Located within the established historic campus of Cranleigh School in Surrey, UK, the new van Hasselt Centre was built using a hybrid structure combining steel and wood. Kerto® LVL (laminated veneer lumber) was used in floor and roof elements to stabilise the steel frame. Use of light and green Kerto LVL products enabled fast installation on the construction site.

Cranleigh has its roots in a community school launched in 1865. As part of the school’s ongoing Masterplan development project, two disused squash court buildings were redeveloped and expanded.

A new building of 2,525 square metres covering the area was constructed around a steel frame with stabilising Kerto LVL elements for the floor and roof. Architects Allies & Morrison were given the task of designing the building.

Fast Light Green Kerto LVL

The design team convinced the school of the feasibility of the steel-timber concept and Kerto LVL Ripa elements. All the coordination and decision making thus took place at the design stage rather than on the construction site. This sped up the work onsite.

“Timber is used extensively in this project and is inherent in its design concept. The structural timber is exposed to view throughout, providing maintenance-free soffits that give character and warmth to the building’s spaces,” says Jack Stephenson from the architects Allies & Morrison. “The RIPA technology allows the ribs and deck to be designed as one module, which increases the span by approximately 40 per cent. The prefabricated timber panels were chosen not only for visual reasons but also for their fast installation and lightness.”

The project was coordinated using BIM (building information modelling) Revit and IFC files. This improved the coordination between the architect, engineer, steel manufacturer, element manufacturer and construction site, minimising mistakes and problems onsite.

Enhanced loadbearing properties

The van Hasselt Centre’s structure consists of a steel frame superstructure, with Kerto LVL elements creating the floor and roof. The Kerto LVL Ripa roof and floor elements used in the Cranleigh School project were manufactured by Belgian company Dupac.

The Kerto LVL Ripa elements design technology is an innovation by Metsä Wood that allows wide spans without supporting partition walls or pillars. It is based on Metsä Wood’s Kerto LVL wooden panels, which feature outstanding dimensional stability. Based on Kerto LVL S-beam and Kerto LVL Q-panel loadbearing components and structural gluing, it enables very long spans for elements of up to 20 metres.

Floor panels were constructed using 37 mm thick Kerto LVL Q-panels as deck and 73 x 358 mm Kerto LVL S-beams as ribs. The roof panels were constructed with 25 mm Q-panels and 45x300 mm S-beams.

The acoustician required a concrete screed on top of the timber panels. Kerto-Ripa was the only structure capable of easy load transfer within the maximum depth. The elements were chosen because they enabled long-span, lightweight, shallow depth and high capacity.

Compensating for frame tolerances

The Kerto LVL elements give stability to the steel frame. As the steelwork had rough tolerances, elements and details were designed to allow for bigger steel tolerances.

To allow varying levels of steel while still retaining the same floor level, a top-hung connection with bearing ribs on the steel was used. The top-hung detail was specially designed for the project, with glued elements and screw reinforcement. Using Kerto LVL in the roof elements allowed big openings, bringing light to the rooms.

Easy connections, speedy installation

The Kerto LVL elements were screwed to the pre-drilled steel structure and to each other. Because most of the details were left exposed, it was essential that connectors were partly hidden or completely invisible. This was achieved through careful design, cooperation with the architect and the use of tailored detailing.

All services in the wooden elements were pre-drilled to limit work onsite and minimise damage.

Up to 60 minutes of fire resistance

The Kerto LVL floor elements were designed to withstand 30 minutes of fire, with an additional 30 minutes achieved through the application of a fire retardant coating, bringing the fire resistance class to R60.

The timber elements of the roof structure in the corridor areas had factory flame protection, with additional flame treatment applied on site.

Prefabrication saves time on the construction site

The elements were entirely prefabricated, a decision that allowed work to be done simultaneously onsite and offsite, thereby accelerating the installation phase. Careful logistics planning – down to the order in which the elements were loaded on trucks to speed up installation – ensured just-on-time deliveries, minimising the need to store material onsite while enabling uninterrupted installation.

The properties of the Kerto LVL elements as reinforcements in a steel frame structure, combined with fast and easy installation, allow fast completion of even demanding structures.