sketches exploring ways to strengthen the structure

23.12.14 | structural design development

At the end of December I started working with the structural engineer Steve Webb, a founding director of the London-based firm Webb Yates. Steve teaches at both the Architectural Association and the Royal College of Art, and tutored me last year during my 1:1 technical studies project – a cast concrete and folded steel sink I built for the farm. I really enjoyed working with him, and his knowledge of Hooke Park (which he’s visited as part of his teaching on the AA’s Interprofessional Studio program) is also an advantage for the project. He has very generously offered to support the project through offering his services as a structural engineer pro-bono; it’s fantastic to have him on board!

By the time I visited the offices for our first meeting, the design had developed since the first iteration I presented back in November. One factor shaping the design is the local planning regulations, and the need to gain planning permission in time for construction to start in early April. I had always envisaged that the project might be built incrementally, allowing for additional time to raise resources, and providing a useful period for reflection and learning between the two phases. This also makes sense from a planning point of view, allowing me to make a relatively simple application for a detached canopy structure this year. In terms of design, limiting the scope of the project to a timber structural frame and a a clay tile roof (without enclosing the space as an interior room) allows for greater material experimentation and innovation within the given time-frame. The completed structure will be used as a sheltered play-area during the winter, with an additional function as an external performance space during the summer.

As such, programatically the project now has many similarities with the Chander Nagar classroom we built in Dehradun. Having designed a simple mono-pitch timber frame for this project, I was eager to adopt a different and more challenging structural system for St. John’s which would take advantage of the facilities at both Hooke Park and Grymsdyke Farm – primarily the CNC routers, which will enable me to make prefabricated pieces with precise joints that can be assembled easily on site. Designing a structure which can be built quickly by a non-specialist team, with minimal use of machinery or site work is also a priority, and so the use of shorter pieces of timber that can be lifted easily is compelling.

I saw two grid-shell structures at the Rural Studio, the Animal Shelter (2006) and Akron Boys’ and Girls’ Club (2007), both of which used pieces of timber connected in a ‘lamellar’ sequence: i.e. the end of one beam is bolted at right angles to the mid-point of a second beam; a third beam is bolted to the mid-point of the second beam; and so on. This is a beautiful technique, pioneered by the German architects and engineer Friedrich Zollinger and Hugo Haring in the 1920s. For my design I’ll be using a slightly different lattice frame for the roof and walls, in which each timber member has a groove notched out so that the next piece can be slotted in, creating a flush surface on which to lay roofing tiles. The grid will be at a 45 degree angle to the ground, creating a diamond base-pattern within which there is scope for almost infinite variation. Unlike lamella structures, which are curved, the double-pitched roof of the canopy will act as a flat folded-plate structure, functioning as a diaphragm, transferring lateral loads to the walls through in-plane shear stress.  The structure needs to be reinforced in the gable ends in order to strengthen the moment connection – essentially bracing the roof and preventing it from pushing outwards and flattening. This could be achieved in a number of ways: a) by adding a collar or haunch, b) filling the gable end with the lattice structure, c) or adding a steel frame around the gable end – which would allow the side walls to be completely open. Although in some ways the steel frame was appealing, as it would reinforce the outline of the structure and act as a sharp contemporary line in contrast with the timber structure and clay tiles, in the end I decided against adding an additional (and expensive) material to the building process and stick to an entirely timber structure.

The canopy is oriented with the front gable-end facing the tree and the surrounding classrooms and needs to be open towards these spaces, so whereas the back of the building will be completely filled with the diamond lattice, the front will be braced with a partial lattice spanning from the bottom outside corners of the facade up to each pitch of each roof. The roof is at a 45 degrees angle, steeper than the 40 degree slope needed to allow water to drain off the clay tiles, but designed to allow the grid of the wall lattice to sit parallel to the roof-line. These decisions provide me with enough information in terms of form and materials to prepare the drawings required for the planning application; the aim for the start of next term is to develop the design in much more detail through 1:1 mock-ups that will be shown at the Work In Progress exhibition in February.