On Saturday, Feb. 6, a magnitude-6.4 earthquake shook the island nation of Taiwan. Nearly all the high-rises in the southern city of Tainan withstood the quake without serious damage, partly because of new building codes enacted after a more severe earthquake in 1999 killed 2400 people. But in the Feb. 6 quake, a 17-story building containing 96 apartments completely collapsed, trapping victims inside and causing the majority of the deaths involved. As of Feb. 11, the confirmed death toll in the quake stood at 55, but many bodies probably remain in the ruins of the Golden Dragon apartment building, which was erected in 1994 before the new building codes took effect. On Feb. 9, the building's developer and two associates were arrested and charged with professional negligence causing death. Prosecutors claimed that shoddy construction was responsible for the building's collapse, saying that cans of polystyrene foam were used as fillers in the reinforced-concrete structure and that steel reinforcing bars were too short.
Building a structure that can resist earthquakes is a challenge that modern structural engineers tackle routinely. Very few steel-framed high-rises are seriously damaged by earthquakes, because the type of steel used in them has a certain amount of "give" which allows the stresses of a shaking foundation to bend but not break supporting members. The only exception is the unusual case when an earthquake's period coincides with a building's resonant frequency and vibrations build up until something snaps.
Reinforced concrete is another matter entirely. Concrete has excellent compressive strength, but it's brittle and doesn't bend easily. If you try to bend it, parts of it are put into tension, and pure (unreinforced) concrete has almost no tensile strength, so it cracks when subjected to the pulling forces that bending causes.
Many decades ago, construction engineers figured out how to embed "rebar"—steel reinforcing bars—in concrete to provide the tensile strength that concrete alone cannot provide. Properly apportioned and applied, reinforcing bars can make concrete-framed structures just as strong as steel ones, with the advantage that setting up molds and pouring beams and floors can be a lot cheaper than assembling a steel frame. So many buildings for which cost is an issue, such as apartment complexes, are made of reinforced concrete.
However, making such a structure earthquake-resistant is a challenge, especially if it was not originally designed that way. A personal anecdote will illustrate this. I attended the California Institute of Technology in Pasadena from 1972 to 1976. That institution began its existence in 1891 as a vocational school funded by businessman Amos Throop. By 1912, the main building on campus was Throop Hall, a reinforced-concrete-and-brick structure that stood until a 1971 magnitude-6.6 quake seriously damaged it. Engineering studies showed that the structure was fatally flawed with regard to earthquake resistance, and would probably collapse in another quake of the same or greater magnitude. So despite its historic associations, it was condemned and fell to the wrecking ball during my freshman year there.
The problem of what to do with existing structures when building codes change is difficult, and municipal authorities rarely condemn buildings that are not obvious ongoing hazards simply because of a building-code change. The Golden Dragon apartment building may have been erected in compliance with the codes as they stood in 1994, but emotions are running high after the disastrous collapse, and the developers will have to argue in court as to whether they behaved responsibly during the construction of their building.
One way to enable reinforced-concrete structures to withstand earthquakes is to make ductile joints between the horizontal and vertical members of the structure. This will allow the building to "follow" horizontal ground movement without imposing fatal strains on the supporting walls. The fact that lightweight material such as plastic foam was used as fill may not necessarily indicate shoddy construction. And the length of rebars is something that may or may not have had anything to do with the building's collapse.
The good news coming out of this tragedy is that more buildings didn't collapse, as for example happened in Haiti in 2010. What few building codes existed there were not enforced, and although there were few structures more than three or four stories high, over 200,000 of them collapsed and the death toll exceeded 100,000. As a rapidly developing nation, government officials in Taiwan did the responsible thing following the 1999 earthquake and imposed building codes that required buildings to withstand a certain level of earthquake shocks. The fact that only one major high-rise collapsed, and that one a pre-1999 structure, says that the new building codes have been largely effective.
In addition to investigating the construction of the ill-fated Golden Dragon, Taiwan officials may want to consider a program of inspections of pre-1999 structures with an eye toward preventing more such tragedies in the event of a larger earthquake. Even if the conclusion is that things are okay, this would be a reassuring thing to find out. And if some other structures are like time bombs waiting to be set off by a large earthquake, the time to find that out is now, not when the next big one hits.
Sources: I referred to news items from Agence France-Press carried by the Australian Broadcasting Company at http://www.abc.net.au/news/2016-02-11/taiwan-court-hears-of-critical-flaws-in-quake-hit-high-rise/7160954and UK's Daily Mail at http://www.dailymail.co.uk/wires/afp/article-3439998/Taiwan-developer-grilled-collapse-quake-building.html. I also referred to the Wikipedia articles on earthquake engineering and the California Institute of Technology.
