at is adopted in order to reduce the damage caused by earthquakes to buildings. It involves detaching the foundation of a structure from the ground by resting the structure on flexible pads known as base isolators. Over the years, a large number of earthquakes have been recorded all over the world. Consequently, there have been unprecedented damages of both environmental and economic nature. Many buildings, bridges, and streets were destroyed due to the violent shaking. For example, the earthquake of 9.0 magnitude that hit Japan’s Tohoku and the resultant tsunami on March 2011 were a tragedy that caused damage worth billions of dollars (Soleimanloo, 2012). The research paper focuses on discussing the use of base isolators to protect structures such as buildings and bridges from severe damage from shaking ground, which is mainly caused by earthquakes and moving trains.
Principles of Base Isolation
Installing rubber bearings in between the foundation and the structure is the key to base isolation. The main principle behind base isolation is detaching the building from a fixed foundation onto a shaking foundation that provides flexibility during earthquakes, thus ensuring the vibration reaching the structure is of minimal magnitude. The base isolators absorb the energy, increasing damping to the system. Base isolation was pioneered by Bill Robinson and Ivan Skinner, making their country New Zealand a leader in using this technique. Base isolation enables a building to withstand heavy seismic waves, protecting the occupants and the building itself. Base isolation can be installed for both new structures and as a seismic retrofit. For example, Pasadena and LA City Halls have been seismically retrofitted using base isolation. Base isolation is suitable for hard soil and low to medium height buildings (Santhosh, Manjunath, & Kumar, 2016).
Types of Base Isolators
There are various mechanisms in earthquake engineering that are applied in order to achieve base isolation. Such isolators include elastomeric rubber bearings, ball and roller bearings, sliding bearings, and springs. In the application of elastomeric rubber bearings, natural or synthetic rubber is bound in between the plates of steel and used to form bearings of horizontal layers. Lead cores are then integrated so as to boost the damping. The steel plates are purposely used to prevent the rubber from bulging. The sliding bearings produce a sliding movement whenever there is earthquake, thereby displacing the destructive forces. Ball and roller bearings are mainly used in machinery isolation. These rollers and balls are enough to provide a damping effect, as well as resistance to service movements (Hongling & Weling, 2010).
Importance of Base Isolation
Earthquakes are the main reason people need to install base isolators. One of many causes of an earthquake is when rock underground suddenly breaks along a fault, causing two earth blocks to slip suddenly past one another. The sudden break produces energy that triggers seismic waves as a result of instantaneous release of energy. That makes the ground shake. Going by the current state of the universe, earthquakes are usually experienced whenever a volcanic eruptions occurs or when tectonic plates collide. It is very common for minor shocks to precede major earthquakes. These minor shocks are commonly referred to as foreshocks. Additionally, after the major shock other minor shocks, called aftershocks, may be experienced. These quakes result in buildings and bridges scrambling down, causing loss of lives and great damage to property worth billions of dollars. This is the reason people need base isolation system to construct buildings and structures that would withstand earthquakes (Dolce & Goretti, 2015).
Cost of Using Base Isolation
There has been a widespread misconception among property developers and investors that the cost of seismic isolation is high. However, it can result in slightly lower overall costs of construction if viewed against the net effect savings. For example, in Auckland a house popularly referred to as the Union house was constructed in 1983. Incorporating base isolation produced approximately 7% savings in the cumulative $6.6m construction cost. In addition. 3 months of construction time were saved because the structural form necessitated reduced seismic force, structural deformations, and ductility demands. Installing base isolation in all aspects of a new structure will add construction cost of less than 3% of the total cost (Nanda & Karim, 2016).
Benefits of Using Isolation Base
There are numerous benefits associated with base isolation. These are realized in savings when an earthquake hits. Base isolation leads to content protection. This is so because in the event an earthquake occurs the contents of the structure are protected against damage, resulting from sudden and violent shakes due to the ability of seismic isolators to control movement. An base isolated building is comparatively safer for its occupants as compared to one that has no base isolators. Additionally, operations continue as normal in the event of an earthquake and losses such as the loss of production time, the loss of revenue, and possible fall in stock value are avoided (Islam, Jameel, & Jumaat, 2014).
The integrity of the building’s internal structure is protected and maintained. Internal structure includes partitions, internal walls, and stairs. The insurance premiums for buildings that are seismically isolated are comparatively lower due to reduced magnitude of possible damages and subsequent loss. For example, owners of seismically isolated apartments in Japan are entitled to 30% discount on insurance premiums. Moreover, seismic isolations do not require maintenance throughout the life of the structure, except in the aftermath of a significant event where the base isolators should be inspected to ensure the load plates and the bolts are in place. The isolators do not need replacement, except for certain cases where the event magnitude exceeds design specification, thereby necessitating removal of a number of devices for the purpose of testing. Eventually, the structural repair costs after an earthquake are greatly reduced or non-existent (Islam, Jameel, & Jumaat, 2014).
Disadvantages of Using Base Isolation
Although the advantages of using base isolation greatly outweigh the disadvantages, there are still several of the latter, usually experienced at the design stage. One such disadvantage is the complexity of seismic design codes. The codes, provided for guidance in the design of structures that are seismically isolated, are complicated, hence making application by the engineers difficult. Another disadvantage is the difficulty of striking a balance of the rigidity of the isolation system. This is even more relevant if the frequency of earthquake occurrence in the area where the building is being erected is not easily determined. This is so because the rigidity of an isolation system used in an area that is frequently hit by earthquakes should be low. This is achieved through making use of thin lead cores. On the other hand, places where earthquakes are seldom experienced do not require high energy dissipation by the isolation system and the use of large lead cores. The other disadvantage is the high initial cost of construction. Although the cost is actually low, compared to the damage that would occur if a structure is not seismically isolated, investors and contractors are usually blinded by the desire to minimize the initial construction costs. Seismic base isolation is also not suitable soft soils (Dolce & Goretti, 2015).
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Difficulty of Adding Base Isolation to Already Built Buildings and Bridges
It is not easy for developing countries to adopt base isolation in the already built buildings and bridges. These countries lack the expertise as well as resources to include base isolation not only in the already built structures, but also when laying the foundation of a bridge or a building. This does not apply to countries such as India, Turkey, and China. These countries should join the US and Japan who use base isolation to protect historical buildings, bridges, and airports, such as Space Shuttle Endeavour in California and the International Airport of Turkey known as Sahiba Gokcen, which is the largest seismically isolated structure in the world (Branco & Guerreiro, 2011).
Retrofitting is upgrading of the already existing structures to become more resistant to earthquakes, which cause the movement of the foundation. leading to collapsing walls and bridges. Installing base isolators under already built structures requires excavating the foundation of the building or bridge in order to detach it from the foundation. Steel reinforced beams are used to replace the structure’s connection to the ground under which isolating rubber or lead is placed (Buckle et al., 2006). The retrofit process can be costly and difficult with regard to the height and the materials used to build the structure, as well as the difficulty of accessing the structure and ease of excavating. Large and more complex buildings with split floors increase the difficulty of the job.
Shake Ground Energy Rescue by Base Isolation
The most appealing base isolator is the rubber bearings, such as the one installed in the Foothill Communities Law and Justice Center. According to the French engineer, Eugene Freyssient, the amount of energy that a rubber base isolator rescuea from the shaking ground or foundation to a structure is determined by the thickness of the rubber pad attached to the steel plate. The horizontal capability of shaking the structure where a rubber isolator is installed is directly proportional to the thickness of the rubber. On the other hand, the vertical capacity of a base isolator is inversely proportional to the thickness of the rubber pad (Alam, Bhuiyan, & Billah, 2012). The rubber bearings are capable of redirecting vibrations in the process of protecting the structure above it. The practice originated in the United Kingdom in the early 1960s as a way to damp the unwanted vibrations caused by trains in such places as hospitals and low cost housing. For instance, the Benaroya Concert Hall in Seattle uses rubber bearings to dispose of vibrations, because it is situated on a train tunnel (Pioldi et al., 2016).
Base isolation aims at making earthquake resistant buildings rather than earthquake-proof buildings, which would be too complicated and expensive to build. This structures will damp most of the ground shaking force, but they will get damaged in severe earthquakes. However, they will not collapse, assuring the safety of the occupants. Base isolation will allow the structure to swing in horizontally with the same amount of energy back and forth without allowing tension to the structure (Kural & Tushamar, 2016).
In conclusion, the above discussion proves that seismic base isolation is a reliable technology of mitigating earthquake effects on structures. There has been great success in the application of this method which is attributed to proper planning and advancement of isolation devices. Extensive research has been conducted on the different proposals of isolation devices. It has also been proven that buildings that are seismically isolated are more reliable and safe than those that haven’t embraced base isolation. However, the high cost limitation on available isolation systems necessitates development of cost effective systems.