Creating the Avenue

9th February, 2018

The new major town centre redevelopment at Bracknell, Berkshire was under discussion and planning for nearly 20 years before finally opening in September 2017. A significant part of the development was the creation of a new multi-storey car park that showed the best of precast and offsite design.

Called the Avenue Car Park, it remains open 24-hours a day and provides 1306 car parking spaces including specific spaces for disabled and electric-car charging. With an anticipated average parking time of two hours it was essential that a large volume of cars could easily gain access and egress, with the various levels easily accessible, and to give drivers a good parking experience with their safety and security paramount. PCE worked in co-operation with Mace, developing a hybrid design-and-build solution, principally using offsite engineered precast concrete components, to provide a solution for the five suspended levels car park structure that would also incorporate an area of multi-level retail space at its front.

Minimising the height of the structure was a necessary planning requirement and the development's central location meant that maximising the amount of offsite construction would reduce the total number of vehicle deliveries required to enter the centre of Bracknell, as well as a reduction in the number of onsite operatives and management needed to construct the structural frame. Mace was awarded the project in February 2015, which resulted in structural design work starting in earnest, with the commencement of offsite production of the precast concrete units in July 2015, thus enabling the erection of the structure to proceed in late November of that year.

The use of three-dimensional BIM techniques was critical during the tender period and following the contract award, enabling PCE to develop the car park element of the structure as an efficient precast hybrid concrete solution using its in-house designed-and-developed long-span GT flooring units. The pre-stressed concrete GT units are designed to meet the required car park structural loading criteria without the need for the casting of any additional onsite reinforced structural concrete over their top surface, which gives a significant reduction in onsite activities and associated disruption, enabling following trades to progress more rapidly.

Due to the car park deck being non-rectangular in plan and the development's principal anchor stores, adjacent to either side of the car park, having differing floor levels, all of the decks were required to fall in two directions. In certain areas, this required design development of the GT flooring units to enable them to be splayed, cast with a 25mm fall across their top surfaces and the use of solid GT units to support reinforced concrete 'cheese wedge'-shaped units. The longest unit was 17.8m by 1.5m wide which included a splayed end, while the solid slabs weighed 22 tonnes.

In certain areas, having a 25mm fall across the width of the GT unit was insufficient to provide the transitions required, which resulted in the bearing corbels of the supporting beams and walls also having to be stepped. Prior to production commencing, two full-scale mock-ups were prepared to prove the structural geometry and adequacy of the non-standard GT units and supporting beams. Compared to other offsite solutions of similar span, the GT floor units provide a reduced overall floor depth and structural dead load, which leads to a reduction in height and weight of the supporting precast concrete bearer beams, as well as a more efficient design of other structural frame components.

For the Avenue Car Park, a consistent high-quality anti-slip brush finish was required for the surface of the car park floors. This was easily achieved with the GT units due to the high-quality control capabilities of factory-engineered concrete.

For the adjoining retail section of shorter and varying spans a hybrid structural solution of precast columns, Delta beams, hollowcore units and in-situ concrete was chosen. Lift cores and the central stair core were formed using flat-pack walls due to the requirement of corbels to pick up the bearing of the adjacent floor units. The two side stair cores were framed with precast columns and beams. A further interesting challenge for the PCE team was the design and delivery of the two curved and handed ramps on the external face of the car park adjacent to Bracknell's Millennium Way which form the up and down access between the floors.

This ramp structure used precast reinforced concrete 'goalposts' to enable the structural bearing of the precast floor slabs to avoid using separate column and beams. Perimeter beams were a combination of straight and curved in plan which spanned between the goalposts to support the pre-stressed concrete plate flooring for the straight sections. Normally reinforced concrete curved and sloping plates at the ends of the ramps were cast from moulds integral with those for the curved beams, ensuring accuracy of fit onsite.

A reinforced structural topping across the plate flooring was also used. Accurate electronic detailing of the different components and close control of the casting accuracy of all the precast concrete units ensured that the ramps' complex geometry was easily achieved.

In total, over 3500 precast units were erected during the 40-week on-site programme. During the project, PCE further developed its electronic QA and paperless control systems to ensure that components were manufactured and delivered on a just-in-time basis and easily erected to the quality expected and with a high regard to all aspects of health and safety.

Nickie Brown, PCE Managing Director, says: "The Avenue multi-storey car park at Bracknell is an excellent example of a design-and-build offsite engineered hybrid concrete structure. We were able to use the latest electronic technology and intelligence, including BIM, to design and detail, share data between all the parties involved and provide control systems. All this has enabled the use of structural concrete to be the most viable and efficient construction material choice for this building."

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