Structural Failures, Part I - An Engineer's Aspect


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Monday, June 22, 2009

Structural Failures, Part I

Engineering a structure in which someone will live, ride, fly, or work is a tremendous responsibility. Generally the first duty recognized by Professional and Chartered engineers is to the safety of the public. As with any profession, mistakes can happen. Unfortunately, in the case of engineering structures, a small mistake can prove to be fatal. Therefore, it is often instructive to study cases where mistakes were made in order to understand and never repeat the mistake.

The following is the first installment in my collection of case studies of Structural Failures.

1907: Quebec City, Quebec--Quebec Bridge Collapse

According to Wikipedia,
"The Quebec Bridge, was included in the National Transcontinental Railway project, undertaken by the federal government. By 1904, the structure was taking shape. However, preliminary calculations made early in the planning stages were never properly checked when the design was finalized, and the actual weight of the bridge was far in excess of its carrying capacity. The dead load was too heavy. All went well until the bridge was nearing completion in the summer of 1907, when the local engineering team under Norman McLure began noticing increasing distortions of key structural members already in place.

Figure 1--Photographs of the lifting of the center span of the Quebec Bridge in place, 1919, considered to be a major engineering achievement for the day. Source: Wikipedia Commons, originally published in "Popular Mechanics" Magazine Dec. 1919.

After four years of construction, the south arm and part of the central section of the bridge collapsed into the St. Lawrence River in just 15 seconds. Of the 86 workers on the bridge that day near quitting time, 75 were killed and the rest were injured."

1919: Boston Molasses Disaster

Figure 2--The Aftermath of the Boston Molasses Disaster. Source: Wikipedia Commons.

Twenty-one people died, 150 were injured, and buildings were destroyed in the 1919 Boston Molasses Disaster. "A huge molasses tank 50 ft (15 m) tall, 90 ft (27 m) in diameter and containing as much as 2,300,000 US gal (8,700,000 L) collapsed. Witnesses stated that as it collapsed, there was a loud rumbling sound like a machine gun as the rivets shot out of the tank, and that the ground shook as if a train were passing by," states Wikipedia.

Wikipedia also says that "The collapse unleashed an immense wave of molasses between 8 and 15 ft (2.5 to 4.5 m) high, moving at 35 mph (56 km/h), and exerting a pressure of 2 ton/ft² (200 kPa).[5] The molasses wave was of sufficient force to break the girders of the adjacent Boston Elevated Railway's Atlantic Avenue structure and lift a train off the tracks. Nearby, buildings were swept off their foundations and crushed. Several blocks were flooded to a depth of 2 to 3 feet (60 to 90 cm)."

Cecil Adams, in a response for straightdope asserts, "Construction of the tank had been overseen, or more accurately gazed stupidly at, by Arthur Jell, a bean counter with no technical background who was unable even to read blueprints. Anxious to complete the tank in time for the arrival of the first molasses shipment, Jell forwent the elementary precaution of filling it first with water to test for leaks. Once molasses was pumped in, the tank leaked so copiously at the seams that neighborhood kids collected the drippings in cans. When an alarmed employee complained, Jell's response was to have the tank painted brown so the leaks wouldn't be so noticeable."

An inquiry found the disaster was due to inadequate construction; USIA paid out more than $600,000 in damages--at least $6.6 million in today's money (Adams).

1940: Tacoma, Washington--Tacoma Narrows Bridge Collapse

Figure 3--Tacoma Narrows Bridge (1940). Source: Wikipedia.

"On the morning of November 7, 1940, the Tacoma Narrows Bridge twisted violently in 42-mile-per-hour winds and collapsed into the cold waters of the Puget Sound. The disaster -- which luckily took no human lives -- shook the engineering community and forever changed the way bridges were built around the world.

Engineer Leon Moisseiff had designed the ultimate in slender bridges. The roadway was a mere 39 feet -- only eight teenagers lying head to toe would fit across the bridge! Moisseiff strengthened his narrow bridge with a solid steel girder beneath the roadway. But soon after it opened, the Tacoma Narrows started behaving strangely. Wind caused the bridge to sway back and forth, and it also sent rippling waves along the deck. The Tacoma Narrows tore itself apart only four months later.

Years later, engineers found that the solid girders actually blocked the wind and caused the slender bridge to twist. The twisting bridge fanned the steady wind into a swirling motion, which caused the bridge to twist even more -- and eventually snap in two. The Tacoma Narrows Bridge was replaced in 1950 by a new bridge stiffened with a truss. Rather than blocking the wind, the open truss allowed the wind to blow through the new bridge."

1954: Mediterranean Sea off Elba--BOAC Flight 781, de Havilland Comet

Figure 4--An illustration showing the recovered (shaded) parts of the wreckage of the de Havilland Comet 1 G-ALYP Yoke Peter and the forward ADF* aerial window in the cabin roof where the initial fatigue failure occurred - after an illustration in Air Disasters by Stanley Stewart. Source: Wikipedia Commons.

According to Wikipedia,
"On 10 January 1954, BOAC Flight 781 a de Havilland Comet 1 (type DH-106), took off from Ciampino Airport in Rome, Italy en route to Heathrow Airport in London, England on the final leg of its flight from Singapore. At about 10:00 GMT, the aircraft suffered an explosive decompression at altitude and crashed into the Mediterranean Sea, killing everyone on board.

The flight was operated by British Overseas Airways Corporation (BOAC) using the aircraft G-ALYP ('Yoke Peter')."

After numerous tests on the recovered wreckage, bookrags claims that,
"...the RAE were able to conclude that the crash had been due to failure of the pressure cabin at the forward ADF window in the roof. This 'window' was in fact one of two apertures for the aerials of an electronic navigation system in which opaque fibreglass panels took the place of the window 'glass.' The failure was a result of metal fatigue caused by the repeated pressurisation and de-pressurisation of the aircraft cabin. Another worrying fact was that the supports around the windows were only riveted not glued, as the original specifications for the aircraft had called for. The problem was exacerbated by the punch rivet construction technique employed. Unlike drill riveting, the imperfect nature of the hole created by punch riveting may cause the start of fatigue cracks around the rivet. The Comet's pressure cabin had been designed to a safety factor comfortably in excess of that required by British Civil Airworthiness Requirements (2.5x P as opposed to the requirement of 1.33x P and an ultimate load of 2x P, P being the cabin 'Proof' pressure) and the accident caused a revision in the estimates of the safe loading strength requirements of airliner pressure cabins. In addition, it was discovered that the stresses around pressure cabin apertures were considerably higher than had been appreciated, especially around sharp-cornered cut-outs, such as windows."

1968: London, England--Ronan Point Collapse

Figure 5--The aftermath of the collapse of the corner of Ronan Point following a gas explosion. Source: Wikipedia, originally published in "The Daily Telegraph," 1968.

On May 16, 1968, early in the morning, 56-year-old cake decorator Ivy Hodge of apartment 90 on the 18th floor of the Ronan Point apartment tower lit a match for her stove to brew her morning cup of tea. There was a gas leak and the resulting explosion, knocked her unconscious.

The pressure of the small gas explosion blew out the walls of her apartment and initiated a partial collapse of the 22-story structure that killed four people and injured seventeen. Miss Hodge and her gas stove were blown across the room by the explosion. She took her stove to her new address after the explosion.

According to the proceedings of the 3rd ASCE Forensics Congress, "The force of the explosion knocked out the opposite corner walls of the apartment. These walls were the sole support for the walls directly above. This created a chain reaction in which floor nineteen collapsed, then floor twenty and so on, propagating upward. The four floors fell onto level eighteen, which initiated a second phase of progressive collapse. This sudden impact loading on floor eighteen caused it to give way, smashing floor seventeen and progressing until it reached the ground."

The ASCE Forensics Congress concluded:
"The investigations found that the Ronan Point apartment tower was deeply flawed in both design and construction. The existing building codes were inadequate for ensuring the safety and integrity of high-rise precast concrete apartment buildings. In particular, the design wind pressures were too low and did not account for the height of the building. The Larsen Nielson building system, intended for buildings with only six stories, had been extended past the point of safety.

The tower consisted of precast panels joined together, without a structural frame. The apartment tower lacked alternate load paths to redistribute forces in the event of a partial collapse. When the structure was dismantled, investigators found appallingly poor workmanship of the critical connections between the panels. The already shaky structure had been further weakened by the inadequate construction practices. The result was described by Levy and Salvadori (1992) as a “house of cards.”

The relatively low overpressure from the gas explosion should have led to localized damage at most, not a partial progressive collapse and the loss of four lives. The evaluation also found that the building was unusually vulnerable to ordinary wind and fire loading."