Sgt. Roland Lee said the closures were just a precaution while the panel was being replaced.
The piece of glass, about a foot long, detached from a larger panel after the section cracked, said Blue Cross spokesman Butch Ward.
"Thank God nothing was hit," he said.
Officials were not immediately sure what caused the break. Ward said technicians would ensure that other panels in the smoked-glass building were safe. The 45-story building was completed in 1990.