October 1, 2006 A new bridge design replaces reinforced concrete columns and bars with steel tubes filled with concrete. The steel and concrete bind, creating strong yet supple columns For the bridge pier's footing, additional structural shapes are embedded in concrete to resist the large bending forces developing at the base of the piers. New or retrofitted bridges over highways and water could withstand fire, hurricanes and flooding.
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BUFFALO, N.Y. -- There are nearly 600,000 bridges in the United States. Millions of people cross bridges every day without giving a second thought to their safety. But many of them could be taken down by a natural disaster like an earthquake or flooding or worse, by a terrorist attack.
"We have already seen, after Hurricane Katrina, bridges massively pushed sideways and out of their piers and, and in a very large scale," says Michel Bruneau, a structural engineer at the Multidisciplinary Center for Earthquake Engineering Research at University at Buffalo in Buffalo, N.Y.
He says it's time to make our bridges safer. "The best way to avoid a disaster in the first place is to have infrastructure that will not fail in a disaster."
Bridges are typically held up by reinforced concrete columns and bars. In Bruneau's new design, steel tubes are filled with concrete. The steel and concrete bind, creating strong yet supple columns. In field tests, a massive blast causes the supports to bend, but not break.
"We definitely think it provides better protection against multiple hazards," Dr. Bruneau says. "It is important to be covered against all hazards, and that is essentially the main change in philosophy that we are trying to impact here."
These multi-hazard-resistant bridges are intended for use over highways and water and can withstand fire, hurricanes and flooding.
The new piers would be used for new bridges, but Bruneau says existing bridges could also be retrofitted with the new design.
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.