The mishap came as the city cracks down on construction safety after a series of accidents. One killed seven people and helped lead to the department commissioner's resignation.
Acting Buildings Commissioner Robert LiMandri called the most recent incident "unacceptable."
"Contractors must prioritize safety and comply with all required safety measures," he said. "We will halt construction when contractors fail to do so and hold them accountable. Development cannot come at the expense of workers and the public."
Tishman Construction spokesman Richard Kielar declined to comment.
The Buildings Department said the 30-by-30 inch piece of steel fell from an outdoor construction elevator carrying materials to the 43-story building's upper floors. The steel was being used as a bridge between the lift and the building for unloading materials.
Workers were using the hoist to bring Sheetrock to the 18th floor. The Buildings Department said Tishman had permission to work on the weekend, but the approval didn't include delivering Sheetrock.
The violations cited included failing to protect the public and property, and failing to secure vertical netting.
The planned tower is expected to house 9,000 of the company's employees when it opens, set for 2009. A Goldman spokeswoman had no immediate comment.
At the same site in December, a crane dropped seven tons of steel and seriously injured an architect in a trailer below. Tishman also was cited in that incident.
LiMandri took over last month after Commissioner Patricia Lancaster stepped down amid scrutiny of recent construction accidents. He vowed to get tough on safety and mentioned stop-work orders as a tool.
Thirteen people have died in construction accidents in the city this year, one more than last year's total.
In the deadliest incident, a 20-story construction crane collapsed in midtown Manhattan on March 15, killing seven people and demolishing a brownstone.
Also earlier this year, a window washer was killed and his brother critically injured when a scaffold plummeted more than 40 stories off a building. Another worker was killed the same day in the Bronx after heavy equipment pinned him while he was digging a hole to lay pipe.