It happened while workers were taking a break at the building at 16 North Bond St. between Mount Vernon Avenue and Oak Street.
The property owner had obtained permits late last year for structural work on the building.
Workers had begun shoring it up to stabilize it when the collapse happened.
As a precaution, neighboring buildings at 14 and 18 North Bond St. were evacuated.
Power to the impacted area has been disconnected as a safety precaution.
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