The president -- along with first lady Michelle Obama, New York Gov. Andrew Cuomo, New York City Mayor Michael Bloomberg, New Jersey Gov. Chris Christie and Port Authority of New York & New Jersey Chairman David Samson -- toured the 22nd floor of the project, which on April 30 surpassed the Empire State Building to become the tallest structure in New York.
The steel beam the dignitaries signed is to be one of the final pieces of the building's framework.
As they prepared to sign the beam, President Obama said, "Don't mess it up anyone."
The president and the others spent a few minutes greeting workers representing more than 60 union trades that worked on One World Trade Center, which will be 1,776 feet tall when completed, making it the tallest skyscraper in the Western Hemisphere.
"We couldn't be prouder of you guys," the president told the workers. "This is what the American spirit is all about"