The New York Building Congress says new construction projects in New York City dropped 31 percent in 2011 compared with 2010.

The building congress says $13.8 billion worth of construction started in 2011. That’s down from $20 billion in the prior year.

The report notes that seven large construction projects valued at $500 million or more started in 2010. Those included the World Trade Center site, Barclays Arena in Brooklyn and the renovation of Madison Square Garden.

New York Building Congress President Richard Anderson says that with so many big-ticket projects starting in 2010, it’s not surprising to see a dip in 2011.

But he says the 31 percent decline is troubling.

On a positive note, residential construction starts rose 24 percent to $2.9 billion last year.