Monday, September 9, 2013

Civil Engineering Failures

The St. Francis Dam Flooding

     Amongst the history of civil engineering disasters, one of the least known is the collapse of the St. Francis Dam and subsequent flooding. Although this disaster claimed more than 500 lives and destroyed millions of dollars of property, very little is heard about it. Even so, it had a major impact on the history of civil engineering and civil engineering regulations today.

     The St. Francis Dam was designed by William Mulholland, the self-taught chief engineer for the Los Angeles Department of Water and Power. He was an experienced dam engineer, having completed several smaller dams before this one. He was also responsible for the design and construction of the Los Angeles Aqueduct, which was the longest aqueduct in the world when it was built in 1913. Without this aqueduct, Los Angeles probably would not have grown to the size and prominence it has today.

     The St. Frances Dam was a curved concrete gravity dam. By definition, a gravity dam is one where that depends upon the force of gravity to prevent it from being pushed aside by the water it contains. Essentially, the force of the water behind the dam is pushing the dam to tip forward upon it’s toe (the most downstream point of the dam). The weight of the dam is acting against that, causing it to rotate downward into the earth from the same point. As long as the weight of the dam structure itself is greater than the weight of the water behind the dam, it will not move.

     Gravity dams are very common due to their high reliability and relative lower construction cost. They can be either solid or hollow, and can be made of only concrete or a combination of concrete and dirt. Other than the weight of the dam, the other major design criterion is to ensure that the toe of the dam is sunk deep enough into the earth to prevent it from sliding forward.

     What caused the St. Francis Dam to fail wasn’t the design of the dam or its construction, but rather a limited understanding of the geological foundation of the dam. In the 1920s, when the dam was built, the technology wasn’t available to properly determine the strength of the paleo-mega-landslide rock formations that the left abutment rested upon. Two of the most famous geologists of the day examined the site and determined that it was suitable for the dam’s construction.

     This was the major contributing factor to the failure of this dam. However, it was not the only factor. Twice during the construction of the dam, Mulholland added an additional ten feet to its height. While he compensated for this additional height in his design, those compensations didn’t bring the dam up to the standards practiced by other dam engineers of his day. According to modern standards, the design of this dam was subpar for a concrete dam of its size.

     Although there was almost no warning of the impending disaster, earlier in the day of the failure, Mulholland examined a leak in the dam. Leaks in concrete dams are not uncommon, and this one was found to be inconsequential. Since the failure of the dam was mostly caused by the subterranean rock, the only way that that leak could have been an indicator, would be if it was large enough to demonstrate that the dam’s foundation was shifting.

     The disaster of the St. Francis Dam’s failure resulted in a number of changes to civil engineering, specifically to the area of dam design and construction, but also affecting all areas of civil engineering: 
New federal standards for the construction of dams. 
Increased inspections of all dams across the United States were mandated. In the initial inspection after this disaster, a full third of the dams were found to need of alterations, repairs or reinforcement. Today, all dams are inspected on a regular basis. 

      Civil engineering examinations and registration traces its roots to this tragedy, which brought to public light the lack of sufficient standards for civil engineers. 

     An increased awareness of the geological factors in civil engineering came to light from the failure of the dam’s foundation. Geotechnical engineering can trace its roots to this disaster. Today, geologic input on dam design and construction is commonplace. 
The importance of peer review of designs was brought to light. Since this time, a project of this size has never been designed and overseen by only one engineer. 

     So, although the collapse of the St. Francis Dam was a disaster that cost many lives, it ultimately became a major part of defining the civil engineering we practice today. The lessons learned from this tragedy have helped ensure that later projects are much less prone to failure.

The Tacoma Narrows Bridge Collapse

     The first Tacoma Narrows Bridge, completed in 1940, probably stands as the greatest civil engineering failure in history. When constructed, it was to be the third longest suspension bridge in the world. Narrow and graceful, the bridge would connect the Kitsap Peninsula to Tacoma, Washington.

     Even before opening, it was clear that the bridge had problems. Workers had noticed undulations in the bridge and named it Galloping Gertie. Since suspension bridges are designed to be flexible, the problem was determined to be inconsequential, as the wave-like undulations would not cause any structural damage to the bridge. However, because of concern for the comfort of commuters on the bridge, a number of measures were designed by civil engineering consultants and implemented to try and reduce these undulations.

     The bridge failed on November 7th of 1940, a mere four months after opening. Fortunately, the only life lost in its spectacular failure was that of a cocker spaniel named Tubby. Structural engineers were left with the question of analyzing how the bridge failed. As with any civil engineering failure, a thorough analysis is a necessary part of avoiding similar catastrophes in the future.

     This bridge was a revolutionary design in suspension bridges, intended to reduce cost, while providing an aesthetically pleasing structure to grace the Tacoma Narrows. Two basic designs for the bridge deck had been proposed, a 25 foot tall traditional lattice-style truss, and 8 foot tall plate girders (which are essentially tall I-beams). Since the girders produced a slimmer, more elegant design at a much lower cost, they were the design decided upon.

     In addition to this design innovation, there was another critical design feature which contributed to the ultimate demise of Girtie. Since traffic was expected to be fairly low, the bridge was only to be two lanes wide, resulting in an incredibly narrow 25 feet width. Between the narrow width and the low profile, it was the smallest cross section ever conceived for any lengthy suspension bridge.

     As with any structure, reducing dimensions also reduces rigidity. This, along with the large sail area provided by the eight foot tall girders, gave even the lightest wind the ability to easily push the 2,800 foot long main span. It was not uncommon for the center of Girtie to be deflected as much as several feet to the side, in addition to the wave-like motion that she was well known for. However, neither of these problems created any structural risk, as the bridge and its roadbed had been designed with some flexibility.

     On November 7th, 1940, amidst a 42 mile-per-hour wind, the Tacoma Narrows Bridge’s movement changed drastically. Instead of the well-known undulating motion, the roadbed of the bridge started twisting, with the opposite ends of the center span twisting out of sync with each other. This put the structure at risk, as the connection of the girders and roadbed had not been designed to withstand this sort of motion.

     A famous film, taken by the owner of a photography shop near the Tacoma end of the bridge provides ample documentation of the bridge’s torsional twisting and ultimate self-destruction. The first failures the film shows are not the roadbed or supporting girders, but rather the suspender cables. These are the vertical cables running from the main cable across the top of the towers, down to the roadbed.

     To understand the importance of these cables, one must understand the theory behind how a suspension bridge works. The roadbed is not intended to be self-supporting, merely to have enough stiffness to prevent winds and the load passing across it to cause excessive movement. The entire weight of the bridge is supported by the towers, which are firmly anchored in concrete piers. Two main cables are draped over the towers, and are attached to massive, heavy anchorages on both shores. From these main cables, suspender cables drop down to attach to the roadbed and support it. All these cables transfer the weight of the bridge roadbed and load to the towers.

     What happened to the Tacoma Narrows Bridge is that as the wind torsionally twisted the bridge as much as 45 degrees, the weight of the bridge needed up being supported by the suspender cables on only one side. That proved to be too much for the cables, which snapped. The resulting transfer of weight to adjacent cables overloaded them, snapping additional cables in a domino effect. Without the cable system to support the roadbed, joints in the roadbed failed, causing it to collapse.

     All of this started because of the natural resonance which is in any object. As the bridge was being designed, this resonance was not properly accounted for. When the wind hit 42 miles per hour, it caused the motion which ultimately led to failure.

     One long-term result of the lessons which were learned from the Tacoma Narrows Bridge disaster is that modern structural engineers now use computer simulations of wind flow programs to better understand and design for the natural resonance of bridges, buildings and other structures. This helps reduce greatly the possibility of another disaster like this in the future.

The Hyatt Regency Walkway Collapse

     While the spectacular demise of the original Tacoma Narrows Bridge can be considered the largest civil engineering disaster in terms of physical size, it doesn’t hold a candle to the cost in human lives of the collapse of the Walkway in the Hyatt Regency. The total lives lost through that disaster were 114, with over 200 more hospitalized due to injuries. This collapse wasn’t an error in the design or calculations to back up that design, but rather in the execution of the design by the fabrication company who built the walkway and lack of project management by the civil engineers on the project.

     As originally designed, there were three walkways, spanning the atrium and connecting the two halves of the second, third and fourth floors. The walkways for the second and fourth floors were stacked one over the other, while the one for the third floor was offset to one side. These walkways were suspended from the ceiling on metal rods, which attached to box beams that crossed underneath the walkways.

     In the original engineering drawings, the same threaded rod ran from the ceiling, down through the fourth floor walkway and continues down to the second floor walkway. Large nuts, backed by washers were to connect the threaded rods to the box beams. The box beams themselves were two steel “C” channels, which were welded together.

     Due to the difficulty of manufacturing a threaded rod that long, and the risk of it becoming damaged in shipment and installation, the steel fabricator who was building the walkway decided to change the design, using two rods in each location, rather than one. The first rod would run from the ceiling down through the upper walkway and attach there. The second rod would run from the upper walkway, down to the lower one, hanging it from the upper walkway’s beams.

     On the surface, this seems like an acceptable solution to the problem, making the design easier to fabricate and install. Since the same size steel rods were being installed for both the upper and lower sections, it would seem that the rods should be able to carry the weight. However, this did not take into account the amount of weight placed upon the nuts on the bottom side of the upper walkway’s beams.

     In the original design, those nuts only had to support the weight of the upper walkway. None of the weight of the lower walkway would be transferred to those nuts, but rather directly to the hanger rod by the nuts on the bottom of the lower walkway’s beams. Although there was an error in the calculations, the nuts on the upper walkway could marginally accomplish their task if you eliminated any safety factor.

     In the modified design, those critical nuts no longer had to carry the load for only the upper walkway, but the entire weight of the lower walkway as well. That added weight meant that the nuts could only carry 40 percent of the dead load. Add the weight of the people jumping and swaying as they watched the dance contest in the lobby, and the nuts didn’t stand a chance.

     To envision the effect of this, think of a rope handing from a tree, with two people handing from that rope. Each person is holding onto the rope, supporting their own weight. However, if the lower person grabs hold of the upper person’s leg and releases the rope, the upper person now has to support the weight of the two of them. The possibility of that person’s arms failing and the two falling is greatly increased.

     In addition to the overload, the holes for the box beams had been drilled through the welds holding the two “C” channels together, the weakest point in the box beam. When the wreckage was examined, it was found that the pressure on the nuts not only caused the nuts to fail, but the holes in the box beams to deform, allowing the nut to pass through.

     Forensic engineering discovered that this failure was actually a series of four failures on the part of the engineering team. The first failure, which may have been survivable, was the miscalculation of the weight of the walkways, and the ability of the nuts to support them. The second failure was a failure of communications between the engineering firm and the fabricator who made the design. Thirdly, nobody ever calculated the difference in loads and stresses made by the design change. Finally, the engineering team never checked the final work by the fabricators, to ensure that they had followed the design in all details.

     The civil engineer’s responsibility doesn’t end when the drawings are completed. Engineering involvement throughout the construction process is essential to avoid similar disasters. Contractors often make design changes, whether it is the replacement of welds with bolts or changing the materials used. These changes are made by non-engineering professionals, based upon their experience, but without the benefit of knowing how to calculate the effects of their change. In some cases, those changes can cause disaster.

     Had the civil engineers who had responsibility for this design completed their responsibilities, this disaster could easily have been avoided.

Injaka Bridge Collapse (South Africa)

     Incorrect positioning of temporary bearings during incremental launching have been identified as the primary cause of the fatal 1998 Injaka Bridge collapse in South Africa.

     But the official investigation report into the collapse, which killed 14 people and injured 19, concludes that bearing problems were made worse by a catalogue of design, monitoring, construction and organisational errors.

     Inexperienced design and construction staff, poor construction quality control and a failure to react to a 'clear warning that all was not well' with the structure, led to the disaster.

     The conclusions are contained in a report to South Africa's director of public prosecution by presiding inspector Larry Kloppenborg and made public last month.

     The report recommends that criminal charges under the Occupational Health & Safety Act be brought against the bosses of designer VKE Consulting Engineers' Pretoria office, contractor Concor Holdings and client the Department of Water Affairs & Forestry.

     Kloppenborg also recommends that similar criminal charges be brought against Johan Bisschoff, who directly supervised VKE's permanent design and against Rolf Heese of Concor who was in charge of the temporary works design.

     The report recommends that any charges brought should also consider the fact that 'the extent of the catastrophic failure of the bridge equally could have caused the deaths of any of the 19 injured persons.'

     The collapse of the bridge on 6 July 1998 was one of the worst construction accidents ever seen in South Africa.

     At 300m long, 14m wide and up to 37m above the river bed, Injaka Bridge was a major structure and the consultant and contractor had extensive experience with such incrementally launched post-tensioned structures.

     The collapse occurred after the contractor had slid out five of the 20, 15m long sections of the 3m deep box section deck.

     The sixth segment was being jacked as the structure collapsed. At that point the concrete deck extended 24.4m beyond pier 2 with the leading edge of the 27m long launching nose projecting 7.1m beyond pier 3.

     Among those killed was Maria Gouws, the VKE engineer responsible for designing the structure. She was on the deck with several guests celebrating progress on the structure.

     The primary cause of the collapse was found to have been the positioning of temporary bearings on which the deck structure slid out during construction.

     These were located inside the permanent bearing positions which were to be under the box section webs. As a result the temporary bearings punched through the structure and caused the collapse.

     Kloppenborg concludes that Gouws and Heese lacked sufficient experience to have been left in charge of the permanent and temporary work for such a complex, incrementally launched structure. Gouws had only around two years' bridge design experience.

     The report highlights the lack of calculation checks carried out on the permanent and temporary works designs, as well as lack of construction and production monitoring. It criticises the designer and contractor for allowing inexperienced staff to have such operational freedom.



source: www.nce
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Koror–Babeldaob Bridge Collapse



     The original Koror–Babeldaob Bridge was a balanced cantilever prestressed concrete box girder bridge with a main span of 240.8 m and total length of 385.6 m (1265 ft). It was designed by Dyckerhoff & Widmann AG and Alfred A. Yee and Associates. It was constructed by Dyckerhoff & Widmann AG, contractor was Palau based Korean company, Socio Construction Co. It was the world's largest bridge of its type, until its record was broken by the 260 m span of the Gateway Bridge in Brisbane, Australia, finished in 1985.

     This led to two studies carried out by Louis Berger International and the Japan international Co-operation Agency. They concluded that the bridge was "safe" and the large deflections were due to creep and the modulus of elasticity of the concrete in place being lower than anticipated.

     On September 26, 1996, the bridge suddenly collapsed and shut off fresh water and electricity between the islands. In addition, the collapse killed two people and injured four more. This caused the government to declare a state of emergency. By request of Kuniwo Nakamura, then the country's Presidentand foreign minister, Japan provided emergency aid as well as a temporary bridge.

Reasons of Collapse

     The 18-year old, Koror-Babeldaob bridge (KB bridge) collapsed abruptly and catastrophically. The failure occurred during benign weather and loading conditions, less than three years after two independent teams (Louis Berger International and the Japan international Co-operation Agency) of bridge engineers had evaluated the bridge and declared it safe, and less than three months after completion of a strengthening programme to correct a significant midspan sag that was continuing to worsen.

     By 1990, a physical phenomenon called creep had caused the midline of the bridge to sag 1.2 meters, causing discomfort to drivers and concern for officials. The Palau government commissioned two studies by Louis Berger International and Japan International Co-operation Agency. They both concluded that the bridge was structurally safe though 1m more of creep would occur in the future. Based on the studies, the Palau government decided to counteract the cosmetic damage caused by creep with resurfacing and reinforcement of the bridge.

     According to British civil engineers Chris Burgoyne and Richard Scantlebury, the reinforcement operation had 4 main components.
The bridge midspan was modified, changing the originally non-weight-bearing hinged joint to a continuous block of concrete.
Eight prestress cables were added to straighten the span.
Eight flat-jacks were added to the center of the structure to add additional prestress, loading the center of the bridge.
The bridge was resurfaced to smooth out the sagging road.

     The most probable progression of the collapse began with a weakening of the Babeldaob side. This caused a shear failure, resulting in all of the weight of the continuous main span coming to rest on the Koror side. This unbalanced moment caused the backspan of the Koror side to fail and tipped the bridge into the channel. The new design of the bridge hastened the bridge's collapse, but did not actually cause the fatal weakening of the Babeldoab flange. The ultimate cause of collapse remains unknown.[9] The repair and resurfacing operation may have contributed to the bridge failure, but since the lawsuit over the collapse was settled out of court, no final cause has ever been definitively published.

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