Plymouth

Drake Circus

This challenging project on the site of the old Bretonside bus station in the centre of the city features a complex steelwork frame. The 12-screen cinema, including IMAX digital screen, sits over two levels of new car parking with a third car park utilising the existing viaduct undercroft.

A condition survey of the existing viaduct was carried out prior to the start of project works. Further repeat surveys were then undertaken over a prolonged period to ensure that any movement of the bridge due to external factors could be understood and incorporated into the design of movement joints. Existing units located beneath the Exeter Street viaduct were demolished to accommodate the conversion of the space to car parking.

The roof of the new structure is comprised of lightweight metal decking supported on proprietary purlins which spanned onto the steel frame. Part of the roof was to be utilised for mechanical plant and in these areas, a reinforced concrete (RC) slab on metal decking was installed to support the plant loading, acting compositely with the steel frame.


Drake Circus

Client: British Land
Location:
Plymouth
Completion Date: October 2019


Architect: Corstorphine + Wright
Other Team Members:
McLaren Construction Group
Sector: Retail & Leisure



Challenge

Challenge

This challenging project on the site of the old Bretonside bus station in the centre of the city features a complex steelwork frame. The 12-screen cinema, including IMAX digital screen, sits over two levels of new car parking with a third car park utilising the existing viaduct undercroft.

A condition survey of the existing viaduct was carried out prior to the start of project works. Further repeat surveys were then undertaken over a prolonged period to ensure that any movement of the bridge due to external factors could be understood and incorporated into the design of movement joints. Existing units located beneath the Exeter Street viaduct were demolished to accommodate the conversion of the space to car parking.

The roof of the new structure is comprised of lightweight metal decking supported on proprietary purlins which spanned onto the steel frame. Part of the roof was to be utilised for mechanical plant and in these areas, a reinforced concrete (RC) slab on metal decking was installed to support the plant loading, acting compositely with the steel frame.

The main frame is supported on shallow pad foundations, bearing onto bedrock (saltash formation) with the lower carpark supported on a ground bearing slab. Retaining RC walls were required to the north, east and west sides of the site.

Lateral stability of the structure was achieved through diaphragm action of the roof decking and floor slabs. Lateral loads transfer to vertical cross bracing of the steel frame and to the foundations. These foundations were additionally designed to resist tensile forces due to uplift by utilising the self-weight of the foundations.

Special client requirements from the landlord, tenants Cineworld and South West Water were taken into account, as well as additional conditions from the acoustic and fire report.

Design and analysis of the main frame was carried out using Tekla Structural Designer (TSD). This included a comprehensive wind analysis, overall stability analysis and individual member design as well as accidental load analysis.

Design and analysis of the 12no. cinema screens was undertaken in SCIA.

Solution

Above the IMAX auditorium is a sky bar supported by a 4m deep truss spanning 25m and weighing almost 20.5 tonnes. A deep truss solution was required to accommodate the column-free zone of the IMAX cinema below.

In addition to the analysis and design of the main frame under permanent conditions, TSD was used to assess and review the temporary stability of the structure during construction, eliminating the need for temporary horizontal bracings.

Structural challenges faced by the team included the following:

- Complex coordination of cinema boxes to align with acoustic requirements
- The many grid transfers to suit the varying space uses resulting in over 50no. transfer beams

- Complex stability system, including both horizontally offset bracing and transferred bracing
- Limited structural zones where long span beams were required such as perimeter auditoria beams
- Building over large public sewers

These various challenges were overcome by early attention to detail, with BIM model transfers between designers and proactive communication.

We had a very good relationship with BHC, the main frame contractors, where a proactive approach to design was adopted which resulted in structurally efficient design solutions.

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