PITTSBURGH (KDKA/AP) – A large stone fell overnight from the corner of one of downtown Pittsburgh’s most iconic buildings, closing the street below.

Grant Street was shut down between between Fifth and Fourth Avenues, and Forbes Avenue is closed from Ross Street to Cherry Way after the three-by-three piece of granite fell from the underside of a cornice on the Frick Building about 2:45 a.m. Monday. The closures are expected to remain in place at least through noon on Tuesday.

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Public Safety Director Wendell Hissrich says it’s lucky the streets were empty when it happened. It’s estimated a piece of granite that size weighs about 120-150 pounds per foot. Nobody was hurt.

“It occurred at the best possible time, early in the morning. If it had been in the afternoon or the morning we might be here talking about a tragedy,” Hissrich said.

So now the question is why did it happen?

The owners of the building are responsible for finding out. Moira Kennedy, of the city’s Permits, License and Inspection Department, says the process has already started.

“We have ordered the owner of the building to obtain a structural engineer’s report,” Kennedy said. “At this point, we are focusing on that corner to make sure that we are developing a strategy to abate it.”

The hope is that structural evaluation on the building will be done by Tuesday, which is good news for motorists.

In the meantime, city officials say downtown drivers should plan ahead.

“If you have to come downtown, be careful where you go, think your trips out, there’s lots of alternative routes,” said City’s Operations Officer Guy Costa.

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Engineers say the damage to the 100-year-old Frick Building was most likely caused by the “freeze/thaw” cycle in our weather.

Building owners may have to put a 21-story scaffold up to keep people safe and allow for repairs.

Grant Street could reopen sometime Tuesday, but in the meantime, pay attention. “No Parking” signs will be posted around the area to help with the traffic flow.

Also, the city says beware; you could be towed if you ignore the signs.

The building inspection should be done by Tuesday, and then a determination will be made on how soon the streets can open back up.

The building is named for industrialist Henry Clay Frick who had it built along with several others downtown. It opened in 1902.

The 20-story building stands about 330 feet high. The building houses law offices and other commercial tenants.

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(© Copyright 2017 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)

Lynne Hayes-Freeland