Apr. 22, 2010 How do engineers prepare buildings to withstand earthquakes? According to experts from the School of Civil Engineering of Barcelona, one approach to this problem is to consider a building as you would a living being. Two UPC-Barcelona Tech research groups -- Risk Management and Control, Dynamics and Applications -- have extensive experience in this field.
An earthquake is the sudden release of energy accumulated in the Earth's crust, brought about by the movement of a fault. This shock causes seismic waves to spread out in all directions. According to a study by the Center for Research on the Epidemiology of Disasters (CRED), over the past decade, earthquakes have been responsible for 60% of all deaths caused by natural disasters -- a total of approximately 780,000 victims.
The consequences of an earthquake can be catastrophic, as we saw recently in Haiti. With more than 220,000 victims, that magnitude 7.0 quake was one of the deadliest on record. It destroyed 97,294 homes and damaged 188,383 others in Port-au-Prince and southern Haiti. On the basis of data analyzed over the past few decades, researchers agree that natural disasters -- of which earthquakes account for 9% -- have not seen an increase in either frequency or power. Social and regional vulnerability, however, have increased sharply.
According to Àlex Barbat, director of the Risk Management research group in the Department of Strength of Materials and Structural Engineering at the UPC-Barcelona Tech: "Statistics show that, over the years, risk, defined as expected losses, has increased in all of the world's seismically active areas, and all signs suggest that it will continue to rise. This is because many urban areas are surrounded by two or three industrial belts, made up of increasingly sophisticated facilities, which are also subject to damage during an earthquake. And let's not forget that many seismically active areas, such as those in Asia, also happen to be some of the world's most densely populated areas."
In light of the fact that it remains impossible to accurately predict earthquakes, Mr. Barbat made the following observation: "Buildings are the structures where most risk is concentrated. When an earthquake strikes an urban area, most losses -- meaning both the loss of human life and economic, cultural and social losses -- are caused by the deficient seismic behavior of buildings." The immediate conclusion that can be drawn from all this is the need to design earthquake-resistant structures.
New techniques The vibrations produced by strong earthquakes can cause serious damage to structural elements. As a general rule, structures hold up due to their strength, their capacity to withstand deformation and their ability to dissipate energy. In conventional design, beams become deformed, are damaged or even break in order to prevent buildings from collapsing. As an alternative to this practice, a new design philosophy known as vibration control has emerged.
In civil engineering, this is an area of knowledge that treats structures as dynamic systems -- that is, as systems subject to external actions that, over the course of their useful lives, respond by exhibiting particular behaviors. These actions often take the form of undesired agitation that causes movements in the structure that should be reduced in the most appropriate manner possible. "In the case of earthquakes specifically, the external action is the movement of the ground, and the dynamic system or structure tends to be a building," explained José Rodellar, director of the Control, Dynamics and Applications research group in the Department of Applied Mathematics III at the UPC-Barcelona Tech.
"In essence," Mr. Rodellar continued, "the idea is to consider the structure as a 'living' system that is subjected to agitations. We are interested in connecting some sort of additional mechanism to the structure in order to improve its behavior and strength in the event of an earthquake."
The first alternative is to use passive control systems such as base isolators or energy dissipators -- that is, to retrofit the structure with devices that will absorb some energy in the event of an earthquake. Today, the most mature and widely used technology is base isolation, which consists in adding special supports that decouple the movement of the structure's base from the movement of the ground. A flexible device is installed that allows the base to move. The friction that this causes dissipates some of the energy that would otherwise directly enter the structure. Another system is to support the structure on flexible elements made from materials such as neoprene. In this method, the deformation of the support elements is what allows the base to move.
Eergy-dissipation systems essentially consist of dampers placed at the joints between structural elements -- for example, where beams meet diagonal supports. During an earthquake, these devices, made from very weak steel, may undergo plastic deformation or even break, but only in very specific, predefined areas of the building.
The ain advantages of these control systems are their simplicity, the possibility of replacing them if they age or break, and the fact that they do not require an external energy source, since they work by reacting to the movement of the structure. Nevertheless, when the force of an earthquake exceeds a certain magnitude, the limitations of these systems become clear.
Another alternative is to employ an active control system, which is designed to apply a force that counteracts that of the earthquake. Active systems differ from passive systems in that they apply force to the structure by means of a real-time process. A series of sensors begin by measuring the structure's response (displacement, velocity and acceleration). The measurements are digitized and sent to a computer that carries out a control algorithm. This algorithm calculates the value of the necessary displacement and governs the mechanism of the actuators, which transform the signal into effective force. Active systems have great potential but are difficult to implement in practice. The main difficulty is a technological one, having to do with the way in which forces can be applied to large structures such as buildings.
This is why much of the research in the field is focused on developing semi-active strategies.
This intermediate line of work consists in designing passive control devices whose characteristics, rather than being fixed at the time of their construction, can be adjusted by a controller in real time on the basis of the structure's response, as measured by the sensors.
Smart materials are materials that respond to specific external stimuli and carry out particular functions as a result of their intrinsic properties. These materials have had a major impact on the field of vibration control.
For example, electrorheological or magnetorheological materials are fluids that can change very quickly, in milliseconds, from liquid to solid (and vice versa) when exposed to an electrical or magnetic field. Magnetorheological fluids contain suspended magnetic particles that are distributed randomly. When an electrical or magnetic field is applied, these particles realign themselves, thereby increasing the material's resistance to vibrations and making it less likely to be deformed in the direction in which the particles are aligned. These characteristics make magnetorheological fluids appropriate for use in the design of semi-active control devices, especially dampers containing these fluids.
Smart materials are most frequently used to absorb vibrations in vehicle suspensions. Attempts are being made to transfer this technology to civil engineering, where it has thus far only been used experimentally. The remaining challenge is to engineer a way to automatically modify the voltage required to cause changes in the properties of the materials, with a view to achieving a rapid, effective and lasting response. As a result, a recent line of research has focused on the design of devices, mathematical modeling and the formulation of algorithms.
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