COMPOSITE CONCRETE FRAME WITH PT SLABS INTERACTING WITH STEEL ELEMENTS

Interspan successfully designed and installed post-tensioned slabs in two buildings in a hybrid concrete-steel frame structure at College Square in the heart of Dublin for Walls Construction.  College Square has been developed by Marlet Property Group.

Post-tensioned slabs were chosen primarily for structural efficiency due to the large spans in both buildings.

College House

College House is a 10-storey commercial structure, while Apollo House is an 11-storey commercial – each floor of each building covering an average area of 1800m². The design involved a combination of steel and PT elements. During the early stages of the project, PT slabs were employed for the main floor areas of College House and College Square, utilising a typical slab depth of 350mm due to the 12m spans and commercial load case.

College House also has an impressive atrium, which cuts the slab edge back into the building in a V-shape, a feature that is continued through the building from levels 2 to 10 roof. The atrium presented an engineering challenge due to the architectural requirement for the corner of the V to be column-less, in combination with the aforementioned loads grid layout, and now an 18m span that needed to be dealt with meant that a solution of 800mm deep x 2400mm wide post-tensioned beams was the most efficient and visually pleasing solution.

Post-tensioning Solution

The post-tensioning solution was selected as it was a more economical and readily available solution to the large beams, which originally had been designed in steel.

Later in the project, due to the introduction of double-height precast walls, the demands on PT beams also increased, necessitating adjustments in their scope. The flexibility of the post-tensioning method ensured minimal impact on element depth while maximising the structural efficiency of the beams. A small increase in the level of prestressing within the beams achieved this.

Interspan suggested and developed a cost-effective hybrid steel and concrete solution.

Lighter steel framing was employed to create a step-back effect on the upper levels of the structures, transferred on a thickened PT slab only where needed. As time progressed, however, steel prices dramatically soared, and so Interspan suggested and developed a hybrid steel and concrete solution with the client engineer, Waterman Moylan, to bring the project back to budget.

This solution showcased the flexibility of post-tensioning and employed 250mm thick steel beams in combination with 225mm thick PT slabs. The top flange of the beams was to be level with the SSL of the slabs to adhere to the headroom requirements of the project; in order for the post-tensioning system to be effective, tendons were installed through the web of the beams in a novel and well-coordinated engineering solution to a commercial challenge. The hybrid concrete-steel frame structure with PT slabs presents a promising approach to modern construction, emphasizing sustainability, innovation, and structural efficiency.

Post-tensioning in combination with steel beams not only proved the correct technical solution, but the in situ nature of the construction also meant that delays were not incurred due to uncertainty in the availability of alternative, more expensive steel solutions.

Overall, the implementation of PT slabs in the hybrid concrete-steel structure provided several advantages, such as reducing embodied carbon, optimising material usage, and enabling creative design solutions for precast façade panels. The extensive coordination and design adjustments undertaken by Interspan showcased their commitment to achieving a successful and efficient construction process.

Design Collaboration

Overall, College Square project showcased the ability of the design teams at Interspan, Waterman Moylan, and Walls to respond dynamically to change throughout both the early design and later in the construction phases of the project and is an outstanding example of collaboration between the teams involved, the flexibility of concrete frame construction, and in particular the flexibility and practicality of post-tensioned slabs and beams to overcome problems and integrate with other methods in a seamless, cost-effective manner.