Fire Strategy Design Considerations for Complex Buildings

What Makes a Building Complex from a Fire Safety Perspective?

In fire engineering terms, a complex building is one whose design, configuration, or occupancy falls outside the assumptions built into standard prescriptive fire safety codes. This typically includes buildings with atria, underground levels, or deep floor plates; tall buildings where evacuation times are extended; mixed-use developments where different occupancy types interact; buildings with unconventional structural forms; and any building where a simple application of Approved Document B or BS 9991 would produce either an unworkable result or one that does not reflect the actual fire safety performance of the building.

Atria and Interconnected Spaces

Atria — open vertical spaces connecting two or more floors — are one of the most common fire engineering challenges in complex buildings. An unprotected atrium creates a direct path for smoke from a fire on one floor to affect all connected floors. Managing this risk requires a dedicated atrium smoke control system, typically comprising smoke extract fans at high level, make-up air provision at low level, and automatic smoke curtains or fire-rated screens to limit the areas from which smoke can enter the atrium.

The design of atrium smoke control systems requires CFD fire modelling to demonstrate that adequate smoke-free height is maintained over escape routes for long enough to allow safe evacuation. This is a performance-based analysis that cannot be completed using prescriptive guidance alone.

Buildings with Deep Basements and Underground Levels

Basements and underground levels present fire safety challenges that are fundamentally different from above-ground floors. Escape is upward against the natural direction of smoke movement. Natural ventilation for smoke control is absent or limited. Firefighter access is more difficult and time-consuming. For buildings with significant basement occupancy — car parks, retail units, plant rooms, or below-ground commercial floors — the fire strategy must address these challenges specifically, typically with mechanical smoke extract systems, enhanced detection, and carefully designed escape routes.

Tall Buildings

Tall buildings — particularly those over 18 metres classified as higher-risk under the Building Safety Act 2022 — require fire strategies that address extended evacuation times, the behaviour of fire and smoke over many floors, the management of a stay-put strategy in buildings where residents may be many floors above a fire, and the specific firefighting facilities required for fire service operations in tall buildings.

For the tallest buildings — those over 30 or 40 storeys — evacuation modelling is increasingly required to demonstrate that the escape strategy is workable, particularly for scenarios where simultaneous evacuation is required. The interaction between the elevator systems, the fire safety strategy, and the evacuation strategy also requires careful coordination.

Performance-Based Design for Complex Buildings

Complex buildings almost always require performance-based fire engineering — the demonstration through engineering analysis that the building achieves an acceptable level of fire safety, even where it does not follow prescriptive code provisions. The tools used include CFD fire and smoke modelling, evacuation modelling, structural fire engineering analysis, and probabilistic risk assessment.

For Gateway 2 submissions for complex higher-risk buildings, the BSR expects performance-based analyses to be robust, well-documented, and — where the building is particularly complex or the performance-based approach is novel — independently peer reviewed by a second chartered fire engineer.