The development at Winchester House, Bracknell, is 19 storeys high with a single level basement and replaces the former 3M 15 storey office building, which was demolished during summer 2015. The scheme consists of 311 high-quality flats, a gym and two business units suitable for a restaurant or retail use. The profile of the building steps back at each level above 10th floor and incorporates roof terraces for residents.
The new landmark building will form a central part of the town centre regeneration along with a new public realm market square.
Pure Structures Ltd were appointed by Comer Homes to carry out the Structural Design of the building.
The building structure is a reinforced concrete frame comprising flat slabs and blade columns and with stability provided by the combined action of shear walls and coupled main stair/lift cores.
The building envelope is a specialist system with feature rainscreen cladding/ribs and integrated balconies.
The main structural elements are described below:
Superstructure floor slabs (2nd floor level to the roof level) are typically 250mm thick reinforced concrete flat slabs. Floor slabs are locally increased to 450/500mm thick transfer slabs at 10th floor level and above where column reframing is necessary due to stepping back of the floor plan to accommodate the external terrace areas.
The 1st Floor slab is typically 400mm thick and enables in-slab transfer of column loads from rectangular sections (above) to 750mm Ø columns within retail areas (below). Where rectangular/circular columns (above/below) are not concentric, transfer beams/slabs are provided similar to the upper floor levels at 1000/1200mm thick.
The retail arcade (tunnel) through the building at ground floor level created a 275m2 column-free zone. To achieve this, the structural floor plate was locally increased to 1200/1400mm thick where a transfer structure is provided to support columns above which are coordinating with the typical residential arrangements up to 12th floor.
Floor slab design was carried out using RAM Concept FEA (Finite Element Analysis). Overall stability of the building has been analysed using an FEA full structure model in Staad Pro. The model was dynamically analysed to the Euro Code 1 wind spectrum of gusting frequencies to establish the dynamic response frequency of the structure.
The basement is contained within a contiguous piled wall enclosure, with facing insitu concrete liner wall.
The contiguous wall comprises 750mm Ø piles and is typically provided with a 1050mm deep capping beam at Ground Level. The capping beam increases to 1500mm deep on the end elevations of the building, where columns are supported directly on the capping beam at Ground Level.
To facilitate basement excavation/raft formation, the contig wall was designed as a cantilever wall and enabling unrestricted access for the construction of the core rafts and internal pile caps.
The building is supported on an arrangement of both CFA (Continuous Flight Angered) and Open Bored Rotary piles supporting a RC raft foundation.
Open Bored Rotary piles were installed from Ground Level within the extents of the Basement area. This reduced the amount of concrete required by pouring the pile to Basement Level and leaving a 4.0m void within the steel pile casing up to pile mat level at Ground floor. This method of piling also enabled the installation 31.0m deep piles.
During the site enabling works and demolition of the existing sub-structure to the former 3M building, 900/1500mm Ø diameter piled foundations were uncovered.
These existing piles were surveyed and their positions overlaid onto the proposed Basement plan/new piled foundation arrangement.
The new foundation piles and raft have been designed to ‘bridge over’ the existing piles where they coincide with proposed new columns and cores and making the existing piles redundant.
BIM (Building Information Modelling)
All drawing production was carried out using Autodesk Revit and Pure Structures’ BIM model provided a valuable tool for the client and contractor to improve the construction process. Collaborative work carried out between the design team was also particularly useful in relation to setting out coordination and clash detection.
Work commenced on site in February 2017 and with completion proposed for the end of 2018.
Full Structural design
Comer Homes Group
Frank Shaw Associates