Post-tensioning provides the right solution for complex cancer centre

Radu Trancau from CCL discusses the post-tensioned slabs installed at the new Clatterbridge Cancer Centre in Liverpool

Post-tensioning concrete slabs were used on the new Clatterbridge Cancer Centre Liverpool

The Clatterbridge Cancer Centre is one of the UK’s leading oncology services.

And the latest addition to its estate is the custom-designed Clatterbridge Cancer Centre Liverpool.

Designed by architectural practice, BDP, and engineered by AECOM; the building was constructed by Laing O’Rourke and the main contractor has worked collaboratively with CCL and other delivery partners to develop a detailed design for the post-tensioned (PT) slabs that answers the very-specific loading, deflection, and vibration criteria of the design.

Slab strategy

Occupying a sloping site, the new cancer centre is an unusual triangular-shaped building with a unitised glazed façade and a stepped terraced roof.

At the front of the building there is a curved cantilevered section, supported by huge raking columns.

The viability of different slab strategies was explored at the early concept stages of the project. And this analysis led to the decision to use RC slabs for the basement and ground floor levels.

The remaining storeys, including the stepped cover slab, have all been designed and installed with PT.

Vibration and loading

The MRI equipment that will be installed on level zero is extremely sensitive and there is a pharmacy located on the top floor where chemotherapy infusions and other treatments will be custom prepared to precise requirements.

The equipment used here is also very sensitive.

To provide sufficient concrete mass for the required vibration resistance, the depth of the slab was increased and the vibration control provided by the slab thickness was further enhanced by stiffening in the critical areas with a set of deep orthogonal beams.

Radu Trancau explains why the system was the best fit for the challenging development

For the top-floor pharmacy, the depth across the whole slab is 525mm, as compared to the 325mm of the building’s standard PT slabs.

Additional tendons and anchors were required for these areas to enhance the slab’s tensile strength, but the use of PT avoided the onerous level of steel reinforcement that would have been required to support a conventional slab of this mass.

On the roof, where there is complex plant, the slab design had to take account of heavy loading, with varying loads in different locations, and the horizontal forces created by large service openings. In addition to requiring an increased slab thickness, the cover slab also had to be cast to falls to aid water management.

Close collaboration was required between CCL, the construction team, and the MEP providers to co-ordinate the location of the PT tendons and anchors.

Indeed, design co-ordination has been one of the key challenges of the project.

Each level has its idiosyncrasies: the cores are a mix of in-situ concrete pours, slip-form and precast concrete, each of which had to be tied into the PT slab.

The raking columns below the cantilevered section at the front of the building also had to be tied into the PT slab, and the design of the slab here had to take account of the significant horizontal forces created by the size of the cantilever and its unusual shape.

A load path had to be established using a strut-and-tie analysis to ensure the forces through the slab were evenly distributed, minimising their impact on the concrete cores.

The impact of horizontal forces was also a design consideration for the level-two slab, where a large atrium means the forces from the steel structure at the void had to be incorporated in the slab design. Here, an edge beam has been installed to help manage the forces and control the deflections.

An edge beam was also installed as part of the solution for managing deflection and enabling a neat finish for the unitised façade.

The fixing brackets have been recessed c.150mm into the slab and the CCL team worked closely with the construction team and the façade designer to co-ordinate the location of the PT anchors and tendons with the façade fixing brackets.

The edge beam on each floor has been used to control deflection and they will be concealed in the ceiling voids, providing a robust structural solution with no visual impact, either inside or outside the building.

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