New Possibilities to Build Bridges to Withstand Earthquakes
The paper has discussed various types of bridges and characteristics of bridges ranging from various countries. In this regard, we have considered different types and varieties of bridges across the world. In addition, reasons behind bridge construction have also been discussed into details. Bridge and other structures destruction by earthquakes across the world has also been highlighted with measures to prevent occurrence of such events explored. In this regard, we have suggested various forms and designs of constructing earthquake resistance and proof bridges. The paper has concluded by providing various recommendations which can be used to prevent future disasters as a result of earthquakes.
1.1 Background and Introduction
The recent past has witnessed widespread dynamics and changes in the environmental around the globe. This has been attributed to various changes in the earth’s movements due to natural occurrences. After various horizontal and vertical earths’ movements, the ground service develops fault’s which causes earthquake. Earthquakes around the world have caused devastating damages especially in prone areas like China, Japan and Indonesia. In these countries, construction work in bridges, buildings and other structures have suffered the greatest setbacks.
Economic growth and development is dependent on various factors which are paramount for consideration by any government. The infrastructural development including communication, transport and social networks are essential for guaranteed growth. The transportation sector plays a crucial role in ensuring economic development and growth is an achievable reality. In this regard, this sector’s development ought to be emphasized to achieve the desired results (Chung &Hamed, 2011).
In this report, we shall discuss the contribution of strong and well built structures in the economic development and growth. Transportation sector forms a major pillar towards economic growth hence its development is of paramount importance. In this regard, bridges are the most essential components under discussion. The report shall consider various types of bridges in the world as well as their contribution. In order to have strong structures, various special structures are used in bridge construction to strengthen them against various disturbances. The recent past has witnessed destruction and collapse of bridges in the event of an earthquake which leaves devastating destruction. Therefore, various designs have been developed and adopted in the construction of bridges to ensure they conform to earthquake proof designs. These designs include use of plastic hedges in construction of integral bridges, use of expansion joints and finally the use of seismic coefficient method.
2.0 Types of Bridges
They are various types of bridges in the world and can be broadly categorized into three types as discussed below.
2.1 Girder Bridges
This represents the most common and widely used bridge type in the world with the simplest form. It consists of a log placed across the creek forming an example of a girder bridge. This form is represented by two major forms of bridges which include box-girders and I-beam girders commonly used in steel girder type of bridges. They are other examples representing Girder bridges which include pi girders which derived its name from the mathematical symbol for pi. The other form is the T shaped form of q Girder Bridge commonly used. When I beam is constructed bearing various curves, the corners and beams of the bridge become subject and prone to twisting forces referred as torque.
2.2. Arch Bridges
The arch bridges are second oldest type of bridges after girder bridges with a classical architecture pose. This are more advanced than girder bridges since their corners make proper use of stones from classic and durability. In its construction designs, the arch bridges do not require fitting piers hence preferred for rivers and valleys. Their architectural designs make arch bridges among the most beautiful form of bridges in the world. Its designs are highly resistance to earth’s activities and resistance to forces. However, they can only be effective when the ground is stable and its foundation solid since both arch of the bridge ought to be fixed in a horizontal direction. There are usually four types of arch bridges including; Tied arches, Hinge-less, Two-hinged and finally Three-hinged bridges (Mohiuddin, 2013).
2.3 Cable-stayed Bridges
This is constructed when a typical cable is usually placed in a continuous span of more than one tower which is erected above available piers. The use of steel cables in construction is preferred due to their flexibility and extreme strength. Many bridges are constructed with this method due to the economic value of steel as well as allowing for both slender and lighter type of structures to be constructed (Mohiuddin, 2013).
3.0 Bridge Structures and Materials
While discussing the various types of bridges, there are some structures and materials that are discussed briefly.
Bearings refer to brittle, fragile and delicate elements that are metallic in nature. They are incorporated during the construction of a bridge in order to offer more support and resistance against seismic waves. Whenever earthquakes occur, bridges that lack bearings tend to collapse and get dislodged. They are damaged and displaced as they fail to offer the required support or seating which engineers refer to as; monolithic bridge structure (IRIS, 2013).
Hinges are restrainers that offer protection to any kind of a structure especially bridges. This is because; they tend to be aligned in a position parallel to the bridge alignment position. They are therefore, part of the bridge as a preventive measure against unseating, collapse and damage.
3.3 Expansion Joints
Expansion joints on the other hand are in-built in the bridges to assist in absorbing heat emitted from any disturbance. This is because; the energy leads to the bridge expanding from the high joules of energy released during an earthquake. These joints are also known as movement joints as they hold bridge joints together while allowing the structure to move in accordance to earthquake activities. In Oregon for example, earthquake bridges that were build without expansion joints would crack whenever seismic waves stressed, exerted pressure or tension leading to the whole bridge collapsing and causing massive damage to the surrounding areas (Moehle & Eberhard, 2010)
4.1 Background and Introduction
Earthquake activities refer to the movements of the earth’s surface which are attributed to massive production and emission of high joule energy. This is mostly due to abrupt changes and movements within the rocks found on the earth surface. When rocks suddenly shift in the earth’s crust surface, the movements are referred to as faults which contribute to earthquakes. Several faults may be located along a fault zone which is known to contribute towards a high magnitude earthquake. An earthquake’s magnitude is measured in terms of its strength and the amount of energy emitted in a seismic wave. The magnitude is measured using the Richter scale while a seismograph measures the seismic waves. Before an earthquake occurs, the rock movements are focused on a direct position above the crust surface which is referred as the epicenter while shearing is a force that pushes rocks into an opposite direction. The shearing forces can therefore, make rocks change shape due to stress while tension makes them thinner due to stretching (Pandora, 2011).
4.2 Earthquake Destruction
Earthquake movements cause severe damage and destruction to bridges, buildings, infrastructures and loss of human lives and property. The severity of bridge damage can be measured or compared by using the Expected Damage Method. This method not only estimates possible damages to be sustained on a bridge, but also record the actual economic loss. Engineers therefore, use this method while deciding which bridges require immediate retrofitting based on the level of expected and actual damage. Retrofitting a bridge is considered to be more expensive than designing and building a new earthquake proof bridge. This is because it involves preventive and repair costs which are often higher than the construction costs involved (USDT, 2009). The table below represents the measures used to assess the earthquake magnitudes, energy, intensity and acceleration on various frontiers. This is helpful especially when studied on time series analysis to determine any eventuality whenever an earthquake occurs.
Source: (USDT, 2009)
4.3 Geological Faults and Eventual Collapse
Geological faults result to loss of the crusts cohesion thus fracturing, breaking and displacing rocks. When the crust bends, compresses and stretches due to internal seismic forces, they result to a shift which occur at plate tectonic boundaries. There are a number of earthquakes around the world known to have had a high magnitude and consequently massive damages. They include; Charleston in South Carolina which occurred in 1886, New Madrid in North America in 1812 and the San Francisco earthquake which occurred in 1906. All these earthquakes recorded a magnitude exceeding 5.0 Richter’s scale with New Madrid earthquake recording the highest at 8.0 out of 12 Richter’s scale magnitude (SCGS, 2010). Its destruction was massive as demonstrated in the figure below.
Source: (Mahesh, 2011)
The above figure represents damages inflicted at The San Francisco Oakland Bridge after the 1906 earthquake. This demonstrated the cost implications that were incurred for the repair of such a tragedy.
The table below represents various comparisons of recorded amplitudes at maximum levels with identical standard torsion seismographes. They are important to assess any abnormal seismography figure capable of causing a high Richter scale magnitude earthquake.
5.0 New Methods to Build Bridges with Earthquake Proof
Japan, Indonesia and China are some of the countries that experience high magnitude earthquakes. Engineers from these and other earthquake prone countries have therefore been tasked with establishing new methods of building earthquake proof bridges. Bridges, dams, buildings and water supply facilities are some of the most damaged structures whenever a high magnitude earthquake is witnessed.
5.1 Use of Seismic Coefficient Method
In order to improve their stability, ductility, strength and durability, the seismic coefficient method should be applied. Through this method, engineers are able to construct bridges able to withstand the highest magnitude of an earthquake which as earlier stated is rated as 12 as shown below.
Seismic waves are known to horizontally or vertically impact a bridge. The bridge foundation should therefore, be build with enough strength to hold the crusts pressure. This is clearly demonstrated by the above figure which represents strong foundation for the bridge. Earthquakes contribute towards different reactions of the soil surface. The soil reaction should therefore, be taken into account to ensure that, despite of the bridge structures being earthquake proof, the soil foundation does not give way when seismic force and pressure impacts. This is because, disregarding soil reactions to earthquake can lead to sliding and shearing movements especially in water facilities. The safety factor should be calculated in consideration of nearby structures, buildings, dams, railways and residential homes. The safety of these structures and people should be increased despite of the economic loss a country is bound to incur. It is therefore crucial to have earthquake resistant bridges by ensuring that their designs are upgraded, raw materials are readily available, structures insolate, stronger, stiff, inelastic and ductile to avoid casing more destruction to the surrounding areas (Mohiuddin, 2013).
5.2 Expansion Joint Method
Due to earth’s movements whenever an earthquake occurs, sophisticated methods of civil construction has been adopted. Such an earthquake proof method is the use of expansion joints located at both end of the bridge as demonstrated below.
The figure above gives a clear demonstration that, even when the strongest earthquake occurs, the bridge has an expansion space. The contractions and expansions as a result of earth’s movements responding to earthquake are absorbed by the expansion bridges preventing further destruction of the bridge. The earthquake effects are greatly absorbed by the expansion joints at both ends of the bridge.
5.3 Wide Length with Plastic Hedges (Integral Bridges)
This methodology of bridge construction is widely used in areas susceptible to earthquakes like in Japan, Indonesia and China. There are several examples of earthquake proof bridges among them being the integral type of bridge. An integral bridge is a structure whose main objective is the provision of resistance to seismic waves in order to prevent bridges and building structures from collapsing. This bridge ought to be more resistant to earthquake stresses and seismic wave forces whether they are constructed with bearings or not.
However, those that are constructed with the bearings, tend to be more earthquake resistance than those without. This is because, bearings are designed with a large withholding capacity as they consist of elements able to sustain vertical and horizontal seismic wave forces. In order to improve the bridge’s performance, it is thus built as a structure with wide lengths coupled with plastic hinges and a high ductility level. An example of an integral bridge is shown in the diagram below. This bridge has an improved performance should seismic waves occur due to incorporation of bearings and restrainers to a seismic wave (Mahesh 2011).
Sources: (Mahesh 2011)
In the above structure, the construction and design is calculated in such a way that, the plastic hedges makes the bridge resistance to any motion and movements caused by any earthquake. Whenever such movements and motions occur, they are held back by the expansion of plastic hedges used especially at the pedestal points.
Integral bridges more than often require high exposure to earthquakes in order for their expansion joints, bearings and other restraining elements to be more effective and efficient. This is especially in countries that are prone to earthquake seismic waves such as China, Afghanistan and Japan.
However, not all integral bridge structures tend to have strong, durable and easily maintained expansion joints and bearings elements. There is therefore a need to built more enhanced integral bridge structures that are capable of withholding the pressure, stress, tension force attributed to seismic waves. More so, these integral bridge structures need to be economical, their elements affordable and raw materials required for their construction readily available (Mahesh, 2011).
6.0 Evaluation and Recommendations
It is clear that, engineers are always seeking improved and enhanced bridge designs though predicting damages and destructions from earthquakes and their expected magnitude before constructing a bridge. These considerations are almost impossible to ignore considering the damages and destructions as demonstrated above. However, through a thorough evaluation of available seismic data, characteristics of earthquakes-prone areas, improved designs, enhanced building criterion through analyzed procedures as well as clear seismic details that are quality controlled; can help in construction of earthquake resistant bridges and other structures. Destructions inflicted on various structures by earthquakes have been documented to be devastating with long lasting effects. Therefore, it is paramount for designers and engineers in the future to consider earthquake proof designs for bridges and other paramount structures.
The provided recommendations are based on the level of vulnerability especially in bridges built in earthquake prone countries. Bridges are destroyed due to their inability to withhold the seismic forces. It is therefore crucial for stakeholders to clearly state the most effective procedure, strategy and route a bridge should be constructed. Through this, an engineer’s priority will be; reduction of the bridge vulnerability at affordable construction costs. The people who live or conduct commercial activities near bridges should be educated through various campaigns to raise awareness with regard to the risks they are exposed to. Among them being potential loss of lives and property which are the greatest whenever an earthquake occurs.
There should be a stipulated maintenance program of all earthquake bridges in each country. Such programs will ensure that, bridges are reviewed, repaired and rehabilitated and as a result, will lead to increased seismic safety. Lastly, stakeholders should conduct studies on seismic vulnerability, risks and potential damages in order to formulate solutions and remedies should an earthquake occur and cause devastating damage and destruction. Therefore, Countries should engage and encourage stakeholders to conduct further studies on earthquakes, seismic vulnerability and impacts on bridges upon seismic wave’s occurrence (ODT, 2009).
The damage caused by an earthquake depends on two factors; the intensity at which the earth’s crust shakes and the quality of bridges among other structures in existence. However, there are other contributing factors that can increase earthquake damages. They include; the location of the epicenter in the crust surface, the type of rocks where seismic waves pass through, the type of soil and the infrastructures heights. The earth’s crust shakes more whenever the waves are bigger, closer and shallower. In order to reduce and prevent bridges from earthquake damages, engineers need to construct them using high quality hinges, expansion joints, bearings and other raw materials. The walls should be stronger while building and bridge heights determined by earthquake frequencies which can be retrieved from Richter scale records. Building of bridges should be an activity that assures they are not only strong but also stable and durable. This is because, although it is hard to predict earthquake movements and magnitude, it is possible to reduce the damage and destruction they cause.
Various ways of building earthquake proof bridges
In order to ensure future bridge construction conforms to measures and designs which are earthquake proof structures, we held interview with an expert in this field posing various question and the responses were documented below.
1. What are the main causes of Earthquake?
Answer: Earthquake is a natural occurrence which is caused by earth’s movements as a result of production and emission of high joule energy. The movements’ causes’ disturbance in motions which eventually causes expansion of faults resulting to earthquake activities. When an earthquake occurs, seismic waves are emitted with devastating effects to various structures including bridges and buildings.
2. What are the effects of earthquake on building structures especially bridges?
Answer: The occurrence of an earthquake caused by disturbance in the earth’s crust, the fault zone expands causing earthquake. When such an event occurs, buildings constructed with various designs have no capability of withstanding the movements. The eventuality of such structures is imminent collapse especially bridges and building.
3. Which structure/materials can be used in construction of earthquake proof bridges?
Answer: In order to construct earthquake proof bridges, various structures and materials should be used to enhance their resistance in case of accidents. There three structures and materials commonly known for this purpose. They include; bearings, hinges and expansion joints. These are commonly regarded as the best structures to be adopted if a bridge has be earthquake proof.
4. Which Designs can be used in construction of earthquake proof bridges?
Answer: In order to ensure bridges constructed withhold seismic waves caused by earthquake effects, various designs can be adopted. They include the use of seismic coefficient method which proves strong foundation for the bridge. Secondly, expansion joint method is the most efficient since it ensures that, in case of movements and seismic waves, the expansion joints hold the bridge together. Lastly is the use of integral bridges which ensures construction of bridges is designed with wide length supported by plastic hedges helpful especially during expansion.
5. What are your recommendations to ensure future constriction of bridges are earthquake proof?
Answer: As I have earlier stated, there are various ways which can be used in construction of earthquake proof bridges and other structures. Future construction should highly utilize bearings as well as hinges in constructions. In addition, use of wide length in construction as well as plastic hedges serves a great deal in ensuring bridges are earthquake proof.
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Mahesh, T. (2011). Economical Design of Earthquake-Resistant Bridges, ISET Journal of Earthquake Technology, 453(42):13-20
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Pandora, R. (2011). Lesson Concept: Withstanding Earthquakes, Retrieved on 5th July 2013 from: http://piascs.siu.edu/Workshop2011/LessonPlans/Earthquakes%20-%20Pandora%20Reyling.pdf
South Carolina Geological Survey (SCGS). (2010). Earthquakes and Seismic Waves, Retrieved on 5th July 2013 from: ftp://ftpdata.dnr.sc.gov/geology/Education/PDF/Earthquakes.pdf
United States Department of Transportation (USDT). (2009). Seismic Retrofitting Manual for Highway Structures: Part 1 Bridges, Retrieved on 5th July 2013 from: http://www.fhwa.dot.gov/publications/research/infrastructure/bridge/06032/06032.pdf
CIVIL ENGINEERING 18
Running Head: CIVIL ENGINEERING 1