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32 TheStructuralEngineer Professional guidance April 2013 Structural failure Interstate 90 Connector Tunnel ceiling collapse

with the failed section being installed in Figure 1 STypical adhesive anchor November 1999. The suspended ceiling and roof hanger plate assembly consisted of concrete ceiling panels and associated steel support framework, and 3/4" it was suspended from the tunnel roof by Sean Brady highlights Epoxy potential similarities stainless steel anchors held in place with epoxy adhesive. Notably, the specifi cation between the recent Sasago produced by the tunnel ceiling designer, road tunnel collapse in Japan Gannett Fleming, did not contain criteria with respect to the long term performance of the and the catastrophic failure anchoring system (Figure 1). of part of Boston’s I-90 The incident occurred at approximately Concrete Connector Tunnel in 2006. 23:00h on Monday the 10 July, 2006, more roof than six years after the D Street portal 5" opened to traffi c. A car travelling eastbound On 2 December 2012, concrete ceiling in the tunnel was struck by about 24t of panels fell from the roof of a 4.7km-long road falling ceiling panels and steel framework tunnel west of Tokyo, Japan. The ceiling that detached from the tunnel roof. The panels were suspended above the tunnel right hand side of the car was crushed, with roadway and the falling panels crushed three the driver suff ering minor injuries and the vehicles and resulted in nine fatalities. The passenger, the driver’s wife, being fatally 0.55" Seal plug tunnel reopened in late December, and while injured (Figure 2). Roof a number of publications speculate that the An investigation into the collapse was 0.9" cause of the failure may be associated with commenced by the National Transportation hanger plate

a failed support bracket or concrete anchor, Safety Board (NTSB), who established that SAFETY BOARD TRANSPORT NATIONAL COURTESY: Sealing washer the fi ndings of an offi cial investigation are yet all 20 epoxied anchors that connected to be released at the time of writing. the ceiling panel’s M1 support beam had 3" Lock washer This collapse was a stark reminder of pulled free from the tunnel roof. Further, an the failure of ceiling panels in the Interstate examination of the hardened epoxy, which Nut 90 (I-90) Connector Tunnel in Boston, was still attached to the failed anchors, Massachusetts, in 2006, and a number suggested that a number of the anchors Anchor Not to of the key fi ndings from this failure are had been pulled some distance out of the scale discussed here1. tunnel roof (displaced) prior to the incident The I-90 Connecter Tunnel was occurring. constructed as part of the CA/T project in Following the collapse, an inspection of the load capacity published by the anchor Boston, also known as the “Big Dig”. The remaining 634 anchors in the D Street tunnel supplier”1. The NTSB also established that, project was regarded as one of the most found that 161 of them had measureable while all of the anchors may not have been costly and complex public infrastructure displacement consistent with them being installed in a manner that would result projects in the US, and it was completed gradually pulled out by the weight of the in maximum performance, “improper or in 2006 at a fi nal project cost in excess of concrete ceiling panels (Figure 3). Given this defi cient anchor installation procedures or US$14 billion. In essence, the project was evidence, the NTSB concluded “by July 2006, practices alone would not account for all undertaken to improve traffi c fl ow in Boston a signifi cant portion of the adhesive anchors of the anchor failures that were observed by providing 259km of highway lanes, used to support the D Street portal ceilings before and after the accident”1. including 8km of tunnel, six interchanges had displaced to the extent that, without The NTSB investigation focused on why and 200 bridges. It spanned a 20 year corrective action, several of the ceiling the epoxy failure occurred and discovered period and was project managed by Bechtel/ modules in the three portal tunnels were at ambiguities associated with the type and Parsons Brinckerhoff (B/PB). imminent risk of failure and collapse”1. specifi cation of epoxy supplied. The epoxy The incident site in the I-90 Connector With respect to the installation of the used in the construction was NRC-1000 Tunnel, known as the D Street portal, tunnel ceiling, the NTSB found that all of Gold epoxy. This was available in Standard was opened to traffi c in December 2000, these anchors had been subject to, and Set or Fast Set versions. Based on tests on and consisted of approximately 800m of passed, a short term proof load test prior to epoxy samples, the NSTB concluded that cut-and-cover tunnel. The D Street portal being put into service. It was also ascertained only Fast Set epoxy had been used in the D ceiling was installed by Modern Continental that the design loads on each anchor “were Street portal. Further tests confi rmed that Construction Company in 1999 and 2000, well below the expected average ultimate while the Standard Set and Fast Set epoxies

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Figure 2 WApproximately 24t of concrete ceiling panels crush a vehicle resulting in a fatality

Figure 3 EDisplaced

PRESS ASSOCIATION PRESS hanger plates PRESS ASSOCIATION PRESS

had similar performance under short term regarding its Power-Fast epoxy was performance to ensure that the actions loading, they were dramatically diff erent inadequate and misleading, with the result taken in response to the displacement were under long term loading situations, with Fast that Modern Continental used the Fast Set eff ective. Had these organisations taken Set epoxy exhibiting signifi cant displacement formulation of the epoxy for the adhesive such action, they likely would have found when subject to constant loading. Based on anchors in the D Street portal even though that anchor creep was occurring and they these fi ndings, the NTSB concluded that “the that formulation had been shown through might have taken measures that would have source of the anchor displacement that was testing to be susceptible to creep under prevented this accident”1. found in the D Street portal tunnels and that sustained tension loading”1. However, it also A further issue cited by the NTSB as a precipitated the ceiling collapse was the poor concluded that “Gannett Fleming approved missed opportunity, was that of inspections. creep resistance of the Power-Fast Fast Set the D Street portal anchors without Although an inspection manual had been epoxy used to install the anchors”1. identifying which epoxy formulation was published by B/PB in 2003, no ceiling The NTSB then set out to identify why being used, even though the company was inspections were undertaken between 2003 the ceiling installer, Modern Continental, provided with information indicating that one and the collapse. The NTSB concluded that utilised an inappropriate formulation for version of the Power-Fast epoxy should be had the Massachusetts Turnpike Authority its application. Fundamentally, they found used for short-term loading only”1. “inspected the area above the suspended “no evidence that Modern Continental So once the Fast Set epoxy had been put ceilings in the D Street portal tunnels, the was off ered a choice or made a conscious into service, was there an opportunity to anchor creep that led to this accident would decision to use one epoxy formulation over detect the potential for a catastrophic failure likely have been detected, and action could another”1 and concluded that “Modern prior to July 2006? The NTSB found two have been taken that would have prevented Continental was not aware, when its such opportunities. this accident”1. employees installed the adhesive anchors In September 1999, seven years before the Like many catastrophic structural in the D Street portal, that the epoxy being failure, and approximately two months after collapses in the past decade, a series of used was susceptible to creep and was installation, a Modern Continental employee errors, oversights, omissions and poor therefore unsuitable for this application”1. working in an adjacent tunnel on the Big Dig, communication (combined with missed A review of the supplier’s records the HOV tunnel, noticed that a number of opportunities to identify the potential for indicated signifi cant ambiguity associated anchors had started to pull out of the roof. failure) culminated in loss of life, loss of with the epoxy supplied. The NTSB found Further inspections over the next two weeks functionality, and undermined the integrity of that in 1997, prior to the D Street tunnel showed this displacement was increasing. B/ a key piece of transport infrastructure. installation, Powers, the epoxy supplier, PB initially suspected these displacements undertook creep testing on the Standard were a result of improper installation by Sean Brady is the managing director of Set epoxy and found that it met the required Modern Continental, and it was agreed Brady Heywood (www.bradyheywood.co.uk standards. Then in February 2000, following between B/PB and Modern Continental to & www.bradyheywood.com.au). The fi rm installation of the D Street tunnel ceiling, an replace the displaced anchors and subject provides forensic and investigative structural evaluation report issued by Powers noted them to further short term load testing. engineering services and specialises in that the Fast Set epoxy was only permitted Crucially, despite concerns being voiced determining the cause of engineering failure for short-term loading scenarios – a situation by various individuals involved, the reason and non-performance. that was quite diff erent to what it would for the displacement was not identifi ed. This article fi rst appeared in the March experience in the tunnel. In a detailed Two years later, in another portion of the 2013 edition of Engineers Australia magazine. review of Powers’ documentation, the NTSB I-90 Connector Tunnel, it was found that learned that “the Power-Fast Fast Set epoxy additional anchors had begun to pull out after had been tested for creep performance in having been proof tested just two months REFERENCES: 1995 and 1996 and had failed to meet the previously. As before, these anchors were 1) National Transport Safety Board (2007) Highway Accident Report, Ceiling standard”1, thus potentially explaining the replaced and the cause of displacement Collapse in the Interstate 90 Connector Tunnel, recommendation for short term applications remained unknown. The NTSB concluded Boston, Massachusetts, July 10, 2006 [Online] only. Ultimately, the NTSB found that “the that “B/PB and Modern Continental should Available at: www.ntsb.gov/doclib/reports/2007/ information that was provided by Powers have instituted a program to monitor anchor HAR0702.pdf (Accessed: March 2013)

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