Research Proposal: Building Construction Collapse

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Collapse of I-35W in Minnesota

How safe are America's older concrete highway bridges? And how long is a concrete bridge expected to remain viable? What are the influences that have a negative effect on the integrity of concrete-steel composite bridge girders? Has there been adequate research into these matters? What are the scholarly journals reporting?

These concerns are not new, but they have become more public and urgent since the collapse of the I-35 freeway bridge (#9340) in Minnesota in 2007. That structural failure has raised new concerns about what effects corrosion -- and bridge design -- can have and does have on the structural integrity of concrete bridge girders. And although engineers have been working with contractors and planners for many years to address issues of bridge safety, much remains to be done.

To the precise point: what were the factors that led up to the collapse of the freeway bridge in Minnesota? And indeed why did the I-35W freeway bridge collapse and kill 13 people in Minnesota on August 1, 2007?

Review of Pertinent Literature

The I-35W freeway bridge was built between 1964 and 1967, according to an article in Engineering News-Record, and there exist a "wealth of documents" prepared over the previous ten years of inspections (Van Hampton, et al., 2007). The reports offered detailed information on the "aging steel truss bridge's condition and recommended possible fixes" (Van Hampton 2007). The bridge was built using steel multibeam approach spans, "concrete slab approach spans and steel deck truss main spans…[and was constructed with] a single 458-ft-long steel arch to avoid putting piers into the water" because those piers would have interfered with traffic on the Mississippi far below (Van Hampton). Engineers, "because of a lack of redundancy," had criticized the design Hampton adds.

This bridge was designed to last about fifty years, and was due to be replaced in 2020. State engineers had conducted load tests and reported that it may have performed better than expected; other reports show state engineers "made a series of deliberate decisions to examine fatigue, repair aging connections and defer important retrofits to the bridge's facture-critical deck truss" (Van Hampton). The state of Minnesota claimed that the inspections conducted on the bridge "exceeded federal guidelines" because it was done every year rather than every two years as "typically prescribed" (Van Hampton).

In 1990, the Minnesota Department of Transportation rated the bridge "structurally deficient" and "functionally obsolete" -- and on a scale of one to nine, the bridge was rated a "four" (Van Hampton). Other problems were noted by the inspectors, like "mist from nearby St. Anthony Falls" and "bird droppings" both which "accelerated corrosion," Hampton explains. The state recommended possible replacement at that time.

Over the past twenty or so years, the state drilled out "numerous fatigue cracks," it repainted the bridge and added steel "stiffeners" and installed a "potassium-acetate deicing system" for $618,000. That deicing system was the largest of its kind at the time; the system (with a 3,100 gallon tank) trained 76 spray nozzles in "strategic locations" on the bridge. Attempts to keep the bridge safe obviously were not successful in the long run, but many studies were conducted, Van Hampton writes, and while there were "many poor fatigue details" on the main and floor trusses it was deemed "fit for service."

The state even hired a San Francisco-based company (URS Corp.) in 2003 to launch a long-term evaluation as to whether the "fracture-critical bridge would collapse if any truss member failed" (Van Hampton). The report that URS Corp. gave the state of Minnesota in June 2006 a report that suggested the Department of Transportation should "retrofit 20 fracture-critical members in the main truss" and also add "internal redundancy" by bolstering with high-performance steel plates and high-strength bolts (Van Hampton). But engineers were reticent to install the steel plates in fear that the retrofitting would "require massive drilling and possibly compromise the structure" and so other options were considered. Those included retrofitting 52 fracture-critical truss members on the main truss and other testing.

As late as May, 2007, Minnesota transportation officials began a "critical inspection" which covered about 50% of the bridge; they found no "unsafe" details and left the other half of the inspection for later in 2007, Van Hampton reported.

And then, at just past 6:00 P.M. Central Time, the 456-ft-long center span of I-35W gave way -- and all the vehicles, passengers, and construction crews who were working on resurfacing the deck of the bridge "disappeared from sight behind a cloud of concrete dust" (McNichol 2007), according to an article in Roads & Bridges. Virtually hundreds of tons of superstructure made a 108-ft plunge into the Mississippi River, killing 13 people and injuring over a hundred people. There were 110 vehicles in the wreckage; soon some 50 agencies responded to the disaster and news of the incident flashed around the world on television and within 24 hours 19 investigators from the National Transportation Safety Board (NTSB) arrived to begin their work (McNichol 46).

In the immediate aftermath of the tragedy, according to the McNichol article, several potential causes were suspected: water, salt, stress and corrosion, "large gussets," bird guano, road salt, "extreme temperatures" during winter and summer, overloading, the 287 tons of construction equipment on the bridge at the time of collapse -- or "other structural support beam problems" (McNichol 49).

All those likely suspected causes notwithstanding, in November 2008 the NTSB issued its final investigative report (Ichniowski 2008). The report cited the "probably cause" as a "design error" that resulted in the failure of the "gusset plates" on the 41-year-old 1,907-ft-long steel-deck truss bridge (Ichniowski). Writing in the Engineering News-Record, Ichniowski explains that the gusset plates -- at that moment in time -- could not carry loads that included deck upgrades, construction materials, equipment and "rush-hour traffic."

In addition to that report, the NTSB also blamed the failure of the quality-assurance procedures implemented by the designer, Sverdrup & Parcel from St. Louis; the designer did not "ensure calculations for the gusset plates on the bridge's main truss were performed" (Ichniowski). But Sverdrup & Parcel did not receive all the criticism; there was plenty left over for federal and state agencies that were responsible for "inadequate design review," Ichniowski writes. The problem goes back to the original design in terms of the "gusset plates" in the main trusses, the NTSB claimed. Those plates should have been an inch thick, but they were only a half-inch thick.

Meanwhile, Leland Teschler, the editor of the industry magazine Machine Design, offers some interesting background into this topic in a generalized narrative. Teschler explains that the Technical Council on Forensic Engineering of the American Society of Civil Engineers (ASCE) became concerned in the 1980s over "a rise in structural failures and performance deficiencies" vis-a-vis bridges and other massive structures. The ASCE noticed one particular problem, that "designers were losing control over how construction projects were executed" (Teschler 2007).

Bridges were being built on a "fast track approach," Teschler writes, because that strategy saved time and money "but made it easier for contractors and designers to misunderstand each other" (Teschler p. 8). The worst part of fast tracking bridge construction was that it "led to lines of responsibility during construction that were unclear," Teschler continues. And moreover, the cost-cutting philosophy extended to the operation of buildings and "other technically sophisticated structures" which resulted in projects being built with "no one around who could properly maintain complicated systems or inspect for hazardous conditions in an intelligent manner" (Teschler 8).

An example of what Teschler is writing about happened with Boston's Big Dig; the wrong epoxy was used to secure the ceiling anchor bolts -- builders wanted an epoxy that dried very quickly to be part of the fast track strategy. As a result,… [END OF PREVIEW]

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