Seismic and wind vibrations can cause damage, discomfort, and even collapse of structures. As a civil engineer, you need to design structures that can resist these dynamic loads and perform well under various conditions. In this article, you will learn some of the basic principles and methods to control seismic and wind vibrations in structures.
Vibrations are caused by the interaction of forces and masses. Seismic vibrations are generated by the movement of the earth's crust due to earthquakes, volcanic eruptions, or human activities. Wind vibrations are caused by the fluctuating pressure and drag of the air flow around the structure. Both types of vibrations can induce cyclic stresses, deformations, and resonances in the structure, affecting its stability, durability, and functionality.
The most used devices are tuned mass damper, post-tensioned connections, buckling restrained braces, eccentrically braces, viscous fluid damper, viscoelastic solid damper, metallic damper, friction damper, and based-isolated, among others.
The response of a structure to vibrations depends on its natural frequency, damping, and stiffness. The natural frequency is the rate at which the structure tends to oscillate when disturbed. Damping is the ability of the structure to dissipate energy and reduce vibrations. Stiffness is the resistance of the structure to deformation. The response of a structure can be classified into three modes: static, dynamic, and resonance. Static mode occurs when the frequency of the vibration is much lower than the natural frequency of the structure. Dynamic mode occurs when the frequency of the vibration is close to or higher than the natural frequency of the structure. Resonance mode occurs when the frequency of the vibration matches the natural frequency of the structure, resulting in large amplification of vibrations.
In order to ensure the safety, serviceability, and economy of a structure when controlling seismic and wind vibrations, there are certain design criteria that must be followed. Strength is paramount, as the structure must be able to withstand the maximum expected forces and stresses from seismic and wind loads. Additionally, ductility is necessary to absorb energy and deform without losing its integrity or capacity. Redundancy is also important, as it helps to distribute loads and prevent progressive collapse in the event of component failure. Isolation is vital for reducing the transmission of vibrations from the ground or air to the structure or vice versa. Lastly, control should be implemented to modify natural frequency, damping, or stiffness to avoid resonance or excessive vibrations.
Designing structures for seismic and wind vibrations requires various methods, depending on the type, size, shape, and location of the structure. Base isolation involves placing flexible devices between the foundation and superstructure to isolate the structure from ground motions. Tuned mass dampers reduce vibrations by installing a mass connected to the structure by a spring and damper. Active control uses sensors, actuators, and controllers to monitor and adjust the response of the structure. And passive control uses devices or materials that do not require external power or input to provide damping or stiffness.
To illustrate some of the design methods used to control seismic and wind vibrations, consider Taipei 101, the iconic Transamerica Pyramid in San Francisco, and the Millau Viaduct in France. Taipei 101 has a 660-ton tuned mass damper at the top of the tower that sways to counteract wind forces and reduce vibrations by up to 40%. Transamerica Pyramid utilizes a base isolation system of 39 steel and concrete columns resting on steel plates and rubber pads to reduce seismic forces and displacements by up to 70%. Finally, Millau Viaduct has an active control system with hydraulic jacks and cables that adjust the tension and position of the deck and pylons, reducing wind-induced vibrations and oscillations by up to 50%.
There are a number of ways to control seismic and wind vibrations in structures. Here are some of the most common methods: * Tuned mass dampers (TMDs). * Active control systems. * Passive control systems. * Base isolation. ?? The best method for controlling seismic and wind vibrations in a structure will depend on a number of factors, including the type of structure, the severity of the seismic and wind hazards, and the budget. It is important to consult with a structural engineer to determine the best method for a particular structure.
Seismic and wind vibrations can cause significant damage to structures, so it is important to take steps to control them. There are a number of ways to do this, including: Using energy dissipation devices. These devices absorb the energy of vibration, reducing the amplitude of the motion. Some common energy dissipation devices include tuned mass dampers, viscous dampers, and friction dampers. The structural design can be modified to make it more resistant to vibration. This can be done by increasing the mass of the structure, using stiffer materials, or adding bracing. Using isolation systems. Some common isolation systems include base isolation and rocking isolation.