Design Type:  Architecture & Interiors

Designer:  Mark Sarkisian, PE, SE, LEED AP

The Rocker Beijing, China Building structures conceived as moving mechanisms rather than static objects provided the inspiration behind the Rocker, a mechanism developed for the New Beijing Poly Plaza, China Poly Group Corporation’s new headquarters. The Rocker was implemented to allow for the suspension of a lantern-like museum structure within the atrium of an office building and for the support of the world’s largest cable-net without interaction with the building’s lateral system. The result allows the top of the building to move freely during an extreme seismic event, which is likely in Beijing. The idea is founded in the concept of the pulley, allowing loads to remain constant in tension-supporting members, which pass through or over the mechanism. Structural analysis showed that the support for the 22-story-tall glass atrium wall could not be reasonably achieved using a conventional two-way cable-net, but could be achieved if the 90-meter-high by 60-meter-wide enclosure was broken down into smaller segments. A cable-stayed system was introduced by using two large diameter parallel strand bridge cables in diagonal fold lines while anchoring to the eight-story suspended lantern-like museum structure. The museum structure acts as a counterweight for the cables, introducing pre-stress and providing the required stiffness to resist out-of-plane loads caused by wind on the cable-net. In addition to the diagonal cables used at the atrium glass wall, two additional cables and a Rocker were introduced at the rear of the museum structure to assist in its suspension. Extensive study showed that when installed straight, the diagonal cables curved towards the façade during tensioning of the cable-net, reacting against the faceted form of the net. The length of each connecting rod between the diagonal cables and the net was pre-calculated to result in planar glass surfaces at the end of all stressing operations. Using the faceted design solution, the typical horizontal and vertical cables are minimized at 34 millimeters and 26 millimeters, respectively. By diagonally connecting the structure at the roof of the lantern to the structure 10 stories higher at the top of the atrium, bridge cable elements would act as mega-braces when the structure was subjected to earthquake loads. Up to 900 millimeters (3 feet) of relative movement between the two roofs is expected in an extreme seismic event. If conventionally connected to the base building structure, earthquake-induced forces would result in the cables and their connections becoming unreasonably large and infeasible. Recognizing that the building sways sideways in multiple directions during an earthquake, the team developed an innovative mechanism that would manage these demands while introducing no additional force into the base building structure. In the plane of the cable-net façade when swaying, one of the two diagonal cables lengthens as the other shortens with the rocker acting as a “reverse pulley,” using steel castings / plates designed to pass between one another and interconnect with steel pins allowing for movements in both building directions. Because the bridge cable sizes were so large (over 200 millimeters or 8 inches in diameter) a conventional drum-type pulley would have required a diameter of 6,000 millimeters or 20 feet. To add to the complexity of the system, the bridge cable lengths were different since the position of the lantern relative to the atrium façade (and, by extension, the internal reinforced concrete shear wall / buttress supports) was not symmetrically placed. Even with the sound theory of the pulley concept, the team had doubts about whether this concept could be achieved with the Rocker. A working model was created to simulate the expected building movement. Springs were connected at the ends of the cables. If they did not elongate during the side-to-side motion, additional forces within the cables would not be present. The experiment found that no significant elongation occurred during movement, confirming that the concept would work and could be installed in the first application of its kind.