Cable-Nets and ''Rocker Mechanisms'' — The New Beijing Poly Plaza

Abstract

Prominently located at a major intersection along Beijing's second ring road, northeast of the Forbidden City, the 110m tall New Beijing Poly Plaza project includes 24 stories of office space, an eight-story hanging museum `lantern' and a 90m tall atrium enclosed by what is believed to be the world's largest cable-net supported glass wall — a challenge amplified by the region's high seismicity. The cable-net wall is 90m high by 60m wide — dimensions that make a simple cable-net supported wall require uneconomical cable sizes and levels of pre-stress. The design is achieved by folding the cable-net around diagonal `V'-shaped, parallel-strand bridge cables, thus subdividing the wall into three planar facets and reducing the effective cable spans. The parallel-strand cables also support the `lantern' as it hangs in the atrium space without any columns extending to grade. Gravity loads from the `lantern' are used to induce high levels of tension or pre-stress in the parallel-strand cables. As the parallel-strand cables diagonally connect diaphragms that are 11 stories apart, lateral drift in the base building causes brace forces in the diagonal cables, increasing the design forces beyond reasonable levels. An innovative `rocker' mechanism is used to isolate the cable hanger system from forces induced by seismic drift. The `rocker' mechanism is architecturally `celebrated' — an exposed articulated joint mechanism made of rigid pin-connected castings which perform as a pulley equivalent. The mechanism allows the `V'-shaped parallel strand cables to behave as a continuous cable running around a pulley, offsetting induced tensile and compressive brace forces in the cables as the building drifts.