Vitrum Building Post-Tensioned Slab Optimisation

Interspan worked with John F Hunt during the tender phase to optimise the structural design for the Vitrum Building at St John’s Innovation Park, Cambridge. The 400 mm slab scheme was redesigned as 350 mm post-tensioned slabs across Levels 1 to 5, saving 530 m³ of concrete and reducing embodied carbon by an estimated 210 tCO₂e.

Project Overview

The Vitrum Building at St John’s Innovation Park in Cambridge is a purpose-designed life science research and development facility. Its structural floors needed to accommodate demanding laboratory loading, stringent vibration criteria and architectural requirements for high-quality fair-faced concrete soffits.

McLaughlin & Harvey was appointed as main contractor, with John F Hunt delivering the concrete frame. Interspan was engaged by John F Hunt during the tender phase to review and optimise the structural floor design before construction. Consulting Engineer – Thornston Tomasetti.

Tender-Stage Design Optimisation

The scheme comprised 400 mm thick reinforced concrete slabs spanning a 9.9 m by 6.6 m structural grid, with reinforced concrete beams introduced in selected areas to support the facade and other structural elements.

Detailed modelling in RAM Concept demonstrated that the floor system could be redesigned as 350 mm thick post-tensioned slabs across Levels 1 to 5.

The 50 mm reduction saved approximately 530 m³ of concrete compared with the original scheme while retaining the required structural performance.

Designed for Demanding Laboratory Loads

The laboratory and research areas were designed for significant imposed loads of 5.0 kN/m² plus a further 1.0 kN/m² allowance. Combining these loads with the reduced slab depth required careful assessment of strength, serviceability and local behaviour throughout the floor plates.

Interspan incorporated shear reinforcement and anti-deflection measures along critical zones. Particular attention was given to punching shear around the supporting columns to ensure the optimised slab design maintained the required capacity and robustness.

Controlling Vibration and Deflection

Life science laboratories can contain sensitive equipment and activities that require tighter vibration control than conventional commercial spaces. The Vitrum Building therefore had low response-factor requirements that needed to be considered alongside strength and deflection performance.

The post-tensioned solution allowed tendon profiles, reinforcement and slab behaviour to be modelled in detail. This supported an efficient floor depth while addressing both the laboratory vibration criteria and the building’s long-term serviceability requirements.

Coordinating the Fourth-Floor Transfer Structure

Additional coordination was required at the fourth floor, where the post-tensioned slabs interfaced with reinforced concrete transfer beams carrying loads from the roof slab.

Interspan coordinated the slab and transfer-beam design to establish a clear load path and resolve the interface between the post-tensioned and conventionally reinforced elements. This detailed coordination reduced design risk before the works reached site.

Fair-Faced Soffits with Reduced Cracking

Post-tensioning was well suited to the project’s fair-faced soffit requirements. The pre-compression introduced into the slabs helps control tensile stresses and significantly reduces cracking when compared with a conventional reinforced concrete in-situ slab solution.

The resulting floor system combined structural efficiency with the high-quality exposed concrete finish required for the building, while the reduced slab depth lowered concrete consumption and overall structural self-weight.

35 Suspended Slab Pours Delivered in 16 Weeks

The programme was fast-paced, with 35 suspended post-tensioned slab pours delivered within 16 weeks. This outcome reflected close coordination and strong collaboration between the Interspan and John F Hunt site teams. The design optimisation delivered both material and programme benefits.

The reduction in slab thickness saved approximately 530 m³ of concrete and was calculated to reduce embodied carbon by an estimated 210 tCO₂e compared with the original scheme. The improvement also supported the project’s targeted SCORS A++ structural carbon performance.

Interspan Scope

• Tender-stage review and value engineering of the structural slab design
• Detailed design of 350 mm post-tensioned slabs across Levels 1 to 5
• RAM Concept modelling of the post-tensioned floor plates
• Punching shear assessment and design of critical shear reinforcement
• Deflection control and laboratory vibration performance assessment
• Coordination of post-tensioned slabs with reinforced concrete transfer beams and roof loads
• Design and delivery support for 35 suspended slab pours completed