Case Studies: Building Collapses

Ronan Point was a 22-storey prefabricated large panel system (PLPS) apartment tower in Canning Town, East London. On the morning of May 16, 1968, a gas explosion followed a cooker failure in a flat on the 18th floor. The explosion pressure was modest, below 70 kPa (10 psi), far below what structural elements should resist. But the explosion blew out the load-bearing external panel walls on the 18th floor. Without those walls, the floors above had no support at the corner of the building. They collapsed. And as they fell, each successive floor below was unable to carry the impact load from the floors above. The corner of the building, from the 18th floor up, fell to the ground. Four people were killed.

The investigation found that the PLPS connections between panels relied primarily on gravity and friction, with minimal mechanical or reinforced continuity. The wall-to-floor connections had been designed to carry vertical load downward but had almost no capacity to transfer horizontal load or to resist being pushed out of plane. Once one panel was displaced, the loads it had been carrying had nowhere to go. The collapse was not caused by the explosion being unusually large. It was caused by the structure having no alternate load path around a failed panel, no robustness against a local removal.

In 1986, Ronan Point was demolished. During demolition it was discovered that many of the panel connections had not been properly made during original construction: incomplete grout filling left connections weaker than even the design intended. The building had been more vulnerable than its drawings suggested from the day it was built.

L'Ambiance Plaza in Bridgeport, Connecticut was a 16-storey residential building under construction using the lift-slab technique. In lift-slab construction, floor slabs are cast at ground level, stacked with bond-breaker between them, and jacked up along steel columns to their final height. The method eliminates formwork at height and reduces construction time relative to traditional poured-in-place construction.

On April 23, 1987, during lifting operations on the west building, a shear failure occurred at a lifting collar connecting a slab to a steel column. The collar and its bearing plate punched through the slab. With one slab unsupported, the entire stack of slabs being held by the lifting jacks above lost stability and fell. Twenty-eight construction workers were killed.

The OSHA and National Bureau of Standards investigation found that the lifting hardware did not have adequate shear capacity for the load. The original structural design required the slab-to-column connection to carry shear forces during lifting that exceeded the design capacity of the collar assembly. The failure was not a random event or a materials defect. It was a predictable consequence of the system design once the actual loads were correctly calculated.

The case prompted the Occupational Safety and Health Administration to adopt specific regulations governing lift-slab construction, including requirements for independent engineering review of jacking procedures and load calculations. It also contributed to a broader reassessment of construction-phase structural analysis as a discipline separate from, but as important as, the design of the completed structure. The temporary condition during construction can be more critical than any permanent loading case.

The Rana Plaza building in Savar, Bangladesh housed five garment factories on its upper floors. The building was originally designed as a commercial structure for shops and offices. It was expanded from four to eight stories over several years without planning permission. The upper floors contained heavy industrial sewing machines and, critically, large diesel generators on the roof, which were started daily because of power outages, transmitting vibration and significant additional dead load to columns that had never been designed for them.

On April 23, 2013, the morning before the collapse, large cracks appeared in several columns. Engineers from the Bangladesh Institute of Engineers inspected the building and recommended immediate evacuation. Banks and shops on the lower floors closed. Factory managers were told, and instructed their workers to return the following morning to meet production targets. On April 24, 2013, the building collapsed. At least 1,134 people were killed and over 2,500 were injured in what became the deadliest structural failure in history outside of wartime and natural disasters.

The structural investigation found that the columns were inadequate for the actual loads being imposed. Reinforcement was below specification, concrete compressive strength was below design, and column dimensions in the upper floors were reduced without engineering justification. The structural deficiencies were severe enough that investigators concluded the building could not have met its original design requirements, let alone support the additional floors and industrial loads it was actually carrying. The generator vibration was a contributing dynamic load on columns already close to their static limit.

The Champlain Towers South was a 12-storey beachfront condominium in Surfside, Florida, completed in 1981. In October 2018, Morabito Consultants prepared a structural field survey report for the condominium association. The report found significant deterioration: cracking and spalling of concrete columns in the pool deck and parking garage, failed waterproofing on the pool deck slab, and corroding rebar exposed through spalled concrete. It recommended immediate repair of the pool deck waterproofing and structural restoration of the deteriorated columns and slab.

Repair work had not been completed when, at 1:22 am on June 24, 2021, a partial collapse initiated in the area of the pool deck on the north side of the tower. Within 12 seconds the main tower followed. Ninety-eight people were killed. NIST opened a full technical investigation that is among the most detailed ongoing forensic structural investigations of a residential building.

The NIST investigation focused on the pool deck slab and the columns below it. The post-tensioned pool deck slab was over the parking structure and supported the pool deck surface. Decades of saltwater infiltration from the beachside environment had carbonated the concrete and initiated corrosion of the reinforcing steel. In columns below grade, corrosion had reduced the reinforcement cross-section. NIST's analysis found that the collapse initiated in the pool deck area, consistent with punching-shear failure of the slab, and propagated upward through the tower as the column capacity at grade level was compromised.

Despite different mechanisms, the forensic investigations of these four collapses followed a consistent methodology. Physical evidence from the debris was mapped, photographed, and sampled. Design drawings were compared with what was actually built. Load calculations used measured material properties rather than assumed design values. Each hypothesis was tested explicitly against alternative explanations.

• Concrete core sampling: at Rana Plaza and Surfside, cores from surviving similar sections established actual concrete compressive strength. Both were below specification. At Ronan Point during demolition, mortar sample analysis showed gaps in the original connections.
• Rebar cross-section measurement: at Surfside, reinforcing steel samples recovered from the collapse debris showed measurable cross-section loss from corrosion. The degree of loss was consistent with decades of progressive deterioration in a marine environment, not an overload on the night of collapse.
• Load path and capacity calculations: in each case the investigators calculated the capacity of the critical element using measured material properties and compared it to the demand. In all four cases the capacity was below demand, establishing a structural deficiency as distinct from an external overload event.
• Documentary review: at Rana Plaza, inspection reports from the morning before the collapse were central to both the structural and criminal investigations. At Surfside, the 2018 Morabito report established that the deficiency was known and documented before the collapse.

Building collapses in the forensic engineering literature cluster around a small number of recurring themes.

• Construction method risk: L'Ambiance Plaza showed that construction-phase structural conditions can be more critical than any permanent state. Construction-phase analysis is now a professional requirement on projects using non-standard erection methods.
• Robustness and continuity: Ronan Point established that a structure must provide alternate load paths around failed elements. Minimum tie requirements in building codes are the direct response.
• Unauthorised modification: Rana Plaza's illegal upper floors and generator installations imposed loads the columns could not sustain. Structural modification without engineering assessment is consistently a finding in building collapse investigations worldwide.
• Deferred maintenance: Surfside shows how known, documented deterioration can reach a critical state if repair is repeatedly postponed. Florida's 2022 legislation requiring regular structural inspections and mandatory reserve funding for repairs is a direct consequence.