The Mississippi River Bridge Collapse
The Mississippi River Bridge Collapse
On the late evening hours of August 1, 2007, thirteen people suffered premature deaths at the collapse of the Interstate I-35W overpass that joins Minneapolis to Saint Anthony Falls. The Mississippi bridge collapse commonly referred to as I-35W or bridge 9340, was an eight-lane bridge that was made entirely out of steel. The overpass located in Minneapolis represents the city’s fifth busiest link with an average of one thousand four hundred vehicles per day. The initial reports projected the blame for the incidence on design flaws that resulted in the overpass being unable to support the amounts of weight it carried per day. The event is marked as one of the most publicly out cried incidences in the field of Engineering due to the flawed design of the bridge and the high number of casualties. This paper will provide an overview of the incidence identifying its cause, the investigation, findings and the impact of the incidence on Engineering practices.
Table of Contents
Table of Contents…………………………………………………………………2
List of Figures…………………………………………………………………..…3
The Bridge Collapse: What Happened?………………………………………………..5
Impact on Engineering Practices………………………………………………..10
List of Figures
Figure 1: Demonstration on the deformation caused by elasticity using a steel clip. (Hardy 7).
Figure 2: Three-dimensional diagram depicting the deck truss portion of the I-35W overpass (Hao 4).
Figure 3: A detailed three-dimensional model depicting a joint U10W and L11W embedded on a global model (Hao 4).
Figure 4: Positioning of rivets on the gusset plates (Hao 4).
Figure 5: Heat signature diagram depicting the stress in the U10W (Hao 4).
Figure 6: Overhead photograph of the collapse Interstate overpass (National Transportation Safety Board 25).
Infrastructure is an important economic tool for any nation. Referencing its importance, the degree of precision and care when constructing such networks need to be high to safeguard human life. The catastrophic failure of the Mississippi overpass hoisted chief safety issues concerning the manner in which constructions were being designed and implemented. This paper provides a basic depiction of the Mississippi River Bridge Collapse that took place on August 1, 2007. The event led to a death-toll of fourteen while one hundred and forty-six individuals were injured. The aim of this paper is to narrate about the occurrence, its cause, steps that identified the cause, findings and the implications of the collapse on engineering practices. The bridge collapse is normally seen as a wake up call to engineers, the need to have an increment in redundancy and an improvement in maintenance systems on infrastructures. The bridge incidence is a depiction of the resultant effects of lacking engineering planning and maintenance procedures.
Minneapolis is a city of bridges. The name ‘Minneapolis’ comes from the American Indian word for water. Situated along the shorelines of the West Mississippi river, the city depends on its vast network of bridges to connect with St. Paul and other neighbor environs. The I-35 Interstate that collapsed is one of the vital North-South U.S connective paths that stretch over 1600 miles. The interstate covers from Lake Superior upstate to Laredo in Southern Texas. I-35 interstate only passes the Mississippi river once at the bridge. The design of the bridge dates back in time to early 1960s, constructed by the Huron Incorporated and Industrial Construction Company. Construction of the conduit began in 1964 and came into completion in 1967. The initial vehicle traffic in its early years was 66000 automobiles pre day, by end 2004, the traffic had multiplied to 144000 per day. Four percent of this traffic is commercial trucks.
In early 2001, engineers from the University of Minnesota carried out an assessment of the bridge. The resultant report identified that the overpass had a poor pivoting system and appeared to be ‘fatigued.’ The interstate bridge used a connection of a main truss and branch truss systems. The analysis informed that if one of the truss branches suffered a severe crack, then the entire truss network would follow suit and collapse. The bridge had earlier undergone repairs in 1977, 1998 and 2007 in attempts to increase redundancy. The safety measurements in the repairs entailed an upgrade of lower deck materials through thickening and installation of a median barricade. In 2005, the U.S department of Transportation also assessed the Mississippi overpass giving the conduit a fifty percent rating in safety..
1.1.1 The Bridge Collapse: What Happened?
According to a surveillance video from St Anthony Falls Lock and dam, at exactly 6.05 pm, a portion of the I-35W interstate overpass collapsed (National Transportation Safety Board. Highway Accident Report 43). The footage revealed a number of one hundred and eleven vehicles present at the bridge at the time of the collapse. Of this number, twenty-five were construction vehicles for workers that were conducting repairs on the overpass. The adjoining spans off the bridge gave way lowering the eight lanes into the river. The structure and deck components of the overpass fell into the river and riverbanks. Refer to figure six. Northern sections collapsed onto unoccupied freight trains that were adjacent to a rail yard. The first responders of the incident were the citizens who witnessed the occurrence. The County’s emergency response bodies reported to the scene seven minutes after the bridge collapsed. According to the Highway Accident Report, by 7.55pm, two hours later, emergency workers had rescued the remaining alive person from the rubble.
1.1.2 Engineering Involved
The I-35 overpass had a length of 1907 feet with fourteen spans. The bridge was pivoted on thirteen piers enabling a flow of traffic of four lanes per direction. The centre deck truss, which was 1064 feet long, crossed the river and was composed of two Warren trusses that were parallel. The main trusses had fifty-six connective nodes each that were joined in vertical, horizontal and diagonal directions. The connective mechanism used to join the nodes was riveted gusset plates. Among the three main trusses, one was made up of steel while the other two were made of concrete slabs. No pier was built into the river; instead, the bridge used a 140-meter steel truss. The engineering design used a concept of moving loads from the thump to the superstructure through the bearings. The longitudinal shifts in load were the bridge’s safety mechanism in supporting weight more than its factor.
The main cause of the collapse was a failure in the Gusset Plate located at U10. A gusset plate is a thick steel sheet used as a connector of beams and girders. The beams are connected to columns or used in the connection of trusses (Hardy 7). To facilitate understanding of the failure, basic concepts of elasticity and plasticity have to be applied. Using a simple metallic clip as a demonstration tool, figure one informs on the processes. The more the clip is straightened, the more deformations appear at the ends. The deformation is referred to as negative plasticity. Once the clip is straightened, and a deformation appears, it cannot return to its original shape (Hardy 7). This state represents irreversible elasticity. Repeated deformations in the clip result in a crack at the curves, eventually, the clip divides to separate portions. This is a smaller representation of what happened on the gusset plates.
The initial designs of the overpass did not allow any stress from plastic deformation on its components. Safety design codes in engineering necessitate a component in the construction to support two times the desired weight to ensure it remains elastic no matter the forces applied on it (Subnamarian 5). The gusset plates at U10 did not adhere to this engineering concept. The plates were less thick by half an inch making the elasticity factor equal to one. This means the plate could only support its original weight and given the force generated by momentum of passing automobiles; the plates kept on being deformed (Subnamarian 5). The original designs suggested the plates were one inch thick, but later assessment found them to be half an inch.
- The Investigation of the River Bridge Collapse
2.1 The Investigation
The investigation used forensic designs similar to those at the time that the bridge was being constructed. The analysis was done by The Federal Highway Administration (FHWA) and Turner-Fairbanks Highway Research Centre (TFHRC) (Hao 3). The two research bodies applied the Finite Element Analysis Approach (FEAA) to highlight how the U10 and L11 nodes suffered deformation leading to their failure. The FEAA was attained by embedding three-dimensional models of the joints into a structural element. Figure 2 shows the graphic model and figure 3 shows how they were embedded into the structure. The models applied solid and shell elements in vital areas of investigation. The complete three-dimensional simulation consisted of rivet locations, the gusset plate and how the nodes were fastened. Refer to figure four. The simulation using specialized computer programs revealed on how the gusset plates gradually deformed from pressure (Hao 4).
After simulation of the plates was carried out, risk analysis was applied to explore the unstable nature of the gusset plates. In this process, an arc formulation solved loads and displacements simultaneously. A sub-model portion of the bridge was used to give more detail. The sub-model consisted of two rivets located in high-stress regions of the bridge modeled with solid components and contact pairs. The rest of the rivets in the sub-model were used as mesh independent holders. Figure 5 shows the sub-model using heat signatures to reveal on the stress areas that resulted in the collapse of I-35 (Hao 5).
The results of the investigations can be summarized in five points according to the Federal Highway Administration. First, it was determined that the main truss and connective sub trusses were initially designed with acceptable safety factors, but the main truss was the flaw component in the network (National Transportation Safety Board 101). There was no evidence that suggested the sub-trusses were accountable for initiating the overpass collapse. The sub trusses were fractured due to the weight of the collapsing bridge and not the initial deformation that the main truss had. Second, referencing loads, the safety of the plates in elasticity was equal to one. The plates at U10 were half an inch less than the original design and could not support twice its weight as necessitated by engineers in construction (National Transportation Safety Board 101). The reason as to why the gusset plates were half an inch less in thickness was never revealed even with perusal of the initial construction plans of 1967.
Third, the overpass collapse was caused by the failure of the gusset plates in the main truss U10 around the area L9/U10 (National Transportation Safety Board 101). The plates underwent plastic deformation reducing their capacity to meet the stresses that were placed on them at the time of crumpling. The fourth finding was that temperature fluctuations played a significant role in directing the stress in the truss members to the main node at U10. Stress from the branch nodes transferred to the gusset plates and main nodes initiating the collapse. Temperature fluctuations were because of moving wind gradients above and below the overpass, moving vehicles on top of the bridge and heat from the sun. The final cause of the collapse was the additional weight that was put on the bridge during its modifications earlier that year. The extra weight only increased the rate of plastic deformation in the gusset plates (National Transportation Safety Board 101).
The FHWA and TFHRC using the results from their investigations gave various recommendations to two governmental bodies that were accountable for transport infrastructure. The first set of recommendations went to the Federal Highway Association. The regulatory body in conjunction with state highway authorities should develop a design quality and design assurance control program. The program should be implemented in all states to standardize infrastructure construction processes. The program will entails procedures and algorithms that will facilitate detection of the infrastructure design flaws before implementation of the designs. The program will be a verification and validation platform assessing the accuracy in specifications dealing with loads. Bridge owners should analyze corrosion levels in gusset plates especially in visual impaired sections of the overpasses (National Transportation Safety Board 101).
The FHA should restructure its bridge inspector training. To attain the restructure, the institution should update the training courses offered at the National Highway Institute. The course should have inspection techniques unique for the examination of gusset plates educating on the points related to plate distortion. The final changes in the FHA were on learning materials that required revisions to incorporate the new set learning course. The Bridge Inspector Reference Manual was to be removed from the syllabus until its revision.
The second set of recommendations went to the American Association of State Highway and Transportation officials. The AASHT was to assist the FHA in its development and implementation of the quality assurance program. The government body should revise its course manual for overpass examination. The manual had to be updated to include guidelines for performing load ratings on new infrastructures before they were opened for use. The manual also had to be modified to include procedures for evaluating the status of gusset plates while calculating load for steel trusses. The AASHT should develop instructions for overpass owners to ensure that building loads and raw materials used in a structure, be it construction or maintenance, do not stress the truss connections because of weight. The final recommendation was that the institution should incorporate gusset plates s recognized structural elements. Rating the plates as vital structural elements would educate bridge owners on their importance. Owners would be able to track and respond to damage, distortion and corrosion on the plates.
- Impact on Engineering Practices
The Mississippi Bridge Collapse led to a massive public outcry on the safety in using public infrastructure. The outcry led to an evaluation of engineering practices with reference to the collapsed overpass. The resultant issues of this evaluation can be categorized into two, before project implementation and after project implementation. The issues highlight the drawbacks in planning and response in multi-threat environments that engineers need to incorporate while building. After project modifications entail scientific procedures while before project adjustments employ educative algorithms.
3.1 Before Project Implementation
Before project modifications, use the rule of prevention in ensuring quality assurance in product development. The adjustments use experiences to formulate guidelines that are applied in initiation of a project. According to Cohen and Stambaugh, the primary causes in engineering catastrophes are human factors, design errors, the poor nature of materials, extreme conditions and combinations of the other factors (Cohen and Stambaugh 51). Apart from the extreme weather, the other three reasons are eradicated through a transformation of human factor. To facilitate this change, engineering ethics is integrated with current course works. The engineer has accountability towards his client. Ethics mandated a review of Bridge Inspector Reference Manuals and the Manual for bridge Evaluation. The two manuals were updated to raise the awareness and skills of inspectors while evaluating public infrastructure.
In the U.S, Human Error Assessment and Reduction Technique (HEART) has been integrated in engineering to ascertain human reliability (Cohen and Stambaugh 54). The technique was first formulated in 1986. The process is an engineering requirement while contracting in the U.S and is used to evaluate the probability of a human flaw presenting itself in a job. The process entails four stages. The first stage identifies the full skills required for a given task. The second stage involves the development of a human unreliability chart to ascertain the degree of human reliability a task necessitates. The third stage compares the required skills and the human unreliability levels. The higher the unreliability level for a task, the better it is to perform it because of low-error probabilities. Deciding on whether to carry out a task represents the final stage of the HEART process.
3.2 After Project Implementation
Modifications on engineering practices after project implementation aim at the integration of critical thinking in planning, maintenance and response stages of development. The adjustments use a top-down approach in project development in attempts of reducing failures and improving quality assurance. The first adjustment was controlled planning using structured brainstorming. Brainstorming is a cognitive process that follows set rules in the determination of new concepts. Structured brainstorming, on the other hand, is a cognitive twelve-step process developed by engineers that facilitate identification of forces or factors that might come up in the middle of a given situation. Structured planning facilitates a group of engineers to carry out the necessary adjustments to the plan to make it solid.
The second adjustment made to engineering is the formation of a back up response map that follows any new plan. Effective response maps guard a project from maximized effects of failure through rapid action. Referencing the Mississippi overpass rubble, effective response would have been in immediate reaction by emergency services. Response map could also have been in secondary lanes or gusset planes to ensure wide weight distribution and support. Back up plans are team developed through an engineering approach of worst-case scenarios. They anticipate primary and secondary implications from malfunction; cause of failure and how to minimize or eradicate them.
3.3 Other Impacts on Engineering
The collapse of the Mississippi overpass has led to the streamlining of the engineering profession making it as accountable as medicine. An engineer is personally responsible for flaws that occur under his supervision. An engineer can be prosecuted for any damages in property, loss of assets or loss of life. For this reason, an engineer must make available a constructible design document that reveals the feasibility of his or her plan. Any structural constructions must be verified and approved by government inspectors before, during and after operations. Structural constructions include erection drawings, electrical systems and structural lifting. Another impact is the specification of duties between a designer and contractor. The two professions and duties have to be separately defined to determine the guilty party given a project flaw. The designer is responsible for developing a project while a contractor is accountable of implementing it.
The Mississippi Highway Bridge Collapse is a depiction of the consequences of poor engineering practices. The overpass had undergone assessments prior to its collapse that all gave the same concern on the conduit’s safety. The gusset plates had an unstable bending ability given its elasticity. The heat, weight and corrosion increased the rate of deformation from the elasticity resulting in the overpass. Concrete material used to reinforce the truss and gullet plates increased the weight force highlighting the poor engineering practices in maintenance. The overpass collapse marked the initiation of engineering changes ranging from modified inspector training to new policies in construction. The modifications necessitate an engineer to employ critical thinking in the planning of a project, development of an effective response map and integration of ethics in construction. Engineer designs must also be verified and validated by a professional government examiner before any constructions begin. The Mississippi collapse is one of many catastrophes that converse on the need of engineers to carry out flaw free designs and detailed maintenance of constructions to safeguard human life.
Cohen, Harold and Hollis Stambaugh. I-35W Bridge Collapse and Response. U.S. Fire Administration Technical Report Series. 2007. 1-60. Web. 22 July 2014. Print.
Hao.S. I-35W Bridge Collapse. Journal of Bridge Engineering. 2010. 1-7. Web.22 July 2014. Print.
Hardy, Devon. Collapse. A Case Study of the Minneapolis Bridge Disaster. 2013.1-18. Web. 22 July 2014. Print.
National Transportation Safety Board. Highway Accident Report. Collapse of I-35W Highway Bridge. 2008. 1-178. Web. 22 July 2014. Print.
Subnamarian, N. I-35W Mississippi River Bridge Failure- Is It a Wake Up Call? The Indian Concrete Journal. 2008. 29-38. Web. 22 July 2014. Print.
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