Ask the Expert: How Past Punching Shear Failures Inform Safer Concrete Design

In this 'Ask The Expert' we sat down with Harriet Cotton, Research & Development Engineer for Forming & Reinforcing at Leviat, to find out more about why punching shear remains one of the most critical considerations in reinforced concrete design. Our conversation includes how clearer guidance, modern reinforcement systems and evolving standards are helping engineers reduce risk and design with greater confidence.

Punching shear failures in major buildings in Seoul, Paris and the UK have demonstrated how underestimated shear forces, inadequate detailing, or long-term deterioration can trigger disproportionate collapse. While punching shear is a localised failure mechanism at slab column connections in flat slab construction, its sudden nature and limited warning make it particularly dangerous, as the structure may have limited ability to absorb the failure and redistribute the load elsewhere.

These historic failures continue to shape contemporary design and assessment practice. With the introduction of the second generation of Eurocode 2, punching shear design is moving towards a clearer, mechanics-based framework founded on Critical Shear Crack Theory (CSCT), providing engineers with a more transparent means of verification that better reflects real slab behaviour. At the same time, the UK’s extensive stock of existing concrete flat slab buildings, many of which are subject to increased loading, change of use or material degradation, brings renewed focus on punching shear assessment as a critical aspect of structural safety.

Why does punching shear remain a critical risk?

Punching shear failure differs fundamentally from many other failure mechanisms considered in reinforced concrete design. Unlike flexural (bending) failure, which is typically ductile and accompanied by visible cracking and deflection, punching shear can occur abruptly once critical cracking develops around a column. When this happens, load transfer is lost over a small area, but the consequences can extend far beyond the immediate connection.

The Sampoong Department Store collapse in Seoul remains one of the most cited examples. Design alterations, reduced slab thickness and increased loading combined to overwhelm slab column connections that lacked sufficient robustness. At Charles de Gaulle Airport’s Terminal 2E in Paris, complex architectural geometry and structural form, coupled with insufficient understanding of load redistribution and long-term behaviour, contributed to collapse. In Wolverhampton, the Pipers Row Car Park failure highlighted how local deterioration of the concrete at the slab-column connection alone, without exceptional loading, can critically reduce punching shear capacity over time.

What these different cases and building types demonstrate is that punching shear failure is rarely caused by a single oversight. It is typically the result of inadequate design, construction deficiencies, changes in use, or long-term deterioration.

What are the modern demands on flat slab construction?

Today’s design trends have intensified the demands placed on slab column connections. Thinner slabs, larger column spacings and heavier floor loads are commonplace, driven by architectural flexibility, sustainability objectives and economic pressures. Post-tensioned flat slabs, which allow reduced slab thickness, add further complexity to punching shear design and detailing around columns.

For new construction, these demands require careful consideration of punching shear from the earliest stages of design. However, the greater challenge increasingly lies with existing buildings. Many UK flat slab structures were designed to earlier standards, with lower imposed loads and limited consideration of future adaptation. Today, these buildings are often expected to accommodate additional plant, heavier occupancy loads or structural alterations.

In parallel, ageing mechanisms such as carbonation-induced corrosion, loss of cover, and freeze–thaw damage can significantly reduce effective capacity. The Pipers Row collapse demonstrates that punching shear failure does not necessarily require high loading as deterioration alone may be sufficient to initiate collapse.

What is the significance of second-generation Eurocode 2?

The introduction of the second generation of Eurocode 2 represents a fundamental shift in punching shear design philosophy. Earlier approaches relied largely on empirical relationships calibrated against test data. While effective for standard scenarios, these methods offered limited transparency and could be difficult to apply confidently to unusual (non-standard) geometries or assessment of existing structures.¹

Critical Shear Crack Theory (CSCT) is a mechanics-based model that more closely reflects observed slab behaviour in testing. This approach is expected to reduce uncertainty in punching shear assessment and allow engineers to design with greater precision, rather than relying solely on conservative empirical rules. The revised Eurocode 2 adopts control perimeters that better reflect punching shear failure and introduces parameters linked to the type of reinforcement provided, reflecting contemporary construction practice.  Although the resulting checks remain complex, the resistance verification formulae capture more of the underlying physical behaviour, supporting more robust assessment of non-standard designs and material properties.

For structural engineers and architects, this shift has two important implications. Firstly, the second generation of Eurocode allows punching shear design to be informed by detailed finite element analysis, creating closer alignment between overall slab behaviour and local verification. It also introduces an option for iterative verification at the perimeter of reinforcement rather than a single-step approach, offering additional flexibility as engineers become more familiar with its application. Secondly, the framework is better suited to assessment work than the first generation, as it allows engineers to focus on how an existing slab behaves, rather than relying on over-simplified rules.

What are the implications for assessment and strengthening?

The assessment of existing flat slab buildings requires a careful and systematic approach. Historic failures underline the danger of relying on original drawings or assumptions without considering changes in loading, material condition and structural behaviour.

Modern assessment practice increasingly combines detailed analysis with targeted investigation to establish realistic punching shear demand and capacity. Where construction deficiencies are identified, strengthening may be required to restore acceptable safety margins. Prefabricated punching shear reinforcement systems and strengthening techniques have been extensively tested and are supported by guidance and certification, offering verified safety performance when correctly designed and installed.

However, the broader lesson lies in approach and design. Effective strengthening relies on a sound understanding of punching shear behaviour, appropriate modelling assumptions, and clear detailing that considers constructability and load transfer. It is not simply the addition of reinforcement, but applying it intelligently within a robust engineering framework.

What digital tools are available to make specification easier?

With punching shear verification becoming more sophisticated under the revised Eurocode framework, digital tools play an increasingly important role. This is particularly relevant in modern projects where slabs vary in geometry, loading and detailing and are therefore difficult to manage manually. The more complex the building, and the greater the number of slab–column connections, the more important it becomes to use software to manage these calculations consistently and reliably.

Specialist design tools address this challenge by automating verification, producing clear documentation, and allowing reinforcement layouts to be optimised. They have become essential in modern practice, where project schedules demand fast, transparent, and code-compliant design.

Our design program simplifies the specification and ordering of a Shearfix System. This easy-to-use program allows the optimum design to be determined and generates a printable calculation sheet, a DXF file and a parts list of the specified layout. The program allows analysis to BS EN 1992 (Eurocode 2). Find out more here.

Improving Practice

Punching shear failures are a sobering reminder that structural safety depends on understanding behaviour, not simply satisfying minimum rules. The evolution of Eurocode 2 reflects lessons learned from past collapses, offering engineers a more rational and transparent framework for both design and assessment.

For anyone working with concrete structures, whether designing new buildings or assessing existing ones, punching shear deserves careful attention at every stage of design and construction. By applying the insights gained from historic failures, using proven reinforcement systems and intuitive digital tools, and maintaining a rigorous assessment mindset, the industry can significantly reduce the risk of sudden, disproportionate collapse in flat slab construction.

¹Muttoni, A., Simoes, J.T., Faria, D.M.V., Fernandez, M. (2022) A mechanical approach for the punching shear provisions in the second generation of Eurocode 2, Homigon y Acero

Download our Leviat White Paper: Considerations for Building Safer Structures with Proven Punching Shear Solutions

This white paper sets out why punching shear remains one of the most critical considerations in reinforced concrete design, and how clearer guidance, modern reinforcement systems and evolving standards are helping engineers reduce risk and design with greater confidence.