Wednesday, November 18, 2009

Will a floating donut design stand?

Posted by Will it stand? at 11:03 AM
In 2007, I spent six months living and working in Copenhagen. There were many cultural differences to overcome, and a lot of new construction methods to learn. Scandinavian design is renown for being modern and forward thinking. Danish architects are leaders in long span structures, sustainable design and creative use of space. I learned to approach projects more creatively in order to achieve the design goals.

One conceptual project that I was involved with was sited adjacent to Parken, the National Football Stadium. The proposed program included a multipurpose arena, a theatre stage, parking, office space, a fitness center and an extensive green roof. Locating all of these services on the constrained site was a challenge for the architectural team. Parking was located below grade, the arena at ground level and everything else above.

Parken Arena

In that scenario, the major challenge was finding an economical way of supporting multiple floors of occupied space above the arena. To ensure unobstructed views in the arena, a 50m (160 ft) free span was required. The first scheme was

The first concept explored a conventional box design, topped with tennis courts and a crown-like perimeter wall. However, when the long span loading concerns were shared, the architects relieved weight by removing structure above the mid-span. The resulting design featured a floating square donut above the box. Developing a structure that retained the visual intent would be difficult, but more economical than the original, more conventional, approach.

Structural design proceeded in two steps: first setting the design of the arena enclosure and secondly supporting the ancillary levels above. The intent was to have both systems vertically supported by the same set of columns, thus avoiding a transfer situation in the arena roof. In one scenario, we considered supporting the entire donut on just four corner super-columns.

Robot Truss Analysis

The truss supporting the donut would need to be three stories tall in order to span the length of the arena. However, this was aesthetically possible, because the truss would be located along the interior face of the building. The design of the truss members depended on the loads, materials, and architectural requirements.

The layout of the diagonals can be chosen so that they are in compression or tension. Eventually, all loads find their way to the top or bottom members, called chords. The design of these members is critical, since they usually see the greatest amount of load. Different design consequences apply if a member is in tension or compression. For a member in compression, the length is especially critical, since this most determines the susceptibility to buckling. Therefore, we would prefer the longest members to be in tension. However, dealing with strange load conditions and providing room for walkways through the truss can disrupt the best laid plans.

In the end, our design mixed a Warren layout (alternating diagonal directions) with a Pratt (diagonals only in tension). The combination was due selected so that the 3-story tall truss would be stable during construction. The one-story warren truss could be erected on the ground and lifted all at once. Then the other two stories would be erected above, using the first floor as an erection platform.
We later looked at several more schemes for the proposed arena. Ultimately, the project was canceled before construction commenced. Nevertheless it presented an interesting exercise in combining universal principles with local preferences.
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