Transforming Bristol Street, Birmingham: Post-Tensioning in Action

Interspan was proud to deliver the post-tensioning solution for a transformative development on Boulevard – Bristol Street, Birmingham, one of several significant projects aimed at revitalising the city’s Southside area.

Bespoke Construction, as the main contractor, and Clark Mulhern Construction, as the frame contractor, will construct 278 residential one- and two-bedroom apartments with communal amenities, including podium gardens and parking, a gymnasium, co-working spaces, and resident lounges, creating a gateway to beautifully designed city living.

Interspan’s Post-Tensioning Contribution

Clark Mulhern engaged Interspan to explore whether a post-tensioning solution could deliver material savings. Interspan proposed an efficient design that significantly reduced the volume of concrete, reinforcement, and associated materials. These material reductions, along with programme efficiencies, made the post-tensioned option the preferred solution. As a result, Clark Mulhern secured the concrete frame package and appointed Interspan to deliver the full design-and-build scope for the post-tensioned slabs.

Phase 1A: Thinner Slabs, Greater Efficiency

– Original design: 260mm reinforced concrete slab
– Interspan’s solution: 225mm post-tensioned slab
– Outcome: Saved 438m³ of concrete across two linked blocks with 6 and 15 suspended floors

Phase 1B: A Leaner Transfer Slab

– Original design: 650mm thick RC transfer slab below a five-storey steel frame with undercroft parking and a 300mm RC podium
– Interspan’s solution: Reduced slab thickness by 50mm
– Outcome: Saved 108m³ of concrete

Post-Tensioning Benefits: Speed, Strength, Sustainability

Interspan’s design integrated seamlessly with slip-formed cores and precast columns, enabling faster construction cycles. Post-tensioned slabs, which are self-supporting once final stressing is completed at 25 MPa compressive strength, allow earlier stripping of formwork, which is critical for maintaining momentum on multi-level construction projects.

The Bristol Street project realised a 285-tonne reduction in embodied carbon across both phases, highlighting the environmental value of well-executed post-tensioning

How Post-Tensioning Supports Sustainable Construction

1. Material Efficiency
   – Thinner Slabs = Less concrete = Less cement = Lower CO₂
   – Reduced Reinforcement: Less steel required due to compressive forces provided by tendons
   – Thermal Mass: Concrete’s high density and moderate conductivity improve energy performance over a building’s life
2. Extended Durability
   – Fewer cracks and deflections mean longer service life, lower maintenance, and less material-intensive repair work
3. Improved Structural Efficiency
   – Optimised slab thickness improves load-bearing efficiency, reducing overall structural weight and material usage
4. Faster, Cleaner Construction
   – Faster strike times = faster build = less energy consumed onsite
   – Lighter loads reduce transport requirements, further lowering carbon emissions

The Bristol Street development is a strong example of how Interspan’s post-tensioning expertise delivers on performance, cost efficiency, and sustainability.

By partnering early with structural and construction teams without compromising durability or speed, Interspan helps unlock smarter, lower-carbon building outcomes.