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The news:
Scientists in California, USA are experimenting with freeway overpasses for seismic safety by applying strong forces to full scale models of structures to assess their reaction and survivability of big quakes.
The background:
When the ground below the structures shakes, the otherwise strong and stable structures experience different set of forces and stresses causing it to fail. Differential settlements, failure of columns by snapping of lateral ties, collapse of un-reinforced masonry, extreme deflections rendering the structure unserviceable are a few issues.
Earlier, the shear reinforcement in structures built in seismic regions, arguably, got a raw deal. The highway overpasses, very high flyovers that criss-cross in complex patterns at intersections in cities like Los Angeles and San Francisco. With increasing knowledge of shear reinforcement failure in columns, the retrofitting of these over passes has been undertaken in a massive scale in the state of California. The retro fit may take the following components.
The size and mass of bridge footings is increased, additional piles driven and anchored securely into the ground.
Thick cables hold sections of the freeway together and secure them to the support pillars.
Support columns of existing bridges are covered with a thick coat of cement grout and a jacket of thick steel to prevent outward buckling of main steel bars and snapping or opening up of lateral ties.
Many overpasses that did not got their retrofit yet failed during the 1994 Northridge quake.
Current Efforts:
As California spends billions of dollars retrofitting freeway overpasses and public buildings, efforts were being made test to their strength in the absence of a real quake. It became imperative to conduct these tests, as the older regulations of structures failed in earthquake safety and the behaviour and survival rate of retrofitted structures is hard to predict in real terms.
Scientists are building elaborate models of freeways and office towers and testing how they would respond to extreme shaking. The Los Angeles Times has reported that researchers spent some $1 million building the foundation of a highway overpass near the Los Angeles International Airport and on put it to the test.
They gave the foundation a heavy lateral thrust of 2 million pounds (more than 900 tons) of force to assess the damage. Surprisingly, it held up the shove better than expected by moving only an inch and a half. Scientists plan to factor such data into future computer models for simulation of bridge behaviour in earthquakes.
Earlier tests include at almost full sized model testing s of every part of the new San Francisco-Oakland Bay Bridge and a seven-story custom built building on a massive shake table at UC, San Diego. These shakings are to replicate the effect of the 1994 Northridge earthquake.
The work is important not only to develop structural regulations and codes but even for architectural designs and insurance industry.
However, there is a catch. There may be huge variation in how the real soil mass and structures built on it will behave in real earthquake. Simple mathematical models based on many assumptions may just not work, particularly because the data represents only a single effort to move one piece of a complex overpass. Also, there is a question of how to correlate the magnitude earthquake with the 2 million pounds or so of force applied. More over, the structures fail not only in foundation.
What to expect?:
The bright side of the news is that Network for Earthquake Engineering Simulation, a network of 15 universities and researchers throughout the country promises on its website to "revolutionize earthquake engineering research and education”. The goal is to combine the results of engineering tests on structures with realistic tests on soil and rock under shaking, to improve the understanding of how the ground and the structures built on it will behave in earthquakes.
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