The accident happened just before 10:00 at the site of WTC Tower 4 at Church and Cortlandt Streets.
According to officials, a crane lifting construction materials snapped, dropping a steel beam onto a flatbed truck parked at the site.
A load of wooden planks may have also fallen. They were scattered across the site, near where the beam fell.
The injured worker was treated at the scene.
No other injuries were reported.
The accident halted Number 1 subway service between Chambers Street and South Ferry. The 4 World Trade Center will be a 72-story tower, 977 feet tall, when completed next year.
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