Construction crews worked all Wednesday to prop up the Manhattan high-rise that suddenly buckled — but what caused the terrifying structural failure still remained a mystery.

Temporary shoring and beams were completely installed on the afflicted East 42nd Street building’s 18th through 23rd floors during the morning after the structure stopped shaking and stabilized, Mayor Zohran Mamdani told reporters during an unrelated press conference.

He said crews were doing the same on the 17th through 24th floors.

Mayor Mamdani said crews are set to work all Wednesday propping up a Manhattan high-rise with buckling support beams as four nearby buildings remain under evacuation orders. NYC Department of Buildings A ‘vacate’ notice is displayed on a door inside of a church across from the former Pfizer headquarters building on Wednesday in NYC. Getty Images

“They’re going to be working through the day to get all the way up to the roof and all the way down to floor nine,” he said, adding, “It is a 37-floor building.”

Photographs shared on the city Department of Buildings’ X account showed the emergency shoring – rows of steel standing solidly next to a problem-causing bent beam.

The harrowing situation at 235 E. 42nd St. – the former Pfizer headquarters being converted from offices to apartments – was discovered Tuesday on its 21st floor, where two support beams buckled at around 8 a.m.

The under-renovation structure was quickly evacuated, as were a swath of streets and several surrounding buildings – and a “frozen zone” was established that barred pedestrian and foot traffic below.

A view of 235 East 42nd Street on Wednesday. AP Photo/Yuki Iwamura

Officials shrank the frozen zone Wednesday to East 42nd and 43rd Streets between Second and Third avenues, but kept four buildings under a full vacate order, Mamdani said.

One more building was under a partial vacate order for a ground-floor restaurant, he said.

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“I want to confirm for all New Yorkers that there has been no additional movement of the structure since yesterday morning,” Mamdani said. “This continues to be monitored as that emergency work is underway.”

But what caused the near-disastrous structural failure remained unknown, prompting the DOB to launch an investigation.

The affected area was just below a “bump out” section added to the building during renovations that horizontally extend its 23rd through 32nd floors, City Comptroller Mark Levine posted on X.

Contractors with MetroLoft were converting the former office building into residences, with 1,600 apartments slated to open in 2027. The job, which began in 2024, was touted as the city’s largest office-to-residence conversion by the project’s architects, the Gensler firm.

Structural engineers told The Post that any probe would likely focus on the added section at the top of the tower.

A model of the compromised building on 42nd Street. NY Post Design

Converting buildings from offices to residences requires meticulous calculation and planning as loads can shift during construction, said Michael Shenoda, chair of civil engineering technology at Farmingdale State College.

“It’s really a matter of the loading that came in after,” he said. The building is over 40 years old. The original design seems to have been sufficient — it’s the changing nature of the loads after that seems to have caused it.”

One cause could be if contractors stacked construction materials beyond what the design called for, said James LaFave, a professor of civil engineering at the University of Illinois Urbana-Champaign. 

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Here’s the latest on the evacuated Midtown building in danger of a ‘localized collapse’:

• Crews work to prop up Midtown NYC high-rise that suddenly buckled — as cause remains a mystery
• NYC Pfizer building developer tries to downplay collapse risk — insists high-rise buckling is a ‘freak accident’
• Buckling building in Midtown NYC will face partial demolition — but even stabilizing it beforehand will be highly risky: experts
• NYC buildings commissioner confident buckling Midtown building is ‘stabilizing’ after emergency work
• Massive ‘frozen zone’ around buckling Midtown NYC building leaves hundreds out on street — with no idea when they can return

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He also said investigators should look into whether the building needed strengthening or if load-bearing beams were mistakenly removed.  

“I think the biggest thing that’s going to come out of that forensic investigation is recommendations about next actions,” he said.

“It could go all the way to requiring a partial demolition of those areas, while keeping others. Or maybe there could be a finding that the same items that was found to be a problem in this column yesterday is a more broad problem.”

The construction site was issued seven violations between July and December 2025, resulting in a total of more than $32,000 in fines, agency records show. 

During one incident, a piece of metal fell off the building from the 33rd floor level and landed on the sidewalk below, records show.

Experts said the shoring measures likely stabilized the building in the short-term to give investigators time to find the cause and engineers a chance to plan a long-term fix.

The emergency work includes adding structural steel shoring columns to the affected 21st and 22nd floors, officials said. 

All light-duty shoring posts — which were installed on the 17th through 24th floors as a temporary measure — will be replaced by longer-term steel columns expected to be delivered late Wednesday, according to officials.