Structural Buckling in Buildings: What the New York City Incident Reveals About How Steel Structures Fail

What Is Structural Buckling?

Structural buckling is a sudden lateral deflection or bending of a compression member—most commonly a column or a beam—that occurs when the applied compressive load exceeds the member’s critical buckling load. Unlike yielding, which is a gradual material deformation, buckling is an instability failure: the member’s geometry changes in a way that causes the load path to shift, which then accelerates the failure.

The mathematical foundation comes from Leonhard Euler’s 18th-century column buckling formula, which defines the critical load at which an ideal column will buckle as a function of the material’s stiffness, the column’s cross-sectional moment of inertia, and its effective length. The key insight from Euler’s work—still used in modern structural design through AISC specifications and ASCE 7 load standards—is that buckling is fundamentally about length and slenderness, not just material strength. A very slender column can buckle at a fraction of the load required to crush the same material in a shorter, stockier configuration.

There are several distinct modes of buckling that structural engineers design against:

• Flexural buckling — the column bows laterally about its weak axis, the most common mode in unbraced steel columns
• Torsional buckling — the column twists along its longitudinal axis, common in open thin-walled sections like angles and channels
• Lateral-torsional buckling — a beam deflects laterally and twists simultaneously when its compression flange is not adequately braced
• Local buckling — a plate element within the cross-section (a flange or web) buckles locally before the overall member does
• Sidesway buckling — entire bays of a frame sway laterally due to inadequate bracing at the floor or roof level

How Modern Steel Buildings Are Designed to Prevent Buckling

The American Institute of Steel Construction (AISC), in its Specification for Structural Steel Buildings (AISC 360), and the American Society of Civil Engineers’ ASCE 7 (Minimum Design Loads and Associated Criteria for Buildings and Other Structures) together form the backbone of how structural engineers in the United States design steel frames to resist buckling.

The design process begins with establishing the effective length factor (K) for every column. This factor accounts for the boundary conditions at each end of the column—whether it is pinned, fixed, or somewhere in between. A column that is fully fixed at both ends has an effective length of 0.5 times its actual height; one that is pinned at both ends has an effective length equal to its full height; one that is cantilevered has an effective length of 2.0 times its height. The effective length determines the slenderness ratio, which in turn determines the critical buckling load.

Engineers then use the AISC column strength curves, which account for real-world imperfections and residual stresses from the manufacturing process, to determine the design compressive strength of each column. They apply load combinations from ASCE 7—dead load plus live load, dead plus wind, dead plus seismic—and verify that every column’s design strength exceeds the maximum factored demand with appropriate safety factors.

Beyond individual member design, the system-level protections against buckling include:

• Lateral bracing at regular intervals along beams to prevent lateral-torsional buckling of compression flanges
• Moment frames or braced frames that provide lateral stability to the entire structural system
• Diaphragm action from floor and roof decks that distribute lateral loads to vertical elements
• Compact section requirements that prevent local buckling before the member reaches its full plastic capacity
• Connection design that delivers assumed end conditions — a pin-assumed connection must not behave as a rigid frame under load

What Conditions Lead to Buckling in Practice

Even in buildings designed to code, structural buckling can be triggered by conditions that fall outside or push against the envelope of the original design assumptions. Structural engineers and forensic investigators typically examine several categories of contributing factors.

Load path disruption. When a structural element is removed, relocated, or significantly altered—as often occurs during renovations, demolitions, or construction sequencing changes—the loads it was carrying must redistribute to adjacent members. If those members are not designed for the additional demand, buckling can result. This is why construction sequencing plans and temporary shoring designs are treated as engineering documents, not field decisions.

Reduced lateral bracing. During construction, before floor decks are poured and lateral bracing systems are complete, steel frames are at their most vulnerable to buckling. Erection bracing—temporary cross-bracing installed during the construction sequence—is required precisely because a partially erected steel frame lacks the system stiffness of the completed structure. AISC’s Code of Standard Practice for Steel Buildings and Bridges addresses erection responsibilities for exactly this reason.

Fire exposure. Steel loses strength rapidly when heated. At approximately 550–600°C (1,020–1,110°F), structural steel retains only about 60% of its room-temperature yield strength. At 700°C (1,290°F), that drops below 30%. Fire-protection coatings—spray-applied fire-resistive materials, intumescent paints, or concrete encasement—are engineered to keep steel below its critical temperature for a defined duration. When that protection is compromised or absent, fire can cause buckling that appears sudden but has been accumulating thermally.

Foundation movement. Differential settlement—uneven movement of foundation elements—can introduce secondary bending moments and axial load shifts into columns that were not designed for those load combinations. In New York City, where subsurface conditions vary significantly between bedrock outcrops and deeper soft soils, this is a documented factor in structural distress events (NYC Department of Buildings, Technical Policy and Procedure Notice TPPN #10/88).

How the New York City Department of Buildings Responds to Structural Distress

The New York City Department of Buildings (NYC DOB) maintains a 24-hour Emergency Response Unit that responds to structural distress calls. When visible buckling, leaning, or cracking is reported, the response protocol involves several stages, each governed by the NYC Construction Codes (Title 28 of the NYC Administrative Code) and the Mayoral Executive Order framework for emergency operations.

The immediate response sequence typically includes:

• Site evaluation by a DOB inspector, who makes an initial classification of imminent danger vs. non-imminent structural concern
• Issuance of an Emergency Declaration if imminent danger is found, triggering authority to require immediate vacate, shoring, or partial demolition
• Engagement of a Licensed Special Inspector or Registered Design Professional (RDP) to assess the distress in detail
• Mandatory written report by the RDP describing the nature, extent, and cause of distress
• Remediation orders specifying what structural work is required and in what timeframe
• Re-inspection and sign-off by DOB before reoccupancy is permitted

The NYC DOB’s Buildings Information System (BIS) maintains a public record of every emergency declaration, violation, and stop-work order. Under New York law, structural distress incidents that result in injury, collapse, or imminent collapse must also be reported to the NYC Office of Emergency Management (OEM).

What Structural Forensic Investigators Look For

After an emergency is stabilized, forensic structural engineers begin the systematic investigation of why buckling occurred. This process draws on the field of structural failure analysis, a specialty governed by guidelines from the ASCE Forensic Engineering Division and the National Institute of Standards and Technology (NIST), which has published detailed investigation methodologies following major structural failures including the World Trade Center collapses, the Champlain Towers South collapse in Surfside, Florida, and others.

Forensic investigators typically examine:

• Original structural drawings and specifications compared against as-built conditions found in the field
• Material testing — core samples, mill certificates, hardness testing — to verify the steel properties actually installed
• Connection details — whether welds, bolts, and shear plates matched the design intent and were properly inspected
• Construction sequencing records — shop drawings, RFI logs, inspection reports during erection
• Load history — what loads the structure has actually experienced, including any modifications, overloads, or unintended conditions
• Environmental exposure — corrosion, thermal cycling, freeze-thaw effects on connections and embedded elements

Special Inspections: The System Designed to Prevent This

The International Building Code (IBC), adopted in some form by virtually every U.S. jurisdiction including New York and California, requires Special Inspections for structural steel construction. These are continuous or periodic inspections by a licensed Special Inspector—a third party, separate from both the contractor and the building department—who verifies that critical structural work is executed per the approved drawings.

For structural steel, IBC Chapter 17 requires special inspection of:

• High-strength bolt installation — verifying bolt type, installation method, and torque per AISC specification
• Structural welding — continuous inspection of moment connections, column splices, and other critical welds per AWS D1.1 (Structural Welding Code)
• Steel frame erection — verifying that plumb, alignment, and bracing requirements are met at each stage of erection
• Cold-formed steel — verifying installation of light-gauge framing systems per manufacturer and design specifications
• Metal deck installation — verifying deck attachment to supporting structure

In California, the CBC (California Building Code) mirrors IBC Chapter 17 requirements and adds additional special inspection categories specific to seismic design categories. The design professional of record (structural engineer) is responsible for preparing the Statement of Special Inspections, which lists every inspectable item, the inspection type (continuous vs. periodic), and the acceptance criteria.

What This Means for Building Owners and Developers

Structural buckling incidents—whether in New York City or anywhere else—consistently share a common feature in forensic post-mortems: the failure was detectable before it became an emergency. Lateral bowing of a column, unusual cracking at connections, unexpected deflections of beams, or sounds during load events are observable precursors that, when noticed and reported to a structural engineer, can trigger intervention before failure.

Building owners and property managers benefit from understanding the basic indicators that warrant an engineering assessment:

• Visible out-of-plumb or lateral bowing of vertical elements
• Cracking at beam-to-column connections, especially diagonal cracking in concrete-encased steel
• Unexplained deflection or sagging in floor or roof systems
• Doors or windows that no longer operate properly on upper floors (a proxy for differential settlement or frame distortion)
• Rust staining at expansion joints or around embedded plates, which can indicate active corrosion of structural steel
• Any structural modification or renovation work undertaken without verified engineering review of the load effects

Sources and Further Reading

• AISC 360-22: Specification for Structural Steel Buildings — American Institute of Steel Construction (aisc.org)
• ASCE 7-22: Minimum Design Loads and Associated Criteria for Buildings and Other Structures — American Society of Civil Engineers
• AISC Code of Standard Practice for Steel Buildings and Bridges (AISC 303)
• IBC 2021, Chapter 17: Special Inspections and Tests — International Building Code
• NYC Department of Buildings — Emergency Response Unit and Buildings Information System (nyc.gov/buildings)
• NIST Technical Note 1917: Best Practices for Reducing the Potential for Progressive Collapse in Buildings (2017)
• ASCE Forensic Engineering Division — Guidelines for Failure Investigation (asce.org)
• AWS D1.1/D1.1M: Structural Welding Code — Steel — American Welding Society