‘It’s like we’re coming out of the ground for the third time!’
This was the recent quote from the temporary works coordinator on Domis and Salboy’s brilliant Viadux scheme.
He had underestimated the chill on a sunny spring day, shivering as we stood 17m above true ground level of Great Bridgewater Street.
I could see what he meant though. Once the transfer deck of the tower is poured, the construction of the 40-storey residential tower becomes very ‘ordinary’.
A development that’s anything but ordinary
‘Ordinary’ is a word seldom used on the Viadux site. Feeding the core and columns of a 40-storey tower through 150-year-old, grade 2 listed railway arches is not an ‘ordinary’ project. It has meant piling, excavating and pouring pile caps of up to a depth of 3.6m, all without disturbing the listed masonry or the adjacent electrical substation… and this was just to get out of the ground for the first time.
The team then had to get the columns and the core through the brick viaducts. This was achieved by forming concrete ‘ring beams’, or ‘picture frames’, with some well-thought-through temporary works and a few diamond drill bits. A little bit of thinking outside of our ‘brick box’ allowed us to get out of the ‘ground’ again. Only this time we found ourselves in the live working service yard of Manchester Central Conference Centre.
From complexity comes simplicity
Whilst the first two occasions were very literal examples of bringing gleaming concrete out of the earth, the final time this exceptionally ambitious structure gets to come ‘out of the ground’ is more figurative. That is, once the transfer slab is formed, the complexity and risk associated with the site constraints (the listed arches, working service yard, party wall commitments to name a few) become significantly easier to manage.
The transfer structure at level 2 of the proposed tower comprises a grillage of 1.75m deep reinforced concrete (RC) transfer beams and slabs.
The structural system allows the loads of the columns, shear walls and part of the main core to be transferred out to bifurcating (y-shaped) columns below. In effect, we’re funnelling the loads into a 11mx6.8m column grid to minimise the number of openings and interventions to the brick arches. This means we needed fewer columns where the skyscraper met the second ‘ground’. The building core is also reduced in size with this approach, so the ‘picture frame’ needed to form the opening within the arches was of a realistic size and structural scale.
Concrete is like chocolate… kind of…
The use of the ‘y’ column was very deliberate and considered because the shape ensures much more of the load acts in compression (pushing/squashing), which reinforced concrete (grade C60/75 in this case) is very good at resisting. This is in distinction to a method that induces bending or torsion (twisting). When you break off a piece of chocolate, we instinctively know to bend it to break it.
Concrete is like chocolate in that sense.
Further, the transfer slab also allows the building to be cantilevered circa 3.5m at the ends to avoid the existing viaducts bridge abutments below. That translates to, more living space in the sky and more of the beautiful brick heritage retained below.
A tabletop fit for a skyscraper
The level 2 beam and bifurcating columns do another important job. Below the second floor, the lateral stability of the building is provided by the reduced core and the bi-furcating columns and transfer beams forming ‘moment’ or ‘goal-post’ frames. These mega columns form mega goal posts, or in simpler terms, a super-stiff table to build our tower on top of.
This optimised transfer structure solution we developed resulted in a circa 450m3 reduction (30%) in concrete use and a 100-tonne reduction in the amount of reinforcement (28%) when compared to an alternative scheme that adopted simpler vertical columns with the transfer beams having to span further.
Working in sync with Manchester Central
It’s not all about design, however. Domis and Mayo Civils have built the transfer beams whilst maintaining the live operation of Manchester Central’s service yard. The project team have developed a (CAT3 checked) temporary works scheme to transfer the weight of the wet concrete to the existing masonry piers until it’s strong enough to stand on its own 11, Y-shaped feet.
So here’s to an important milestone in a brilliant building. A building we see breaking new ground by (often literally) punching through existing ground and many many constraints.