Since we introduced the project in Part 1 – The Introduction of this series of articles, we want to focus this article on the primary structure of the Foundations and the Steel Frame. Since the last article, these have both been installed. But first, let us start with the design:

Structural Design
The fundamental structural form is a lightweight roof and lightweight insitu concrete first floor supported on a structural steel frame on pad foundations. The structural steel frame is based on a regular 7m x 5m grid, and the use of the first floor is varied and, in some parts, undefined, so the design was made for a live load of 5.0kN/m2 with an additional allowance of 0.5kN/m2 for lightweight partitions. This will provide a very flexible space to meet the future ambitions of the client. This first floor is an insitu concrete composite floor with the concrete cast on a proprietary metal decking and bonded to the structural steel support beams. This allows the concrete to enhance the strength and stiffness of the steel support beams through composite action. The two reasons for adopting this form of the floor were to reduce the steel beams’ weight and provide a diaphragm at the first-floor level. Also, with the length of the building at around the limit of a single structural element, this enabled better control of thermal expansion and long term shrinkage effects within the slab.

Stability is provided through diagonally braced bays in four locations in each direction up to the first floor. The first floor acts as a rigid diaphragm to allow the columns to carry the roof as a series of cantilevers. The pitched roof is stiffened and braced using the combination of longitudinal braced bays and lateral sway cantilever columns. Deflections being limited to 1/300th of the height in all directions at each level and differentially to 1/360th of the bay width.

The frame was analysed and designed as a full 3D frame using MasterSeries Engineering software (CSCS) to comply with British Standards that Guernsey Building Regulations still accept. Wind loads were determined within the software and combined with Dead loads and Live loads in 63 different load cases. The analysis and design software enabled us to meet the deflection and strength criteria and minimise the quantities of steel and concrete. The entire structure requires a total of 36T structural steel, 166T of mass concrete and 22.3T of reinforced concrete in the design.

The ground was found to be good, a dense silty/sandy clay type material at around 0.8m below finished floor level, but with a high water table; water was found to be at about 400mm below ground level. We originally designed each pad as shallow reinforced concrete founding at around 1100mm below FFL, but the particularly wet conditions leading up to the time of construction demanded that we omit the reinforcement and redesign the foundations as mass concrete pads at approx 1600mm below FFL. The contractor was able to keep the excavations dry during the much simpler concrete pours. The two internal braced bays on Grids 5 and 8 had no easy mass concrete equivalents, so these had to be retained as reinforced concrete.

Steel Frame Erection
The frame was erected on the foundations very quickly. It was fabricated in the UK by A J Lowther & Son and transported back through a local Steel Fabricator, Guernsey Metals [email protected], who erected it. Having been fully modelled in 3D by Termak Ltd, from which the fabricator could accurately cut and fabricate every part, the process went without a hitch with no on-site adjustment necessary and all components fitting the first time – well done to you all.

Composite First Floor.
Once the frame was lined and levelled, the Steel Decking was fixed in place using rows of through ‘deck welded studs’ into the steel beams below. This was a specialist job, and Guernsey Metals arranged for a specialist team from the UK with their specialist welding equipment to carry out the work. The entire floor was welded down in a few days with over 1400 studs.

To view an immersive look of the site, follow this link.

Next month: T & G  Guernsey will begin the installation of the Ground Slab, concreting of the First Floor and the installation of the Cladding