In Part 1 of this article, it was noted that there are more than 617,000 bridges in the United States. Of those, more than 46,000 are rated as “structurally deficient.” In Part 2 of FreightWaves Classics: Fix U.S. bridges!, several of the more high-profile bridge collapses are outlined.
Tacoma Narrows Bridge – 1940
The first Tacoma Narrows Bridge collapsed in 1940. It had been built to span the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula in Washington state. The bridge opened to traffic on July 1, 1940, and collapsed into Puget Sound just slightly more than four months later, on November 7, 1940.
When it collapsed, the Tacoma Narrows Bridge was the world’s third-longest suspension bridge by main span, trailing only the Golden Gate Bridge and the George Washington Bridge. The bridge’s total length was 5,939 feet; its longest span was 2,800 feet and it was 195 feet above the waters of Puget Sound.
The bridge’s roadway twisted and vibrated violently under 40-mile-per-hour winds on the day of its collapse. Since the bridge’s “spectacular” collapse, it “has attracted the attention of engineers, physicists and mathematicians.”
Construction on the bridge began in September 1938. As the bridge’s deck was built, it began to move vertically in windy conditions; the construction workers who were building it nicknamed it “Galloping Gertie.” The bridge’s motion continued after it was opened to traffic, despite several measures to decrease its swaying. None of the measures worked, and the bridge’s main span collapsed on the morning of November 7, 1940. Its deck “oscillated in an alternating twisting motion that gradually increased in amplitude until the deck tore apart.”
The bridge’s towers and cables were dismantled after the collapse and sold as scrap metal. Rebuilding the bridge was stalled for nearly a decade (in large part due to World War II). In 1950, a new Tacoma Narrows Bridge opened in the same location as the first bridge. During its construction, the builders used the first bridge’s tower pedestals and cable anchorages. The span that collapsed into Puget Sound serves as an artificial reef.
The bridge’s collapse is often a lesson in physics textbooks. The bridge collapsed because “moderate winds produced aeroelastic flutter that was self-exciting and unbounded – for any constant sustained wind speed above about 35 mph, the amplitude of the (torsional) flutter oscillation would continuously increase, with a negative damping factor (i.e., a reinforcing effect, opposite to damping). The collapse boosted research into bridge aerodynamics-aeroelastics, which has influenced the designs of all later long-span bridges.”
Silver Bridge – 1967
The Silver Bridge connected Point Pleasant, West Virginia and Gallipolis, Ohio, spanning the Ohio River. A two-lane bridge, it was built in 1928; when it collapsed it was just under 40 years old. The bridge carried as many as 4,000 vehicles over the river daily. In 1929, the Engineering News Record called it “the first of its type in the United States” because of its use of eyebars (55-foot-long sections of steel) instead of steel wire cables.
The Silver Bridge’s original design did use conventional wire cables. According to the West Virginia Department of Transportation (WVDOT), the eyebar chain design was chosen because it was less expensive. The Engineering News Record also stated that, based on the bridge’s design, “any adjustments in the chains, hangers or trusses after erection” would not be possible.
On December 5, 1967, eyewitnesses heard what sounded like a gunshot. However, it was the bridge, and In less than 20 seconds, the bridge “folded like a deck of cards,” according to the WVDOT. When the bridge collapsed, 64 people and 32 vehicles fell into the river; 46 people died.
In the aftermath of the collapse, an investigation found it was due to a small stress crack inside the loop of an eyebar. The crack was attributed to corrosion, and “practically impossible to detect.” Laboratory tests concluded that, “With the north… chain thus broken, the structure’s design made total collapse… inevitable.”
With the bridge gone, an economic loss of about $1 million per month (about $7.65 million today) occurred. In 1968 President Lyndon B. Johnson ordered a federal-state program to reconstruct the bridge. The Silver Memorial Bridge spans the Ohio River where the first bridge stood; it was completed in 1969 and still stands today.
Sunshine Skyway Bridge – 1980
The Sunshine Skyway is a cable-stayed bridge that spans Florida’s lower Tampa Bay and connects St. Petersburg to Terra Ceia. Today’s Sunshine Skyway opened in 1987 and is the second bridge of that name on the site. The four-lane bridge carries Interstate 275 and U.S. Route 19 through Pinellas, Hillsborough and Manatee counties.
The original two-lane bridge opened on September 6, 1954. A similar structure was built parallel and to the west of the two-lane span in 1969. This created two lanes in both directions and the bridges were brought up to interstate highway standards at the time the second bridge was built. However, the second bridge’s opening was delayed until 1971 because its south main pier (support column) had cracked due to insufficient supporting pile depth. When it was opened, the newer span was used for southbound traffic, while the original span carried northbound traffic.
A sudden and severe squall hit Tampa Bay south of St. Petersburg on May 9, 1980. The 20-ton freighter MV Summit Venture was moving across the bay at the time. The storm caused it to temporarily lose its radar; the harbor pilot and its crew struggled to navigate the bay’s shipping channel through “fog, torrential rain and hurricane-force winds.”
Unfortunately, at 7:38 a.m. the ship rammed two of the Sunshine Skyway Bridge’s piers, causing a 1,200-foot-long section of the southbound span to fall into the water. Six cars, a pickup truck and a Greyhound bus plunged 150 feet through steel and iron beams into Tampa Bay, according to NPR. The accident killed 35 people.
John Lerro was the harbor pilot who had been steering the ship when the squall occurred. He was later cleared of wrongdoing by both a state grand jury and a U.S. Coast Guard (USCG) investigation. It was explained that a microburst hit the freighter suddenly with torrential rains and 70 mph winds as it was in the middle of a turn in the shipping channel near the bridge. Although Lerro put the ship’s engines into full reverse and ordered an emergency drop of the anchor, the ship’s bow still hit the two piers with enough force to cause a portion of the roadway to collapse.
The ensuing disaster was the second to occur at or near the bridge in 1980. In January the USCG Cutter Blackthorn collided with the tanker Capricorn near the bridge. The Coast Guard cutter sank and 23 crew members died.
A new Sunshine Skyway was completed in 1987 and is used today. The new bridge has a number of safety features, including: a higher span; the channel underneath it was widened; and large concrete barriers, nicknamed “dolphins,” were installed so that ships could not hit the bridge’s supports.
Cypress Street Viaduct – 1989
Interstate 880’s Cypress Street Viaduct, which was described as a “concrete-and-steel leviathan,” collapsed during the 6.9 magnitude Loma Prieta earthquake that struck Oakland, California on October 17, 1989.
A viaduct is defined as a “long bridge-like structure carrying a road or railroad across a valley or other low ground.” The viaduct was also known as the Cypress Structure or the Cypress Freeway. It was a 1.6-mile-long raised two-deck, multi-lane (four lanes per tier) freeway built of reinforced concrete. It had originally been part of the Nimitz Freeway (State Route 17), and later was a component of Interstate 880.
The Cypress Street Viaduct began carrying traffic on June 11, 1957. It replaced an earlier single-deck viaduct that had been built in the 1930s as one of the approaches to the San Francisco-Oakland Bay Bridge. It was located along Cypress Street between 7th Street and I-80 in the West Oakland neighborhood.
The concrete roadway’s two-tier design was common in California. It incorporated complex design calculations (during an era when highway engineers did not have computers) to gauge how much steel was necessary to support the structure. After the viaduct collapsed, engineers and geologists determined that “its collapse was inevitable.”
During the earthquake, much of the upper deck collapsed onto the lower deck, killing 42 people. From 16th Street north to the MacArthur Maze, the viaduct collapsed “due to ground movement and structural flaws.”
Before the earthquake, the roadway’s upper deck was used by southbound traffic; the lower deck was used by northbound traffic. While some sections of the roadway were supported by two columns on either side, other sections were built with only a single supporting column. According to engineers who examined the wreckage after the earthquake, the design could not withstand the earthquake because “the upper portions of the exterior columns were not tied by reinforcing to the lower columns, and the columns were not sufficiently ringed to prevent bursting.” When it was designed and built such structures were not analyzed “as a whole.” Therefore, “large structure motion contributed to the collapse.” Moreover, the structure was built on filled land on top of bay clay. Filled land is “highly susceptible to soil settlement during an earthquake, and bay clay exhibits larger ground motion.”
Of the 63 total deaths attributed to the earthquake, 42 died in the bridge collapse. That total may have been much higher; however, traffic was lighter than normal because the San Francisco Giants and the Oakland A’s were playing a World Series game when the earthquake struck. Many had left work early or stayed late to watch the game on television.
After the earthquake stopped, residents of the neighborhood and workers began exploring the collapsed structure to rescue those trapped in the shattered structure. The upper tier had collapsed onto the lower tier.
According to an article in The Washington Post, “factors that led to the collapse included the structure’s ‘hinge joints,’” which added flexibility to the viaduct. The Post reported: “Hinge joints were included in the freeway’s column design to absorb weight and vertical movement caused by traffic. But during the Loma Prieta earthquake the ground also shook laterally, creating stress the hinge joints could not withstand. Few steel reinforcing rods snaked up through the joints. The upper and lower columns are honeycombed with these rebars, but the hinge joints contain only a few. During the quake, the columns acted like weak legs on a wobbly table. The lateral movement caused them to snap outward at the weakest point – the hinge joints – freeing the upper deck to pancake down onto the lower one.”
I-35W Mississippi River Bridge – 2007
In 2007, the Interstate 35W bridge over the Mississippi River in Minneapolis, Minnesota carried an average of 140,000 vehicles daily. During the evening rush-hour on August 1, the center span of the eight-lane, steel truss arch bridge suddenly collapsed. That led adjoining sections to crumble. Occupants of 111 vehicles and 18 construction workers fell as far as 115 feet into the river or onto its banks. Thirteen people were killed and 145 were injured in the bridge collapse.
The ensuing investigation by the National Transportation Safety Board found that the bridge suffered a catastrophic failure; its metal gusset plates were “too thin to support the weight of the span, along with rush-hour traffic and the construction equipment on the deck at the time of the accident.”
The bridge’s 14 spans were 1,907 feet long in total. The three main spans were of deck truss construction; all but two of the 11 approach spans were steel multi-girder construction. The two exceptions were constructed of concrete slabs. The bridge’s center span was a single 458-foot steel arched truss over the 390-foot channel.
In 1990, federal government inspectors had rated the I-35W bridge as “structurally deficient,” citing significant corrosion. When the bridge collapsed, about 75,000 other U.S. bridges were classified as “structurally deficient.”
In 2001, the University of Minnesota’s civil engineering department studied the bridge. Cracking in the cross girders at the end of the approach spans was found.
The bridge was again rated “structurally deficient” in 2005 and in “possible need of replacement,” according to the U.S. Department of Transportation’s National Bridge Inventory database. Subsequent inspection reports noted problems. An inspection on June 15, 2006 found cracking and fatigue on the bridge. The day after the bridge collapsed (August 2, 2007), Minnesota Governor Tim Pawlenty stated that the bridge had been scheduled to be replaced in 2020 – 30 years after it was first rated “structurally deficient!”
After the collapse, a replacement span for the bridge opened about 13 months later (in September 2008).
The disasters cited above happened across the United States over about a 70-year period. They are just representative samples of many more bridge-related disasters that have taken place. In the wake of the Silver Bridge collapse, Congress passed legislation mandating federal bridge inspection standards. In other words, the bridges that are “structurally deficient” are known.
As the House of Representatives debates the infrastructure legislation, House members should at least consider the state of America’s bridges and the fate of their constituents.