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Tuesday, 15 March 2011 07:30 |
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Earthquake Devastates Philippines
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By Cathy Clark and Jim Taylor
On July 16, 1990 at 4:26 p.m. local time, a severe earthquake registering 7.7 on the Richter scale struck the northern Philippines. The earthquake caused damage over a region of about 7700 square miles, extending northwest from Manila through the densely populated Central Plains of Luzon and into the mountains of the Cordillera Central.
Over 5,000 people were reported dead or injured, and in excess of 2300 infrastructures were either destroyed or seriously damaged. While the quake was devastating, it was not an unusual occurrence in the Philippines; since 1950 alone there have been six major earthquakes at various locations in the archipelago, having magnitudes ranging from 7.3 to 8.3.
STRUCTURAL DAMAGE
Buildings were decimated by ground shaking, soil failure and liquefication (causing them to settle into the ground), and landslides.
Nearly all multistory buildings in the Philippines are constructed of reinforced concrete frames, supporting slab floors. Short-column failure was evident in many buildings observed to have the classic diagonal cracking where the column was acting as a short shear wall and could not carry the loads. Many unreinforced masonry infilled walls separated from the concrete frames and collapsed.
In the heavily shaken regions, two general types of disastrous failure to multistory, larger reinforced concrete buildings were observed--failed first stories and total building collapse.
First-story (or Soft-story) Failures
The ground floor of a building is frequently the weakest part of the structure. It is seldom enclosed on all four sides by walls capable of resisting shear forces, and it is also generally taller than upper floors. Ground floor shops, stores, lobbies, or garages normally allot most of their front wall area to doors or plate glass, leaving one side of the building with no shear resistance. Bending and shear forces induced by strong ground shaking are therefore concentrated in the ground-floor columns. As a result, the building may fail by collapse of only its first story, with the stronger upper section of the building remaining intact.
Multistory Failures
Many multistory building failures or “pancake” collapses (typically with structures of six to ten stories) were observed in the city of Baguio. One such collapse included a nine-story hotel which killed over a dozen occupants on the ground floor. This type of damage has been observed repeatedly in numerous earthquakes throughout the world where design and construction deficiencies exist.
(Snipped)
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