The substandard construction methods that are suspected of - TopicsExpress

The substandard construction methods that are suspected of triggering the deadly collapse of an eight-story building in Bangladesh on Wednesday are a common problem in developing countries, where construction materials can be expensive and building inspections infrequent, experts say.The catastrophic collapse happened around 9 a.m. local time in an industrial suburb of the Bangladesh capital city of Dhaka (map). Ranza Plaza housed four garment factories, as well as some shops and a bank. (See Bangladesh pictures.)More than 150 people are confirmed dead, and more than a thousand injured. Many are still trapped in the rubble, buried beneath broken concrete slabs and twisted steel rods.Scenes from the disaster show rescue workers and volunteers digging through the rubble by hand and clinging to makeshift ropes made from knotted, colorful strips of fabric as they search for survivors.Officials have blamed the collapse on shoddy construction methods. The upper four floors of the plaza, for example, were reportedly constructed illegally without permits, and a crack was seen on the building exterior a day before the collapse.The building was not built in compliance with the [safety] rules and regulations, Bangladesh Home Minister Muhiuddin Khan Alamgir told CNN.Stern legal actions will be taken against the people who built the structure defying the codes or laws. Uneven Footing:The exact cause of the collapse has not yet been determined, but Henri Gavin, a civil and environmental engineer at Duke University, speculated that the buildings foundation was substandard.It could be that one edge of the building was on much softer soil than the other, so that part of the building settled down a little bit more, Gavin explained. That could easily lead to an instability that would precipitate a collapse.Another possibility is that weight on the top factory floors—where the crack was spotted—was unevenly distributed. (Also see pictures: Sinkhole Swallows Buildings in China.)If this building had very large open spaces the way a lot of factories do, and if the floors had long spans without lots of [reinforcing] columns ... then the building could start to lift one way or the other if heavy equipment was not spaced evenly throughout the floors, Gavin said.When designing a building, engineers are supposed to consider different combinations of how loads are placed in the structure. The intention is to require the engineer to consider as many cases as possible, Gavin said.Such modeling is easy to do—if one has the right computer and software. In developing countries such as Bangladesh, however, calculating different load distributions can be a time-consuming process, and as a result might be skipped. Construction Problems:Poor building design is only one part of the problem, however. The best building design in the world is for naught if a construction firm doesnt follow the plans precisely.That may have been the case with Ranza Plaza, which appears to have been built largely out of concrete. (Learn about megastructures on the National Geographic Channel.) Concrete buildings require large amounts of reinforcing steel, called rebar, to prevent excessive cracking. Depending on the country, steel can be costly.In developing countries, steel is relatively expensive in comparison to the labor and concrete, said Dan Jansen, a civil engineer at California Polytechnic State University.In the U.S., steel is not that huge a factor. Its easy to add more steel to make [the building] more ductile and stronger, and so we do it here.But in developing countries, less steel is often used than is recommended because of the cost.Reducing or changing the reinforcing steel without the building officials approval is never acceptable whether youre in a developing country or the U.S., Jansen said.From looking at photos of the collapse, Jansen said he suspects not enough rebar was used in the buildings construction.The way it collapsed, and the fact that so much of it came down, suggests there was a lack of redundancy, he said. The amount of reinforcing steel used didnt allow it to transfer the load from one section to another, and thats why so much of it came down.In addition to possibly being under-reinforced, the concrete mix may not have had enough cement, said Gavin of Duke University.Many of the casualties from the 1999 Kocaeli earthquake in Turkey were in medium-rise concrete apartment buildings, he added.Investigations following this earthquake revealed that the concrete had more sand and less cement than required by typical design standards. _____________________________ [from : nationalgeographic]