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San Francisco’s new transit hub, a three-block-long edifice that floats like a cruise ship above downtown streets, was open only six weeks before the discovery of cracks in two massive steel beams prompted officials to close the terminal.

On Tuesday, the managers of the project said it would be weeks before the Salesforce Transit Center would reopen. Engineers hope to discover what caused the cracks by analyzing samples of the beams. For some insights we spoke with David Williams, a structural engineer who specializes in large infrastructure projects.

Q. One of the stipulations in the construction of the Transbay terminal was that materials had to be sourced in the United States. Given that so much steel is produced outside America these days, was this a potential factor in the quality of the beams that fractured?

A. It’s extremely limiting for the project managers to buy this type of American steel. It’s available, but you have a very short list of suppliers and it’s often a special order. It’s like going to get a heart valve from someone who does one every six months instead of every six days.