New York City has bet heavily on converting aging office buildings into apartments to help ease a housing shortage. But the threat of a partial collapse Tuesday by one such conversion in progress highlighted the significant challenges of those construction projects.

The former Pfizer building, located on East 42nd Street, was undergoing work to become roughly 1,600 apartment units with amenities such as a rooftop pool and a fitness center.

But on Tuesday, structural columns buckled and floors sagged, prompting the evacuation of that building and several others nearby, according to New York City officials. Nearby bus routes that pass were also delayed or partially suspended.

Office-to-apartment conversions have surged since the pandemic emptied out aging office buildings, offering cities a way to add desperately needed housing. And New York City has championed these conversions, offering tax incentives to turn older vacant office buildings into livable spaces.

But the projects often require extensive structural, plumbing, mechanical and redesign work that can make the project more complicated than building new housing from scratch.