Chapter 9: Calculated HeightsStructural competency in higher-risk buildings
Cundall's Alex Carter reflects on structural safety, competence and accountabilityImage: 100 Broad Street © Howells
The response from the industry to the tragic events at Grenfell are still emerging. The recommendation from Phase 2 of the Grenfell Tower Inquiry is currently being adopted with changes such as the Building Safety Regulator transitioning away from the Health and Safety Executive, and a review of the definition of Higher-Risk Building (HRB) being visibly progressed. The latest data circulated by the Building Safety Regulator indicates that Gateway 2 decision periods for new build HRBs are reducing to 12 to 14 weeks and Building Assessment Certificates (BACs) are now being issued with reassuring regularity.
The competencies required for structural engineers involved in design, construction and certification of HRBs have been defined in detail over the past three years. Initially by the Engineering Council and more recently with the Institution of Structural Engineers (IStructE) and Institution of Civil Engineers (ICE) launching the HRB registration scheme for Structural Engineers.
The IStructE/ICE assessment covers 21 sub-competencies in five areas which collectively focus on.
Understanding technical issues around HRB design loading
Ensuring compliance with Building Regulations
Identifying and quantifying structural risks
Communicating the above to building occupants, building operators, the BSR and with the wider industry.
Since 2017, 15 higher-risk buildings have been evacuated due to structural concerns, affecting over 5500 residents across England. This provides a clear demonstration of impact of structural engineering on building occupants and the relevance of the assessed competencies.
HRB structural designers now hold the accountability and responsibility to articulate and justify how their work is achieving compliance with the functional requirements of the Building Regulations. This is significant progression, although it has been hard for
A successful Building Assessment Certificate approval is reliant on a competent structural engineer engaging with PAPs and the BSR and thus building occupants to articulate the status of ongoing structural safety."
the industry to calibrate the approach expected, the Construction Leadership Council guidance issued over the past year has distilled this down into a three-step approach:
Identify – every structural aspect of the project that requires compliance with Building Regulations
Clarify – which standard or approach will be used to demonstrate compliance, with an explanation of how it is appropriate
Justify – how the functional requirements have been met with a clear and comprehensive narrative.
When viewed in this context, the requirements are neither unreasonable, nor obstructive, but they are driving a major change in procurement with main contractors and key supply chain members intertwined in the submission process.
The 100 Broad Street project in Birmingham stands out for its innovative configuration, prioritising structural efficiency to ensure financial viability in a regional city context. The client embraced previous Cundall research on structural efficiency, influencing the building’s evolution from a traditional shoulder-and-tower form to a fluid, stepped design. The central, square core acts as a robust box, enabling efficient MEP distribution alongside longer corridors at lower levels, while also providing effective resistance to torsional forces
caused by the building’s stepped elevations. Wind tunnel testing was crucial in validating the applied wind loads on the complex geometry and the building’s dynamic response. Ultimately, this case illustrates the importance of a competent structural engineer in both selecting alternative loading approaches and clearly communicating their decisions.
While the 100 Broad Street case study highlights the technical choices made by competent HRB engineers, the IStructE/ICE process assesses a much wider range of competencies.
The Building Safety Act is applicable to the existing stock of circa 12,500 buildings in England and, arguably, this is where the largest risks to building occupants in England is currently located.
The Building Assessment Certificate (BAC) regime for existing occupied HRBs has developed significantly over the past 18 months. Engagement between Principal Accountable Persons (PAPs) and the Building Safety Regulator (BSR) after the initial application stage has highlighted how essential it is to fully understand the risks associated with each building. The safety case submission should provide confidence that the Accountable Persons (APs), those who own the building or have legal obligation to repair the common parts, have identified the building's reasonably foreseeable fire and structural risks and are managing and controlling them.
Anecdotally, many initial BAC rejections by the BSR related to a failure to satisfactorily
demonstrate an understanding of structural safety risks. Whilst an evidence-led approach is important, proportionality governs the response and mitigations applied. Although a thorough understanding of structural risks is required, the intent is to consider key risk areas like:
Structural collapse of residential unit
Multiple-floor collapse
Whole-building collapse
Balcones or attached components
Undermining or foundation failure
Aggressive conditions
Accidental incidents such as gas explosion or impact damage
Scenarios linked to how the building was built
Late in 2025, the BSR issued correspondence to all PAPs specifically identifying the significant structural risks due to historically-designed transfer slabs. This provides a recent real-world illustration of the potential multiple-floor collapse structural risks that apply to occupied HRBs and that competent structural engineers are required to engage with, assess and support ongoing management.
In the past 18 months, Cundall has assessed structural building risks on over 100 buildings up to 50 storeys tall and constructed between the 1960s and early 2020s. The majority use in-situ concrete frames for the primary structure but the work has included multiple structural steel, lightweight loadbearing steel frames and volumetric modular frames. Whilst we entered into the work anticipating that we would find very few structural risks that required active mitigations, we’ve encountered risks of this type in a notable percentage of those reviewed. There are several reoccurring themes that are observed as hindering the robust assessment of structural risks in existing HRBs.
A lack of key structural record drawing in building Health and Safety Documentation
A lack of primary evidence to demonstrate that the structural frame achieves the required fire resistance period
A lack of information from the original designer articulating the structural system and it’s notable features (such as transfer systems).
The result of the above is that many building owners do not have the data available to fully understand their building and the associated structural risks.
A successful BAC approval is reliant on a competent structural engineer engaging with PAPs and the BSR and thus building occupants to articulate the status of ongoing structural safety. Typically, the expertise of using appropriate risk frameworks and tools, such as graphical risk matrices, has been on the periphery of structural competence. Similarly, the rigour of articulating and justifying structural design approaches was part of the design process but often lost in need to produce quick designs efficiently. It is argued that the regulatory Gateway and BAC processes have quite rightly reset the expectations and focus on these aspects of structural design for new build while it will remain a challenge to the industry on existing buildings.
This brings us back to the 21 sub-competencies that are assessed in the Chartered Structural Engineer (HRB) registration, whilst they have a broader reach and a different focus than other competency benchmarks. They provide a clear route for the structural engineering community to demonstrate that it is actively participating in the change required to avoid a repeat of the Grenfell disaster and the subsequent structural issues in HRBs that have since emerged.
www.cundall.com
