The Alnwick Garden Trust

Alnwick, Northumberland

Architect: Hopkins Architects
Structural Engineer: Buro Happold
Principal Contractor: Sir Robert McAlpine

Alnwick Gardens Pavilion

Built to resemble a contemporary greenhouse the structure features a steel and timber barrel vaulted roof covered in ETFE foil pillows. The building houses a restaurant, a retail area, meeting rooms and a covered atrium where visitors can enjoy a view of the spectacular cascade water feature. The steel and timber frame for the building and the roof along with the glazed feature walls was supplied and installed by SH Structures.

The feature timber barrel-vaulted roof was somewhat of a departure from our usual projects however our approach was no different and we carried out the modelling and detailing of the complex diagrid structure before using a timber specialist to produce a kit of parts based upon the drawings provided by ourselves. This is perhaps the first time a steelwork contractor was involved in the detailed design of a timber roof but demonstrates how our expertise developed around steel structures can be adapted to other types of structure.

The use of steelwork within the diagrid shell enabled the stresses in the members to be controlled such that the sizes of the solid timber rafters were acceptable. A further example of integration of steel within the structure is the columns. Here the stability forces were resisted by cantilever bending of the columns, with the major axis being stiffened by a steel flitch plate. This helped control the deflections of the roof, thus reducing the bending in the rafters, and hence controlling their size. This was crucial in reducing both the member sizes and rise of the arch to create the aesthetic form required by the architect, client and planners for such a sensitive site, the overall height of the building being controlled by protected views across the Capability Brown designed landscape and the height of the listed wall.

The roof was made up by the cast nodes, tie rods and struts, closure ladder beams and the elegant façade wind posts. All elements were fully exposed and great care was taken in detailing and fabrication to achieve the finished result. The design life for the frame is 100 years, and great attention was made to the detailing and corrosion protection to ensure this.

Now viewed and enjoyed annually by an ever increasing number of visitors the project secured us a Special Award in the Structural Steel Design Awards and our first WoodAward. The Steel Design Awards judges’ described the project as ‘A fine example of a multi-material solution, which is highly effective and delights the eye of every visitor.’