The elliptically shaped plan and faceted exterior of the 161.2 m (530 ft) tall Poly International Plaza iconic tower in Beijing was inspired by pebbles and ripples in the natural water features that abound in the landscape surrounding the site. A column-free exterior diagrid system evolved as the most appropriate and best integrated structural system for this design. The diagrid module, four stories tall and 18 m (60 ft) across between nodes, was fabricated using concrete filled steel pipes and works with an interior reinforced concrete core to resist gravity and lateral loads. With nodes occurring at every alternate floor, intermediate levels were suspended from nodal levels above to avoid loading diagrid members in bending between nodes. The juxtaposition of diagrid modules on elliptical shape of the tower made three dimensional helical load paths possible, which, working with the global buckling stability afforded by the three dimensional form of the diagrid made it possible to introduce large architecturally exciting atriums into each end of the plan. Local and global buckling analyses, non-linear lateral analysis, finite element modeling of the diagrid nodes and investigation of hoop stress effects on slabs and perimeter members were some of the advanced studies performed on this non-prescriptive structure. Scaled testing of prototypical portions of the diagrid system was performed to evaluate its behavior under dynamic loads and verify the performance of the proposed joint construction. The paper presents the design challenges of this near complete project in an area of high seismicity.
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