BACKGROUND: An earthquake engineer at the University of Buffalo has developed a new "multi-hazard" design for bridges that will make them more resistant to collapsing from the impact of earthquakes and terrorist attacks. Michel Bruneau, director of the Multidisciplinary Center for Earthquake Engineering Research says his design for bridge piers -- the columns that support a bridge's superstructure -- is intended for small to medium sized bridges.
THE DESIGN: The new bridge-pier design uses steel tubes filled with concrete, but without reinforcing bars. The steel and concrete bind together, forming a composite structure, giving the piers strength. It also means they can bend without breaking when subjected to seismic forces or explosive blasts. Most bridges built today are supported by conventional reinforced concrete columns, but these would be likely to break or weaken after a major blast, leading the bridge to collapse.
WHAT CAUSES QUAKES: An earthquake is a vibration that travels through the earth's crust. It can be caused by any number of things, including meteor impacts, underground explosions (from a nuclear test, for example) or collapsing structures, such as a mine. But most naturally-occurring earthquakes are caused by the movement of the earth's tectonic plates. The earth's surface is made up of large plates that slide over the underlying layer. At the plate boundaries, plates can move apart, push together, or slide against each other.
WHOSE FAULT IS IT ANYWAY: Wherever plates meet, there will be faults at the boundaries: breaks in the earth's crust where the blocks of rock on each side are moving in different directions. There are many different kinds of faults, but in all of them, the various blocks of rock push together tightly and produce a lot of friction. If there is a large enough amount of friction the plates can become locked, increasing the pressure until the plates suddenly give way and snap forward suddenly, sending out a series of seismic waves. These fault lines are the main source of earthquakes.
WHAT IS ELASTICITY? Different materials can withstand different amounts of deformation, a property known as elasticity. Most materials are elastic to some degree: when they are deformed or bent by an infusion of incoming energy, they will bounce back to their original shape. But elastic materials all have their limits. Metal springs and rubber bands are very elastic. Plaster and glass are not very elastic; instead, they are brittle and snap with even a small deformation. Energy, like momentum, is conserved, but in a collision, it can turn into different forms of energy, such as heat or noise. How much of the energy is converted depends in part on both the relative toughness and elasticity of the materials involved in the impact. There is no such thing as a perfectly elastic collision, but if there were, all of the energy would be transferred to the target with nothing lost to heat or noise, for example.
The American Society of Civil Engineers contributed to the information contained in the TV portion of this report.