Tuesday, June 23, 2009

Structural Failures, Part II

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 second installment in my collection of case studies of Structural Failures.

1970: Melbourne, Victoria, Australia--Westgate Bridge Collapse

Figure 6--Westgate Bridge Collapse. source: rocketboom.com.

At 11.50 am on 15 October 1970, two years into construction of the bridge, the 112 metre (367.5 ft) span between piers 10 and 11 collapsed and fell 50 metres (164 ft) to the ground and water below. Thirty-five construction workers were killed.Many who died were beneath the structure in workers' huts which were crushed by the falling span. Others were working on and inside the girder when it fell. Wikipedia states, "The whole 2,000-tonne mass plummeted into the Yarra River mud with an explosion of gas, dust and mangled metal that shook buildings hundreds of metres away. Homes were spattered with flying mud. The roar of the impact, the explosion, and the fire that followed, could be heard some distance away."

Quoting from trivia-library.com,
"The Disaster: Construction problems, too, were pyramiding. One big headache was the almost 400'-long span between piers 10 and 11. It was decided to assemble 2 sections of the span on the ground, then hoist them into place and bolt them together. A not uncommon method of assembly, and usually successful when done with utmost care. But when the 2 sections of this particular span were brought together in August of 1970, the north half section was 4 1/2" above the south half section.

Rather than take the sections down for correction, engineers decided to put an 8-ton weight on the high section to bring it level with the lower. On September 6 there was a major buckle. Work came to a halt, followed by a month of deliberation. Then engineers decided to unbolt the 2 sections on each side of the buckle. They theorized that the weight of the high section would cause it to lower and match the level of the lower section. Then it could be rebolted. Operation "unbolt" began at 8:30 A.M. on October 15. At 1st it appeared successful. The high section sank to within 1 1/8" of the lower, but before the sections could be rebolted, the buckle became greater.

At 11:50 A.M. the huge 2-section span could tolerate no more tampering--the stresses were too great. With a terrible grinding roar, bridge, men, and equipment tumbled 160' into the river. More than half the workmen on and under the 2-section span lost their lives.

Aftermath: In the investigation that followed, only the suppliers were deemed to be blameless. Designers, contractors, and engineers were dismissed, to be replaced by counterparts who, hopefully, would complete West Gate Bridge without further mishap."

1976: Idaho--Teton Dam Collapse

Figure 7--Water pouring out of the reservoir of the Teton Dam in Idaho following its catastrophic failure on June 5, 1976. Source: Wikipedia Commons.

"The Teton Dam was a federally built earthen dam on the Teton River in southeastern Idaho, USA which when filling for the first time suffered a catastrophic failure on June 5, 1976. The collapse of the dam resulted in the deaths of 11 people and 13,000 head of cattle. The dam cost about USD $100 million to build, and the federal government paid over $300 million in claims related to the dam failure. Total damage estimates have ranged up to $2 billion. The dam was never rebuilt." According to Wikipedia.

The "Teton Dam Failure Case Study" was presented at the 3rd ASCE Forensics Congress. They concluded that, "the failure of the Teton Dam could have been avoided. Early investigations into the geology of the site showed that the rocks in the area were almost completely of volcanic origin. These volcanic rocks consisted of basalt and rhyolite. In the footnotes to the geological survey of January 1971, the rhyolite is defined as “lightly to locally highly fractured and jointed, relatively light weight” (pp. 4-7, Independent Panel, 1976). This was also the condition for other possible sites located upstream of the site where the dam was constructed. These materials are usually avoided due to a history of erosion and deposition. The reason for the extensive foundation was the poor quality of the underlying material, including the grout curtain. The grout curtain failed to do its job of preventing these materials from being easily washed away.

The panel noted that the design did not provide for downstream defense against cracking or leakage, and did not ensure sealing of the upper part of the rock under the grout cap. The grout curtain was not constructed in three rows, and the reliance on a single curtain was judged to be “unduly optimistic.” The dam and foundation were not instrumented sufficiently to warn of changing conditions."

The case may be best summarized in the words of the panel report – “the Panel concludes (1) that the dam failed by internal erosion (piping) of the core of the dam deep in the right foundation key trench, with the eroded soil particles finding exits through the channels in and along the interface of the dam with the highly pervious abutment rock and talus, to point at the right groin of the dam, (2) that the exit avenues were destroyed and removed by the outrush of reservoir water, (3) that openings existed through inadequately sealed rock joints, and may have developed through cracks in the core zone of the key trench, (4) that, once started, piping progressed rapidly through the main body of the dam and quickly led to complete failure, (5) that the design of the dam did not adequately take into account the foundation conditions and the characteristics of the soil used for filling the key trench, and (6) that construction activities conformed to the actual design in all significant aspects except scheduling.” (pp. iii-iv, Independent Panel, 1976).

In the design and construction of earth dams, it is necessary to select proper materials that are sufficiently resistant to piping and to ensure that they are compacted to the proper density. If a grout curtain is used, methods must be available to ensure that it is continuous and forms a seal with the underlying rock. The design should incorporated adequate defense against cracking and leakage. Finally, dams must have sufficient instrumentation to provide early warning of piping and impending failure.

As a final comment, this case stands as a warning against overconfidence and hubris. “As every dam engineer knows, water also has one job, and that is to get past anything in its way” (p. 93, Macauley, 2000).

1981--Kansas City, Missouri--Hyatt Regency Walkway Collapse

Figure 8--View of the collapsed walkways, during the first day of the investigation of the Hyatt Regency walkway collapse. Source: Wikipedia Commons.

From Wikipedia: "On July 17, 1981, approximately 2,000 people had gathered in the atrium to participate in and watch a dance contest. Dozens stood on the walkways. At 7:05 PM, the walkways on the second, third, and fourth floor were packed with visitors as they watched over the active lobby, which was also full of people. The fourth floor bridge was suspended directly over the second floor bridge, with the third floor walkway set off to the side several meters away from the other two. Construction issues led to a subtle but flawed design change that doubled the load on the connection between the fourth floor walkway support beams and the tie rods carrying the weight of the second floor walkway. This new design could barely handle the dead load weight of the structure itself, much less the weight of the spectators standing on it. The connection failed and both walkways crashed one on top of the other and then into the lobby below, killing 114 people and injuring more than 200 others."

According to matdl.org,

Figure 9--Original and as-built hanger details. Source: matdl.org.

"Originally, the 2nd and 4th floor walkways were to be suspended from the same rod (as shown in fig-1) and held in place by nuts. The preliminary design sketches contained a note specifying a strength of 413 MPa for the hanger rods which was omitted on the final structural drawings. Following the general notes in the absence of a specification on the drawing, the contractor used hanger rods with only 248 MPa of strength. This original design, however, was highly impractical because it called for a nut 6.1 meters up the hanger rod and did not use sleeve nuts. The contractor modified this detail to use 2 hanger rods instead of one (as shown in fig-2) and the engineer approved the design change without checking it. This design change doubled the stress exerted on the nut under the fourth floor beam. Now this nut supported the weight of 2 walkways instead of just one (Roddis, 1993).

Analysis of these two details revealed that the original design of the rod hanger connection would have supported 90 kN, only 60% of the 151 kN required by the Kansas City building code. Even if the details had not been modified the rod hanger connection would have violated building standards. As-built, however, the connection only supported 30% of the minimum load which explains why the walkways collapsed well below maximum load (Feld and Carper, 1997).

While Kansas City did not convict the Hyatt Regency engineers of criminal negligence due to lack of evidence, the Missouri Board of Architects, Professional Engineers, and Land Surveyors was not as timid. It convicted the engineer of record and the project engineer of gross negligence, misconduct, and unprofessional conduct in the practice of engineering. Both of their Missouri professional engineering licenses were revoked, and they lost membership to ASCE. Also the billions of dollars in damages awarded in civil cases brought by the victims and their families dwarfed the half million dollar cost of the building (Roddis, 1993)."

1985: Japan Airlines Flight 123, Boeing 747

Figure 10--Illustration of JA8119, during breakup of the vertical stabilizer. Source: Wikipedia Commons.

"Japan Airlines Flight 123 was a Japan Airlines domestic flight from Tokyo International Airport (Haneda) to Osaka International Airport (Itami). The Boeing 747-SR46 that made this route, registered JA8119, suffered mechanical failures 12 minutes into flight and 32 minutes later crashed into two ridges of Mount Takamagahara in Ueno, Gunma Prefecture, 100 kilometers from Tokyo, on Monday 12 August 1985. The crash site was on Osutaka Ridge (おすたかのおね Osutaka-no-One?), near Mount Osutaka. All 15 crew members and 505 out of 509 passengers died, resulting in a total of 520 deaths and 4 survivors.
It remains the deadliest single-aircraft accident in history (Wikipedia)."

According to Wikipedia, "The official cause of the crash according to the report published by Japan's then Aircraft Accidents Investigation Commission is as follows:

1. The aircraft was involved in a tailstrike incident at Osaka International Airport on 2 June 1978, which damaged the aircraft's rear pressure bulkhead.

2. The subsequent repair of the bulkhead did not conform to Boeing's approved repair methods. Their procedure calls for one continuous doubler plate with three rows of rivets to reinforce the damaged bulkhead, but the Boeing technicians fixing the aircraft used two separate doubler plates, one with two rows of rivets and one with only one row. This reduced the part's resistance to metal fatigue by 70%. According to the FAA, the one "doubler plate" which was specified for the job (the FAA calls it a "splice plate" - essentially a patch) was cut into two pieces parallel to the stress crack it was intended to reinforce, "to make it fit". This negated the effectiveness of two of the rows of rivets. During the investigation Boeing calculated that this incorrect installation would fail after approximately 10,000 pressurizations; the aircraft accomplished 12,319 take-offs between the installation of the new plate and the final accident.

3. When the bulkhead gave way, the resulting explosive decompression ruptured the lines of all four hydraulic systems. With the aircraft's control surfaces disabled, the aircraft became uncontrollable."

1986: Space Shuttle Challenger Disaster

From Wikipedia,

Figure 11--
Kennedy Space Centre, Florida - Space Shuttle Challenger launches from launchpad 39B at the start of STS-51-L. The mission would end in disaster with the destruction of the vehicle 73 seconds later, with the loss of all seven crew members. Source: Wikipedia Commons.

"The Space Shuttle Challenger disaster occurred on January 28, 1986, when Space Shuttle Challenger broke apart 73 seconds into its flight, leading to the deaths of its seven crew members. The spacecraft disintegrated over the Atlantic Ocean, off the coast of central Florida, United States at 11:39 a.m. EST (16:39 UTC).

Disintegration of the entire vehicle began after an O-ring seal in its right solid rocket booster (SRB) failed at liftoff. The O-ring failure caused a breach in the SRB joint it sealed, allowing pressurized hot gas from within the solid rocket motor to reach the outside and impinge upon the adjacent SRB attachment hardware and external fuel tank. This led to the separation of the right-hand SRB's aft attachment and the structural failure of the external tank. Aerodynamic forces promptly broke up the orbiter.

Figure 12--The iconic image of Space Shuttle Challenger's smoke plume after its breakup 73 seconds after launch. The accident caused the death of all seven crew members of the STS-51-L mission. Source: Wikipedia Commons.

The crew compartment and many other vehicle fragments were eventually recovered from the ocean floor after a lengthy search and recovery operation. Although the exact timing of the death of the crew is unknown, several crew members are known to have survived the initial breakup of the spacecraft. However the shuttle had no escape system and the astronauts did not survive the impact of the crew compartment with the ocean surface.

The disaster resulted in a 32-month hiatus in the shuttle program and the formation of the Rogers Commission, a special commission appointed by United States President Ronald Reagan to investigate the accident. The Rogers Commission found that NASA's organizational culture and decision-making processes had been a key contributing factor to the accident. NASA managers had known that contractor Morton Thiokol's design of the SRBs contained a potentially catastrophic flaw in the O-rings since 1977, but they failed to address it properly. They also disregarded warnings from engineers about the dangers of launching on such a cold day and had failed to adequately report these technical concerns to their superiors. The Rogers Commission offered NASA nine recommendations that were to be implemented before shuttle flights resumed."

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