\u00a0hoisted into place by cranes to start construction of\u00a0a beam and supported there by the\u00a0cranes, one at each end.\u00a0 The end sections are cross-braced\u00a0horizontally and diagonally\u00a0to\u00a0a previously constructed beam.<\/p>\n

A\u00a0third crane lifts the middle section into place which is\u00a0bolted to the end sections.\u00a0 The middle section is then\u00a0cross-braced to the previously constructed middle section.\u00a0 The cables from the\u00a0cranes can be seen on the right in many of the pictures and\u00a0in a video.<\/p>\n

How the bridge failed<\/strong><\/span><\/p>\n

The bridge failed when the middle section of three of the beams bent sideways – buckled is another way of describing what happened to the beams.\u00a0 The failure appears\u00a0as smoothly curved beams in pictures on-line and in the newspaper.\u00a0 These would be failed beams #4, #5, and #6 counting from the left in many of the\u00a0pictures.\u00a0 Cranes\u00a0and cables can be seen in the pictures supporting the three sections of beam #6.<\/p>\n

If you look closely at the pictures you can see that it is basically the middle sections that have buckled and curved.\u00a0 You can also see that the end sections are cross-braced\u00a0but there is very little\u00a0cross-bracing in the middle sections, and what bracing is there has\u00a0also buckled and failed.<\/p>\n

The beams reportedly failed\u00a0without warning as crews worked on them.\u00a0 It was also reported that work on the beams was stopped the previous Saturday night due to\u00a0extreme high winds.<\/p>\n

The bridge beams could fail if the\u00a0cable of the third crane connected to the middle section of beam #6 moved to the\u00a0left pulling the top of\u00a0the beam towards beams #5 and #4.\u00a0 Beams #5 and #6 would be pushed over by the small amount of cross-bracing at the middle sections until the\u00a0pushing force needed was too great\u00a0for the cross-bracing and it buckled and failed too.\u00a0 The\u00a0force needed would increase because beams offer increased resistance as they are bent.<\/p>\n

The\u00a0middle sections of the three beams on the\u00a0left, #1, #2, and #3, did not fail because they were adequately braced.\u00a0\u00a0Neither did the end sections of beams #4, #5, and #6 fail, like the middle sections did, because they were adequately braced.\u00a0 The end sections show a little movement\u00a0as a result of being\u00a0“dragged” along by the middle sections.<\/p>\n

What caused the bridge failure?<\/strong><\/span><\/p>\n

What would cause the cable to move to the left initiating the failure of the middle section?\u00a0 Well, a crane consists of a large boom with a cable hanging down at the end with a\u00a0hook or, in this case, a\u00a0sling on it.\u00a0 A boom moves\u00a0a little when the wind blows same as the cable does.\u00a0 We\u00a0see cables moving often enough when we go by a crane and look up.<\/p>\n

Although\u00a0a boom moves a lot less in the wind than a cable, it still\u00a0moves.\u00a0 If the\u00a0boom moved back and forth a little\u00a0in the extreme wind for hours on end this would cause the cable to periodically\u00a0tug on the inadequately braced middle section.\u00a0 It’s not\u00a0difficult\u00a0to imagine the middle section\u00a0being brought to the point of\u00a0failure by this periodic tugging – a form of fatigue.<\/p>\n

If the crane\u00a0operator climbed up into his cab\u00a0Monday\u00a0morning, started\u00a0his crane up, and\u00a0moved\u00a0his boom a little\u00a0for whatever reason that could also cause the cable to tug on the middle section and\u00a0contribute to the initiation of the failure.<\/p>\n

It’s also possible the workers came in\u00a0Monday\u00a0morning to work on the\u00a0bridge and found that the beams had failed, rather than came in, did some work, as stated in the newspaper,\u00a0then the bridge failed.\u00a0 There are errors in the news reporting suggesting this possibility.\u00a0 Notably, the number of beams that failed, reported as four when only three appear in the pictures.<\/p>\n

There would be tugging on the end sections of\u00a0beam #6 as well because\u00a0the crane booms would be moving in the wind at these locations\u00a0too.\u00a0 However, the end sections did not\u00a0buckle and fail because they were adequately cross-braced.<\/p>\n

The bridge might\u00a0not have failed if ….<\/strong><\/span><\/p>\n

If the crane’s cable was not connected\u00a0and tugging on the middle-section, it’s likely the bridge would not have failed.\u00a0 The wind would not have been strong enough to push a large beam like this sideways even though it was inadequately braced.<\/p>\n

An initial hypothesis<\/strong><\/span><\/p>\n

The bridge failed because the middle crane’s boom moved in the wind – possibly also due to the crane operator’s actions, causing the cable\u00a0to periodically tug at the middle section of\u00a0beam #6 and eventually cause it to bend.\u00a0 This caused\u00a0the middle sections of\u00a0beams #5 and #4 to bend as well because they were connected to #6 by\u00a0some cross-bracing.\u00a0 The cross-bracing was inadequate to resist the force from the tugging\u00a0indefinitely and eventually failed too.\u00a0 The middle sections of beams #3, #2, and #1 did not bend and fail because they were adequately cross-braced.<\/em><\/p>\n

Revising the\u00a0hypothesis<\/strong><\/span><\/p>\n

This hypothesis\u00a0might be revised after more evidence came in from additional forensic investigation such as the following:<\/p>\n