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The New York State Department of Transport commissioned a project called the Utica-Rome Expressway Project, and the contract was awarded to an external contractor, Toiga Construction Company of Herikimer. The project involved the design and construction of a major expressway between the cities of Utica and Rome (NYS Route 49), as well as four bridges spanning over the expressway. MJ engineering and Land Surveying were sub-contracted to design the footbridges over the soon to be constructed expressway. The single span 170 ft (~52m) pedestrian footbridge was between 20 and 25 ft (approximately 6m to 8m) above the ground below. The bridge consisted of a single steel tub girder, spanning between the two abutments on either side of the proposed road. The concrete deck was then to be cast in-situ once the steel tub girder was in position. The proposed concrete slab was 7.5in (~190mm) thick and 168in (~4.25m wide), with a volume in the region of 42m3 of concrete.
Figure 1 - Cross-section of bridge
On the 10th of October, the construction workers were in the process of pouring the concrete slab with an automatic screeding machine. Just prior to the failure the construction workers had passed the halfway point on the bridge with the screeding machine when the bridge twisted to such a degree that the steel tub girder failed allowing the entire span of the bridge to buckle and collapse onto the ground below. At the time of failure, ten workers were on the bridge and the concrete was still wet, as curing only took place after the pouring was complete. The failure killed one worker, and injured the nine other workers, two critically and multiple bone fractures for the seven others.
Causes of the accident
The failure of the Marcy Bridge was due to lateral torsional buckling. Buckling is the distortion along the longitudinal axis of the beam this occurs when a critical force is applied. As Leonhard Euler proved with his formula, the slenderness of a member in relation to its length reduces the force required to cause a member to fail.
Euler's formula: where 'PCR' is the critical force,
'E' is Young's modulus of the material, 'I' is the area moment of inertia of the cross-section,
and 'L' is the length.
This formula shows how the length has a major impact on the critical buckling force.
Figure 2 - Bending Moments
Lateral-torsional buckling occurs when the tensional force below the member and the compressive force above the member reach a critical load where the member moves from a state of stability to a state of instability. This movement to instability results in the forces moving from being solely vertical and are transferred into a combination of horizontal and vertical forces cause the member to buckle about its weaker axis.
However, engineers do not normally consider lateral torsional buckling. If you consider a standard beam, the minor axis plays such a negligible role there is not a possibility of failure about the minor axis. In the case of the Marcy Bridge collapse, lateral torsional buckling occurred because the steel tub girder's minor axis is located nearer the bottom of the girder which means any force applied vertically from the top will create what is known as a transverse force. The definition given by thefreedictionary.com is "Forces applied perpendicularly to the longitudinal axis of a member. Transverse loading causes the member to bend and deflect from its original position, with internal tensile and compressive strains accompanying change in curvature." (The Free Dictionary)
Figure 3 - Side View of Failure
Exponent Inc., one of the companies hired by the main contractor to identify the cause of the failure, through a number of tests and investigations, established that the tub girder, which was supposed to support the deck upon it, did not have sufficient strength to carry the wet concrete without it failing through 'global lateral-torsional buckling'.
Figure 4 - Computer simulation of torsional buckling
"Design guides by US Steel (1979) and Bethlehem Steel (Heins and Hall, 1981) recommend the use of top flange lateral braces..." (Shen-en, de Leon, Dolhon, Drerup, Parfit, & ASCE, 2009)
These design guides where concerned with the lateral torsional buckling but the design recommendations were not included in the Marcy Bridge design. The guidelines were not part of any bridge design codes at that time and the guides were not specifically targeted at the pouring on concrete onto a bridge. The designers may or may not have been aware of these publications but still failed to produce a design that was safe to build.
Considering a similar failure occurred in Sweden a few months prior to the failure and the guides available at the time it may have been advisable for one or more of the contractors, sub contractors or Department of Transport to investigate or be aware of what could happen with lateral torsional buckling. There is also the possibility that they were not aware of the failure, the guidelines, or they chose to believe that the project deadlines were more important than a full-scale review of the design. The fact that Toiga Construction had already constructed three similar bridges along the expressway without incident perhaps gave them a confidence that it would not happen on the fourth one and the designs where suitable and safe.
Identification of Responsibilities
The responsibility of the Marcy Bridge collapse lies mainly with the sub-contractor hired for the design part of the project. MJ Engineering and Land Surveying was the sub-contractor for this part of the project and were responsible for the design of the bridge. They failed to identify the need for bracings for the bridge prior to the concrete deck hardening and providing a resistant force to the twisting motion of the bridge. Tioga Construction Co. (the main contractor) was found to be liable in a lawsuit, brought against them by one of the injured workers, for both using a flawed design and poor training. It has never been disclosed whether Tioga Construction recovered this money from the designer, though his lawyer did say his client was looking for compensation from the sub-contractor/designer. The Department of Transport (NYSDOT) who oversaw the project are not blameless. It had previously been recommended that the Department of Transport stop hiring external contractors and to do the work themselves as they had the skills within their workforce to complete similar projects.
The main ethical issue is really a question, is ignorance an excuse? The NYSDOT and the main contractor should have been involved with the design and the companies should have had warning systems in place to help identify issues both on the site and with other similar constructions.
Lessons to be Learned and Recommendations
The most important lesson learned from this is to consider the global lateral torsional buckling force when doing a design. Changes in the bridge design standards and the creation of the programme UTrAp 2.0, which allows the 'Analysis of Steel Box Girders during Construction', are steps that were taken to prevent further failures. This programme has retrospectively analysed the Marcy bridge collapse and it would have identified and possibly prevented the failure of the bridge and death of the construction worker.
Other lessons learned from this disaster are to be constantly aware of failures around the world and a continuing analysis of failures can assist in preventing multiple failures of similar structures. On-site safety and a network of warning systems would help to minimise fatalities if an error occurred.
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