The Viadux project is one of the most ambitious skyscrapers ever built in Manchester. Now in occupation, the Salboy-led development has opened new doors to what’s possible when it comes to the merging of heritage assets and unmistakably modern design.
With the second phase of the project now approved at planning, an even more ambitious tower will begin to take shape above the Castlefield viaduct. Bearing many similarities to Viadux 1, Viadux 2B will raise the stakes even further, going above and beyond the 40 storeys of its predecessor to 76 storeys.
It will be one of the UK’s tallest buildings outside of London and will offer apartments, branded residences and a luxury hotel. This is a bold, high-profile project, but behind the architecture and the aesthetic grandeur is an equally ambitious piece of engineering.
Building tall on a sensitive site
As with Viadux 1, the new tower sits directly above a series of Victorian brick viaduct arches. This is somewhat of a double-edged sword. On one hand, the arches are a defining architectural feature that is central to the wow-factor of the project. On the other hand, they’re a considerable structural constraint that requires a shrewd and considered engineering design.
Like Viadux 1, preserving these arches and integrating them meaningfully into the public realm is central to the success of Viadux 2b. An added challenge in this phase is doing all that whilst accommodating a relatively slender tower.
To preserve the arches and limit the number of openings provided through them, we designed a dramatic transfer structure at the base. This allows the loads from the 70+ storeys above to be carried down through just a few carefully placed mega-columns that thread through openings in the arch crowns. These openings are strategically located to avoid damaging the structural integrity of the masonry.
The solution is both structurally efficient and spatially elegant. The exposed trusses are a key part of Viadux 2B’s unique aesthetic and are central to its visual identity.
Complex foundations and sensitive ground conditions
To give adequate support to a structure of this height in such a constrained location, we had to think carefully about the engineering required for the substructure. Beneath the arches lie heavy masonry piers. Down further still, there was a challenging geotechnical profile just below ground level with significant variability in the underlying strata.
We worked closely with geotechnical specialists to design rotary cored piles with substantial pile caps and core bases, modelled iteratively to limit ground movement. The key challenge was ensuring that any settlement from foundation loading would not lead to cracking or differential movement in the retained viaduct structure. We used sophisticated soil-structure interaction modelling to simulate this behaviour and calibrate the stiffness of the piles.
Dynamic performance and lateral stability
At 76 storeys, the building’s dynamic response to wind was a central concern, not just for serviceability, but also for the comfort of the residents. We collaborated with three separate wind and dynamics specialists to understand and mitigate these effects.
We worked with Simpson Haugh to improve aerodynamic performance and reduce vortex shedding. A large part of this was achieved through the addition of chamfered corners and a subtly ribbed façade. This design decision significantly lowered wind loads, allowing for a lighter core and slimmer slabs. This ultimately reduced the level of concrete required and hence also reduced embodied carbon and construction cost.
The core itself presented a lateral stability challenge. Rectangular in plan and slender in one direction, it lacked stiffness under torsion and lateral load. We overcame this by integrating wing walls into the structural system, effectively widening the core’s resistance path. At lower levels, where large penetrations were needed for hotel amenity spaces, the team worked collaboratively to place the openings conveniently while maintaining structural efficiency.
Transfer trusses and load management
From levels 3 to 5, a series of exposed concrete encased steel trusses manage the transition between the upper residential/hotel floors and the column grid required to preserve the viaduct below. These trusses span large distances and carry enormous loads, transferring an entire tower’s worth of weight.
The location and geometry of the trusses were driven by four competing demands: structural efficiency, robustness, aesthetic integration, and architectural layout. It was our close collaboration with the wider project team that empowered us to achieve a solution that satisfied all demands, optimising steel tonnage while delivering a visually striking structural expression at the podium level.
The use of composite columns allows for a “top-down” construction, where once the steel within the core members has been erected, construction of the floors above can start. The truss members are then encased with concrete for them to achieve their full capacity prior to experiencing their full load.
Optimised slabs, reduced carbon
Material efficiency was a running theme across the Viadux 2b project. Even the floor slabs were carefully optimised through multiple design iterations. Thinner slabs reduce self-weight, which in turn reduces the loads on foundations and the amount of concrete required in the core and columns. That cascade effect adds up quickly over 76 storeys, reducing cost, carbon and construction time.
Iterative, collaborative and considered
The Viadux 2B project brings together multiple strands of expertise, including superstructure design, geotechnical engineering, dynamics and advanced analysis. Through the coordination of all of the above, we’ve been able to meet an incredibly complex brief.
If Viadux 1 demonstrated how to work cleverly around heritage constraints, Viadux 2B shows what’s possible when that approach is scaled up.
