麻豆影院

A Piece is a Piece, No Matter How Small: How the smallest parts led to the 1989 collapse of the Oakland Bay Bridge, Matthew Moore

听

听

In the early evening hours of the seventeenth of October 1989, a 7.1 magnitude earthquake known as the Loma Prieta Earthquake rocked the San Francisco, California Bay Area. This intense seismic event was responsible for sixty-three deaths and 3,757 injuries. The double decker Oakland Bay Bridge, on the northern side of the San Francisco Bay, suffered a partial collapse and structural failure. A fifty-foot top section of the eastern truss collapsed down onto the bottom deck over bridge pier E9.1 The collapse was responsible for one death and over a month of closure. While the Loma Prieta was rated as a severe and violent earthquake, the building and structures of San Francisco were designed to hold up to it. Most structures performed well, including Candlestick Park, where the 1989 World Series was being played at the time, but infrastructures, such as the Bay Bridge did not. This was due to the failure of not large, but very small parts. Although San Francisco was hit with intense seismic activity during the Loma Prieta Earthquake of 1989, the Oakland Bay Bridge suffered a partial collapse solely for faulty engineering reasons, including the lack of earthquake resistant riveting and rebar-supported concrete pillars.听

Construction of the Oakland Bay Bridge began in 1933 and opened November twelfth, 1936, six months before the Golden Gate Bridge.2 It was designed as a double level bridge that incorporated both passenger car and train decks across its span of almost 10,000 feet. The middle point on the bridge rests on Yerba Buena Island, making the bridge a two span suspension type. At the time of construction, the bridge was one of the most advanced of its type.听 With 120 million pounds of reinforced steel and 450,000 cubic feet of concrete, the sheer size of the bridge and its construction made for a complicated engineering process and the chance for things to go wrong at many stages.3听

To complicate things further, over its nearly fifty-six year history leading up to 1989 and the earthquake, no major retrofits or renovations had been done on the bridge. Granted, the bridge was indeed holding up well and proved to be sturdy, but engineers knew that the earthquake resistant properties of the bridge were degrading as it aged.4 Engineers from Caltrans, the Bay Area鈥檚 transportation division, reported cracks and chips in the concrete support pillars built on the bridge piers. The collapsed section of the bridge sat directly over pier E9, pointing to stress from the concrete pillars. Those concrete pillars were reinforced with rebar, a type of metal pole, but were incorrectly positioned inside the concrete to handle lateral seismic activity. The rebar concrete was superior at supporting vertical stresses, such as from the weight of the double decks on the bridge above. However, when the intense and sudden lateral movements of the earthquake began, the pillar鈥檚 strength was greatly reduced. Take a stack of Jenga blocks, for instance, and imagine stacking a pile of books directly on top of it. The wood blocks of the tower would hold up impeccably under the stress of the weight directly above. But if the books or tower were to be jostled from side to side, the tower would either collapse or become extremely unstable in no time at all. This same sort of thing happened on the Bay Bridge the evening of October seventeenth, which weakened the support structure to the point of bringing down the top deck into the lower deck. While the improper rebar concrete was a major factor to blame, it is only one piece to the puzzle that created this disaster.听听

In the fashion of cause-and-effect, the faulty rebar concrete led way to another important piece of the bridge to fail. Up on the decks were the suspension beam structures that gave the bridge its iconic look and created a skeleton like tunnel that encased the traffic decks. Supporting and holding together these beams were millions of rivets. These eliminated the need for welding every piece together as smaller pieces could be riveted instead.听 While rivets are strong, they do not match the integrity of welds, so more rivets had to be used over each beam. The resulting structure resembled a lattice of metal strips connecting together beams. Again, this type of load bearing product was excellent at supporting vertical loads, but any lateral motion would risk compromising its integrity. A good example of how the lattices were methodized is to think of a scissor lift. The lift does well going up and down, but suddenly bumping one from the side could tip it over. The crisscrossing supports up the middle of the bridge鈥檚 trestles acted in this way but remained stationary, as they were riveted into place. The tactic of welding in the 1930鈥檚 was to perform hot rivets. The fast and efficient way to install the millions of rivets needed used heat to make the rivets extra malleable before being punched into the metal. While this was a quick method, it left one huge safety concern. Unlike the hardened steel being used as tresses and trestles, the rivets could not be hardened, for they needed to be soft during the hot riveting process. This led to weaker pieces of the bridge trying to support hardened steel beams. While it was an acceptable engineering decision for daily operations, the earthquake resistivity of a hot riveted bridge degraded sharply over time.听

On that fateful day in October, the combination of old rivets and concrete pillars that were not earthquake resistant, led to the collapse of the Bay Bridge section at pier E9. The inspectors of the bridge had made notes prior to the disaster outlining the potential structural faults they found. However, multiple accounts depict inspections being rushed or abbreviated as the bridge underwent routine maintenance.5 The cracks and chips that had been found in the concrete were declared normal due to settling of the bridge鈥檚 pylons on the ocean floor of the Bay. On the contrary, those cracks should have never passed the inspections, as the chips that accompanied them pointed to a weakness in lateral movement. In addition, on the pillar sections closer down to the water of the Bay, the cracks exposed access for salt water into the metal rebar reinforcements. The salt progressed rust of the supports, but this was never cited as a concern to the structure.6 When the Loma Preita earthquake hit, the concrete pillar鈥檚 weight bearing capabilities were sharply reduced, putting immense stress on the metal rivets. This abrupt shift of weight initiated what became the collapse at section E9.听听听

While this collapse and engineering disaster on the Bay Bridge was abhorrent, the fact that only section E9 collapsed is worth recognition. The rest of the bridge remained almost unfazed and structurally sound during the earthquake. Citing the fifty six year old structure鈥檚 age, the hardened steel and settled concrete supports could have easily turned into a pile of rubble. When bridges are first built, they have not settled yet, leaving cushion room for earth vibrations to dissipate in the concrete and steel structure of the bridge.7听 More than half a century of settling had occurred to the Oakland Bay Bridge prior to the 7.1 magnitude earthquake, making hardly any natural dampening existent. Just off the end of the bridge, on land, was a double decker section of freeway known as the Cyprus Street Viaduct that completely collapsed under its own weight, killing sixty four people.8 The Cyprus Street Viaduct suffered the same kind of rebar concrete weakening as the Bay Bridge. However, this weakening caused the entire Viaduct to violently collapse when the rebar and concrete sheered. If the Bay Bridge had encountered such fate, there would have been countless more loss of life. The resilience of the bridge in light of the collapse at section E9 was certainly shown on that October evening.听