Civil Engineering November 2022 | Vol 30 No 10

which posed additional challenges. This resulted in some of the foundations having the underpinning piles installed eccentrically to avoid existing services. Strap beams were introduced to resolve the eccentric loads. Once micro-piles and the pile caps were cast, strengthening of the columns fol- lowed. This was done by casting reinforced concrete collars around the columns. The columns were roughened and had dowels installed, which were tied to the reinforce- ment cage around the column. Self-compacting concrete was used for the collar concrete, with a small gap at the top between the slab and the strength- ening concrete. The gap between the slab and collar concrete was filled with a high strength epoxy, caulked between the slab and the collar to ensure load transfer from the column above, through the slab, to the column below. The strengthening work had to be carried out with minimal interference to the functioning of the building and vehicle movement along Bute Lane, and in the basements. Although the parking bays next to the columns and foundations that were being strengthened were tem- porarily blocked, most of the basement parking area remained accessible. Bute Lane remained functional throughout the duration of the project, with occasional lane closures during the pumping of the concrete when slabs were being cast. The existing ground/podium level was ribbed slab with a build-up of no-fines concrete (200 mm to 800 mm deep), as well as landscaping and a water feature. The landscaping and the water feature covered a 1.5 m step in the existing slab. This step was situated across the middle of the proposed new building, which the architects did not want to be visible in the new design. To achieve the same level across the interior of the building, the new ground level was raised above the existing ground level. This was done by placing sleeper walls at 2.1 m centres, with a com- posite deck on top. The existing ribbed slab’s capacity to support the walls and the deck was checked, and it was deter- mined that the slab would be overstressed. To reduce the load on the slab, the existing no-fines concrete under the footprint of the new build-up was removed. Its purpose had been to drain water seeping through the landscaping, and it was no longer required for drainage in this area. The removal of the no-fines concrete resulted in the existing slab having sufficient capacity for the new load. Steps and ramps were introduced to tie the new ground level to the existing external levels. The void between the new ground level and the existing level was used for services and reticulation of the stormwater system. AESTHETICALLY STRIKING STRUCTURE The new building above the podium level has three floors and a roof. The various floors are characterised by their curvy slab edges, with the plan layout differing at each level. Each floor has varying levels due to the different uses and floor finishes and this, together with the curved floor edges, com- plicated the design of the post-tensioning. The post-tension cables were set back from the slab edges because a triangular aluminium edge feature was to be installed after the construction of the slabs. Post- fixed anchors were used to connect the feature frames to the concrete edge, and the cables had to be set back to avoid them being damaged. Curved down-stand beams of varying depths were supporting the façade and shopfronts below. These beams were colour coded on the drawings to enable the contractor to easily understand the levels. The roof has a 7.2 m cantilever and designing it as a post-tensioned slab was found to be inefficient and costly. To over- come this, the first 2 m of the cantilever was constructed as a post-tensioned slab, with the last 5.2 m consisting of steelwork and a profiled roof sheeting. FACILITATING SUCCESS WITH 3D MODELLING The drawings for the existing building had all been completed on 16 November 2022 Civil Engineering The building includes a medical wellness centre, gym, crèche, retail, and a rooftop entertainment area with a running track