24 Crimscott Street

The primary challenge was related to using mass timber for the nine-storey building and exposing as much of this timber as possible throughout the leasable space.

This introduces significant challenges for demonstrating fire safety where any timber is exposed as the effects of this are not captured in current guidance documents, yet exposed timber impacts the fire dynamics, severity, and duration.

A new framework developed by the Structural Timber Association allowed the team to identify appropriate methods for demonstrating fire safety in tall timber buildings, such as 24 Crimscott Street, and demonstrate their compliance with the building regulations with respect to internal fire spread.

The approach is performance-based and includes the use of fire engineering analysis tools that capture the impact of exposed timber, such as fire modelling, structural fire engineering, and fire risk assessments.

Following the tragic events of Grenfell, Regulation 7 was introduced in the UK, which bans the use of combustible material in the external wall of a building. This includes mass timber. The proposed glazing system for 24 Crimscott Street incorporates timber mullions.

A careful review of the cladding ban was undertaken with respect to glazing. Since the ban was introduced, it has been clarified that curtain wall is considered to be window frame and glass, which is exempt from the ban. This means the timber mullions are not banned from being used.

However, the performance of the building must still be demonstrated with respect to Regulation B4(1). The detailing of the curtain wall at each floor will continue to be reviewed as the design progresses to ensure that in the event of a fire, compartmentation is maintained and there is no opportunity for vertical flame spread up the exterior of the building.

Following the cladding ban for combustible materials in the UK, the City of London has developed “Policy D12,” which introduces additional guidance to follow more stringent regulation governing the design of 24 Crimscott Street. This also requires adherence to Policy D5, which is for Inclusive Design and ensures all occupants have a dignified evacuation procedure.

Review of Policy D12 clarified that most of the design requirements overlap with those of the Approved Document B, which is the prevalent guidance used in the design of buildings for fire safety in the UK.

The presence of Policy D12 does not add more stringent design requirements but simply regiments the implementation of AD-B and requests a “fire statement” in the form of a standalone document as early as at the time of planning application.

Entuitive developed this fire statement early in the design process to assist with building approvals and show compliance with both Policy D12 and D5.