the gas-fired turbine is part of the structure that controls electricity production and the operation of the building. the central part of the building, containing four turbines, was built in two pieces, with the interior supporting columns and the exterior walls and roof built in one piece. in addition, a fire-resistant glass wall runs the length of the building, and a fire-resistant curtain wall separates the public and private areas.
the girders, which were fabricated at an offsite location, were assembled on site. eight of them were used to support the roof and would have had to fail for the entire roof to have collapsed. thats not what happened. the main girder spanning the length of the building collapsed in the middle, with the floors above it collapsing over it.
all the problems that were found so far have been minor issues, although there have been some larger problems, such as the steel members in the building had to be replaced during construction because of corrosion, says michael g. murphy, a fire protection engineer who has been monitoring the investigation. the fact that they were able to build a $2 billion facility and construct a building that has survived a test of its strength has been a testament to the strength of the design, murphy said.
the great thing about the girder is that it was an indication of the steel. the steel is good, he said. but we don’t know if there are other defects. at the same time, when you have a defect, you want to make sure there aren’t other defects.
theres no estimation yet how long the repairs will take, and there is still more testing to do. the nature of the failure is prompting the contractors, as well as a peer review panel, to investigate other parts of the building that might be susceptible to cracking, as well, said michael engelhardt, a professor at the university of texas at austin, who is chairing the peer review panel.
the authorities have said the cause of the cracking has not been determined. the issue has prompted a call for increased oversight of the center. the cracks at the transbay center are the latest in a series of construction problems that have dogged the project.
in 2014, after the authorities hired the firm of miller, boynton & associates to examine the condition of the building’s foundations, the authority hired the building’s original engineers to do an assessment of the building’s steel and concrete. that report found no cracks in the slab, but it also said that some of the connections of concrete columns to the slabs in the roof and on the lower floor were not designed to be tight and could have problems with corrosion.
a year and a half ago, the citys design and construction authority issued a report concluding that during the terminal construction process, beams were damaged by being struck by large steel plates that were part of the concrete pouring process. the plates were used to keep the concrete from cracking, but they were too thick to be removed and, as a result, were left inside the concrete, said john goodwin, the executive director of the transbay joint powers authority, which built the terminal.
dhalwala said the problem is that many engineers dont understand the limitations of the structural codes and dont know what would constitute a fracture. they dont understand that a small crack could cause a building to collapse.
dhalwala said the models that he uses show what a crack would look like. in reality, however, a crack doesnt have to be perfectly horizontal, meaning it doesnt have to go through the entire web. it can actually be a tiny vertical crack, he said.