It is rare, but there have been cases of bridge collapse due to earthquakes – nowadays, bridges that are in high-risk areas for earthquakes are built with a structure that can withstand or suffer minor damage when an earthquake would occur. But before we could understand the science behind a good suspension bridge, we had to suffer through trial and improvement.
On the 17th October 1989, the Loma Prieta Earthquake hit San Francisco Bay Area with a Richter scale of 6.9 for moment magnitude and 7.1 for surface wave magnitude. It only lasted for 15 to 20 seconds, however there was a death toll of 62 and an average of 3757 people injured. Not only this, but the earthquake left a median of 7500 people homeless due to damage to housing and buildings in the San Francisco Bay Area.
Not only this, but the Oakland Bay Bridge suffered from the quake – the upper deck of the eastern truss collapsed causing it to block the lower deck. Unfortunately, this caused an unlucky car to drive off the bridge, resulting in death. As a result of the damage, the bridge was closed only for a month to repair the damage and make changes to the structure to ensure the safety of commuters. However, the bridge is extremely popular as it allows city workers to commute from the suburbs into San Francisco.
Oakland Bay Bridge Eastern Deck Collapse
The Oakland Bay Bridge was built in 1971, and was also modified in case of the occurrence of an earthquake in the mid 70’s. Nonetheless, the section that did collapse during the earthquake had not been reinforced during the modification, so this explains this happening. The bridge underwent a Seismic Retrofit, meaning that the bridge was modified to withstand an earthquake in the future.
What reinforcements were made during the retrofitting?
The Oakland Bay Bridge was built before the standards of bridges in the San Francisco Bay Area were altered. This meant that the bridge needed to be strengthened anyway and to prevent deck separation. The section of the bridge that collapsed was renowned for it’s overall strength and stiffness throughout the whole bridge, yet became the only deck throughout the bridge to collapse. The bridge was already designed and modified to withstand the shocks from the earthquake and absorb the energy. Moreover, the bolts that reinforced the span of the bridge to the deck of the pier broke due to the horizontal activity, ending in a collapse.
– The interim retrofit included the replacement of rivets with stiffer bolts, the addition of concrete collars surrounding the areas where the bridge was effected the most; all of this occurred in the East Span of the bridge reinforcing the critical areas throughout.
– On the west side of Oakland Bay Bridge, there were modifications made to the ramps at the Transbay Transit terminal. It also included the regeneration of the mainline interstate 80.
– The anchorage between Yerba Buena Island and San Francisco were strengthened as well as the towers and piers; this took place in the West Span.
– Lastly the West Viaduct’s pier structures were also strengthened due to high strength steel rods, concrete jackets, pier pedestals and anchor rods.
The conclusion made from the Oakland Bay Bridge Collapse is not to dismiss any area of a bridge; to make sure that there are dampers or a device to absorb the energy from the earthquake; make sure that materials used have a high strength to withstand jolting movements and that areas holding up the bridge are reinforced, and objects so small as bolts are prevented from falling out.
Another great example of a bridge collapse due to an earthquake is the Kobe Bridge Collapse in Japan due to the Great Hanshin Earthquake in January 1995. Similar to the Loma Prieta Earthquake, it measured 6.8 on the Richter scale and lasted approximately 20 seconds. This earthquake resulted in more deaths and a lot more damage than the previous example.
The bridge I will be referring to is the Hanshin Expressway Bridge. On the 17th January 1995, the lives and ground was shaken by a powerful and destructive earthquake that caused around 5,500 deaths, and 26000 people injured. The reasons for its severity is because the area in which the earthquake hit Japan hard was just on top of the Nojima fault – tectonic plates were pushed underneath another due to their moving closer together. Japans location is where the Pacific, Philippine Sea and Eurasian plates meet. But this is not the only area in the world that has a possibility of intense earthquakes; take San Andreas Fault in California – the population there is also under a great threat of earthquakes. Nevertheless, this means that building must undergo specific structural rules and guidelines so that if an earthquake were to occur, no or minor damage would occur, because the buildings and bridges would have a way of absorbing and withstanding the energy from the earthquake.
Why did the bridge suffer such a violent collapse?
Well, the bridge, or expressway as it is named, was 630 metres long and high loads, (lots of cars, Lorries and heavy duty vehicles), were enforced on the bridge. The expressway linked Kobe and Osaka, like the Oakland Bay Bridge, for commuters mostly, and during the earthquake, twisted sideways due to the shaking of the earthquake, and then resulted in it collapsing on its side in five places throughout the bridge. There was also a collapse of a 550 foot section of the expressway bridge and was said to be due to a shortage of steel reinforcements; there was an observation in that places throughout the bridge that lacked steel reinforcements and was mainly concrete collapse and suffered severe damage, whereas places with reinforcements endured the earthquake and remained unblemished. Moreover, the reinforced concrete columns fell due to the energy from the earthquake where the bridge ended up in premature shear failure.
So, making my own observation and comparing both cases – they both lacked reinforcements mainly. This means that materials and structure had to be revised due to the heavy loads it was undertaking so that if another earthquake were to occur, it would endure the earthquake or suffer minor destruction.
Damage to the Hanshin Expressway Bridge:
– There was damage to the steel bearings, that were used to keep the bridge stiff and withhold heavy weight.
– The bridge fell to the side when the reinforced concrete columns collapsed due to ‘insufficient confinement’.
– The steel piers failed soon after the collapse of the columns, as the steel buckled, (bending and kinking of an object resulting in it snapping or breaking); this damaged the foundations and the expressway fell in its side.
Hanshin Expressway Bridge Aftermath of Kobe Earthquake 1995
Changes Made to the Hanshin Expressway Bridge:
– Steel bearings were replaced with stronger beams called elastomeric beams.
– The Reinforced concrete beams were reinforced even more with a completely different structure so that they are stiffer with a high Young’s Modulus.
– The columns are also slender compared to their previous appearance.
– Places where there was mostly concrete was replaced with reinforced concrete and elastomeric beams.
So overall, bridges and buildings have to be constructed with stiff materials that can withstand and absorb the energy from powerful earthquakes. Materials such as concrete appear weak during seismic activity due to its brittleness. If the foundations are made from a material that is weak during seismic activity, then the structure is more likely to collapse. A structure is only as strong as its foundations. The structure must be reinforced in all places to prevent any failures or destruction.
We can never tell when an earthquake is going to occur, so it is better to prepare for the worst. And in places like San Francisco and Japan, maybe the aesthetics of a bridge doesn’t matter and it’s all about its durability and reliability.