On the morning of July 7, construction workers at 235 East 42nd Street in Midtown Manhattan noticed something wrong. Two structural columns on the 21st floor of the former Pfizer headquarters, which is in the middle of being converted into 1,600 residential apartments in the largest office-to-residential conversion in New York City history, had begun to buckle. The floors between the 21st and 26th stories were sagging under the redistributed load. The building was immediately evacuated and was still moving hours after emergency responders arrived. Mayor Zohran Mamdani described it as an extremely serious situation. The FDNY set up a frozen zone spanning several blocks and evacuated nine adjacent buildings, including a hotel and a school with 400 children. Nobody could safely go inside to assess what was happening. So instead, the city did something that would have been impossible a generation ago. It sent in the technology.

Drones were the first tool deployed. The FDNY used them to monitor the building’s exterior and relay visual information to structural engineers on the ground who needed to understand how the columns were deforming before they could develop a plan to shore them up. The drone’s role in this context is essentially that of a flying eye. Modern inspection drones carry high-resolution cameras capable of capturing millimeter-scale surface detail, thermal sensors that can detect heat anomalies indicating stress concentrations or hidden damage, and in some configurations LiDAR scanners that generate three-dimensional point cloud models of a structure’s exterior geometry. At 42nd Street, where the primary concern was whether the building was continuing to move and whether additional columns were showing signs of stress, drones gave engineers a real-time visual feed from positions that no human could safely occupy. The FDNY specifically credited drone monitoring as providing critical technical information and visuals for partner agencies as they developed their response plan. Buildings can be assessed without a single person entering the structure, which in a scenario where a partial collapse is being actively discussed is not a minor advantage.

What drones provide in visual resolution, tiltmeters and inclinometers provide in precision movement detection. These are devices that measure angular displacement, the degree to which a structural element has deviated from its original orientation, with sensitivity fine enough to detect movements that are invisible to the naked eye. A tiltmeter attached to a column or floor plate will register a change in angle measured in fractions of a degree and transmit that reading continuously to a monitoring system. In a building where the concern is whether an already-buckled column is continuing to deform, that granularity matters enormously. The difference between a column that has buckled and stabilized and one that is still actively moving determines whether engineers can safely enter the building and begin installing emergency shoring. Mayor Mamdani confirmed at a midday press conference that since arriving on scene, officials had witnessed additional movement in one of the compromised columns. That information almost certainly came from some combination of drone visual monitoring and tiltmeter readings rather than from anyone standing next to the column in question.

Accelerometers function on a related but distinct principle. Where tiltmeters measure angular deviation from a reference position, accelerometers measure dynamic movement, the vibrations, oscillations, and micro-tremors that a structure produces as it responds to load changes, wind, thermal expansion, and the ground conditions beneath it. In a healthy building under normal conditions, the accelerometer data is predictable and consistent. When something structural changes, the pattern shifts in ways that trained models and experienced engineers can interpret. MEMS-based accelerometers, the solid-state sensors now standard in serious structural health monitoring applications, can detect accelerations as small as fractions of a g at sampling rates that capture rapid changes as they happen. In the context of a building with two buckled columns redistributing load to the remaining structural elements, accelerometers on those columns and on the floors above and below the failure zone provide continuous evidence of whether the redistribution is stabilizing or progressing toward a more serious failure.

LiDAR, which stands for Light Detection and Ranging, takes the monitoring capability a step further by creating detailed three-dimensional maps of a structure’s geometry at a given point in time. The technology works by emitting pulses of laser light and measuring the time it takes for each pulse to return after bouncing off a surface, building a point cloud that represents the shape of the object being scanned with millimeter-level accuracy. When LiDAR scans of a building taken at different times are compared, even very small deformations in the structure become visible. A column that has moved half an inch, a floor that has sagged at its center, a wall that has deviated from plumb by a fraction of a degree: all of these changes show up clearly in the comparison between two LiDAR scans that would look identical to the human eye. At 42nd Street, where engineers needed to understand not just whether the building was moving but where and how much, LiDAR-equipped drones and ground-based scanners gave them a geometric reference that no visual inspection alone could provide.

None of these technologies are exclusive to emergency response. They represent the leading edge of what the building industry calls structural health monitoring, a discipline that is slowly moving from reactive crisis management to proactive continuous surveillance. Modern skyscrapers are already being equipped with permanent wireless sensor networks that collect tilt, vibration, strain, and displacement data continuously and feed it to cloud analytics platforms that flag anomalies and track long-term trends. The Empire State Building has sensors monitoring its sway behavior. The Burj Khalifa uses an integrated monitoring system that tracks structural performance in real time across its 828-meter height. Major bridges including the Golden Gate run continuous accelerometer networks that detect changes in resonant frequency that can indicate fatigue or damage developing in the structure long before it is visible. The I-35W bridge collapse in Minneapolis in 2007, which killed 13 people, has been cited repeatedly in the structural engineering literature as an event that sensor networks might have detected in advance, and it significantly accelerated the adoption of continuous monitoring in bridge infrastructure.

The construction environment presents a specific challenge that makes all of these technologies particularly valuable. A building under renovation or conversion is structurally dynamic in ways that a completed building is not. Load paths are being changed as elements are removed and added. Temporary supports are carrying loads that the permanent structure was never designed to carry. Additions to existing structures, like the eleven stories being added to the nine-story building adjacent to 235 East 42nd Street, introduce new loads on structural systems that were originally designed for a completely different configuration. The DOB had received multiple safety complaints about the 42nd Street site since May 2025, including reports of falling debris and a worker injury.

The question that the structural engineering community will be asking in the investigation that follows is whether earlier deployment of continuous monitoring technology would have detected the deformation that led to Tuesday’s buckle before it reached the crisis threshold. The answer, based on what the technology can do, is almost certainly yes. The same sensors and monitoring systems that are now being used reactively to understand a building that is already in distress can be deployed proactively at the beginning of a complex construction process to establish a baseline and flag deviations from it as they develop. The 42nd Street incident is, among other things, a demonstration of how powerful these technologies are when deployed in an emergency. It may also turn out to be a catalyst for deploying them earlier, in the conditions that precede the emergency, across the growing pipeline of complex adaptive reuse projects that are reshaping how American cities are built.

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