Structures

The Jagged Edge House is designed to look like a large shard of schist on the cliffside

The "Jagged Edge house" is the kind of dwelling that leaves people amazed. Once it is constructed, it will teeter on a rocky outcrop, and through the perimeter walls, which are entirely made out of glass, its residents will enjoy 270° views of the pine forests and lakes that fill the southernmost tip of New Zealand's South Island.

The man behind this spectacular concept is structural engineer Warrick Weber, director at Weber Consulting, based in New Zealand.

He says: "Jagged Edge is designed to look like a large shard of schist, emulating the surrounding geology."

Inside the house, the project team has achieved another tour de force by creating a space that is uninterrupted by structural columns or internal walls, but which is topped by a slab of concrete. So the obvious question is: how does such a structure hold together?

Weber says: "The design concept at the heart of Jagged Edge is deceptively simple. It's a tension structure supporting a large concrete roof with the tension cables anchored back into the natural rock."

The idea might be simple, but the structural solution had to respond to a challenging environment. The first element the project team had to take into account was that the location they chose for the Jagged Edge house, Closeburn in Queenstown, is a seismically active area. That meant that the house had to be made earthquake-proof.

Weber's main concern here was how to deal with vertical forces created by earthquakes; these would cause the roof structure to bounce up and down at its apex. The structural solution was to use tension cables to anchor the roof slab to the ground floor. To minimise the load the designer created a triangular shape that is lighter at its apex.

The apex is supported by tension cables that are anchored back to a kingpin - a large steel element that holds all the cable together. This kingpin distributes the load of the tension cables into six rock anchors, extending eight metres into the rock, which are capable of taking a combined pull-out load of 750 tonnes.

The second set of tension cables, which span between the roof and the ground floor, are anchored directly into a large concrete foundation beam that runs around the perimeter of the building. These cables are spaced at 1500 mm intervals and are tensioned to eight tonnes each to support the glass panels. They are anchored to the roof's apex, where they act to counteract bounce.

The house offers sweeping 270° views on the surrounding nature

An unobtrusive solution will be adapted to support the glazing. The secondary cables have a fitting attached to support the glass panels, which are 100 mm in front of them.

The panels are then joined with a structural silicon sealant.

Now that the concept for this striking-looking house has been outlined, the project team has only to see it being erected. But every precaution has been taken to make the enterprise as safe as possible. Weber says: "Geotechnical investigation were carried out to make sure that the rock into which cables are going to be anchored was suitable to take the very large loads needed."