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Preventing Resonance in Bridges and Buildings

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Sammi Chen

on 24 May 2017

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Transcript of Preventing Resonance in Bridges and Buildings

Preventing Resonance in Bridges and Buildings
design by Dóri Sirály for Prezi
Resonance the reinforcement or prolongation of sound by reflection from surface or by the synchronous vibration of a neighboring object. In simpler terms, the conditions which the frequency of a wave equals the resonant frequency of the waves medium. Mechanical resonance occurs when there is transfer of energy from one object to another with the same natural or resonant frequency. An example of this would be a child-swing system.
What is Resonance?
Strong vibrations can cause lots of damage to structures and can be used to break materials apart. To limited unwanted vibrations and preventing structures from resonating with frequencies during earthquakes, features and modifications such as dampers are designed to help us in that way. They help save buildings from damage costs, and lives of people. Understanding how vibrations work can help us prevent dangers and faults in structures and natural disasters.
Over the year engineers have discovered ways and have made design modifications to bridges and buildings to help limit undesired vibrations. This helps structures from shaking too much and causing them to be unsafe or from collapsing due to strong natural forces. One way to limit vibrations include the use of dampers. Damping is the reduction in the amplitude of a wave as a result of energy absorption destructive interference. Seismic dampers are a type of dampers and are mechanical devices to dissipate kinetic energy of seismic waves penetrating a building structure. Tune dampers are another kind of dampers, also known as a harmonic absorber, is a device mounted in structures to reduce the amplitude of mechanical vibrations. Their application can prevent discomfort, damage, or outright structural failure. They are frequently used in power transmission, automobiles, and buildings.
Ways to Prevent Vibrations
Design Features
What Causes Resonance?
Historical Incidences
In Physics we learned about sound and vibrations. How vibrations and waves affect us and the environment, when they interact with each other, how to apply the uses of sound waves, and so on. The focus of this to see how natural disasters cause vibrations, and their effect structures like bridges and buildings. Also how to prevent vibrations from causing damage to structures and how to apply these in other ways.
Structural Vibrations and Resonance
One of the most famous incidence of structural damage is the Tacoma Narrows Bridge. it was a suspension bridge that spanned the Tacoma Narrows strait of Puget Sound between Tacoma and the Kitsap Peninsula. it collapsed on November 7, 1940 because of areoelatic flutter. Areoelatic flutter is the response when energy added to a structure vibrating exceeds the energy lost due to damping causing large vibrations. Basically more energy was added to the vibrations that can be lost during natural damping of the structure. it occurs when wind exerts more a force on a structure due to the aerodynamics of the structure and the elasticity of the structure allows it to vibrate.
Natural disasters such as earthquakes can cause lots of damage to environments, buildings, and people. The Earth's crust is made up of several large tectonic plates that fit together and they are always slowly moving. they push, pull, and slide against each other, these plates have jagged edges that catch on each other when moving, creating pressure. It causes the tectonic plates build enough pressure to point plates slip along the fault lines causing earthquakes. once they slip faster than usual seismic waves are created that can travel along or into the earth. when these waves passes the earth's surface they shake the ground causing an earthquake. Other natural forces that can cause structural damage such as strong winds and tsunamis. Tsunamis are caused by underwater tectonic plates and producing earthquakes under the water.
- A diagram of Resonance occurs
Similar to how modifications can be made to structures and bridges to prevent unwanted vibrations, many structures have design features to help prevent them from resonating with the frequencies during earthquakes. Some common ways such by engineers and builders includes base isolation. It is a collection of structural elements which should substantially decouple a superstructure from its substructure resting on a shaking ground thus protecting a building or non-building structure's integrity. It is one of the most popular means of protecting a structure against earthquake forces. Another way that is used elevated building foundation, which is a kind of seismic base isolation technology which is made to to be a major part of a building's superstructure. Dampers are also commonly used to help control vibrations in buildings, are used when designing structures to withstand earthquakes.
Vibrations in Buildings
Resonance occurs when a system is able to store and easily transfer energy between two or more different storage modes, such as kinetic energy and potential energy. When the amplitude of an object's oscillations are increased by the matching vibrations of another object. For resonance to occur there has to be an object with a natural frequency. The object can be can a mechanical device or an electronic circuit. An object's natural frequency is the frequency ids to oscillate at when disturbed. Objects can have more than one frequency, these would be called harmonics. It would also require a forcing function at the same frequency as the natural frequency, in mechanical it would be a variable force, while in electrical it would be a variable electric force. Final there has to be a lack of damping or energy lost. In order for an object to resonate, mechanical or electrical energy has to build up in the object. Anything which removes these form of energy tends to interfere with resonance.
Large vibrations can cause lots of damage to buildings and structures.They are can caused by earthquakes but in most places it is caused by wind. In the past most tall buildings were built with concrete, which resisted compression but was not very flexible. If a concrete building started to sway it could crack and the building could fall. To prevent this engineers strengthen buildings with massive thick walls and steel bars that can bend slightly. Skyscrapers were later built with Girders, which were a series of metal pillars that acted like as a frame. The walls called curtain walls acted as weatherproof rather than support. However as they got taller, wind is forced into narrower streets increasing wind speeds. Engineers must consider increase of wind speeds when designing buildings. Over the years engineers have discovered different ways to to withstand vibrations caused by different forces and new methods are being tested and discussed. With the help of new technology and materials, engineers are constantly trying to improve how we can create structures.
What Are Vibrations?
Vibrations may be regarded as any motion that repeats itself after an interval of time or an oscillation of the parts of a fluid or an elastic solid whose equilibrium has been disturbed, or of an electromagnetic wave. It is phenomenon where oscillations occur about an equilibrium point. Examples of vibratory motion such as the swing of a pendulum, the motion of a plucked guitar string, tidal motion, the swaying of tall buildings due to strong winds or earthquakes, etc. Vibrations can desirable such as the motion of a tuning fork, the reed in a woodwind instrument, etc., but in many case vibrations are undesirable, wasting energy and creating unwanted sound. there types of vibrations, free vibrations, forced, damped vibrations. Vibrations and sound are closely related, as sound is generated by vibrating structures.
Structural vibration occurs when dynamic forces generate by compressor, plumps, and engines cause deck beams to vibrate. this can lead to piping failure, poor equipment reliability, and safety concerns. the vibrations occur due to structures being mechanically resonant. Resonance occurs when dynamic forces coincide with the natural frequencies of the structure. At resonance the forces are amplify and can cause beams to vibrate above safety levels. When designing structures it is important to think about the effect of vibrations as it is a very important factor to consider. Vibrations can cause excessive wear of bearings, materials cracking, fasteners to become loose, noise, and so on. The natural frequencies of a structure are very important to structural and mechanical engineers when designing for human comfort, structural, operational requirements, and also against resonance.
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