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Self-Mass Damper at Tokyo Swatch | |
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Self-Mass Damper at Tokyo Swatch Background In autumn of 2004, an international competition was held by Swatch Group Japan for their mixed retail and office tower in downtown Ginza, Tokyo. Subsequently, architect Shigeru Ban's unorthodox design — creating a public atrium space at the ground level, with individual glass lifts that provide direct access to the boutiques placed throughout the vertical retail space — proved victorious. Supporting the architect from a structural engineering front, as part of the competition submission, Arup proposed a seismic response damping system inspired by the pendulum movement of an antique clock. This paper will focus on the development of this idea, which eventually evolved to the realization of the Self-Mass Damper (SMD). Overview of Structure The key structural features of this building include: 1) creation of multiple atriums throughout the building; 2) implementation of an innovative seismic passive control system, the Self-Mass Damper (SMD) system; and 3) the sculptural undulating roof at the top level. The structural system is the outcome of an endeavor to resolve the unique spatial layout while satisfying Swatch Group's demand for a highly seismic-resistant structure. To summarize the overview of the structure from bottom to top: Due to the stiff soil layer located 14 meters (46 feet) below grade in the Ginza district, the building uses a raft foundation system. The existing basement structure was hollowed out and the basement walls used for temporary shoring. Numerous openings were required at the ground level to accommodate nine elevators, two stairwells, and a car lift to access the parking spaces, leaving very little floor plate area left to transfer the lateral forces into the surrounding basement walls. To resolve this force transmission, the ground level slabs were reinforced with steel plates of six to 12 millimeters (0.24 to 0.47 inches) thick. From the ground level, a three-story retail atrium extends through the longitudinal direction of the building — effectively carving away the much-needed moment frames at the base of the structure. To reinforce the lateral stability of these bottom few levels, three sets of core framing were introduced into the system. The general superstructure system consists of a rigid steel moment frame placed at every 2.4 meters (7.9 feet) along the longitudinal direction. To counter the seismic response of the building, SMD systems were located at floors 9, 10, 12 and 13. In contrast to the box shape of the building, architect Shigeru Ban envisaged a light, free-form roof floating above the structure. The geometry of this roof was determined by a series of form-finding techniques available in GSA, software developed by the research and development division of Arup. In accordance with the architect's image of a woven object, the structural scheme consisted of a pair of stacked steel plates extending in three directions. Requirements for Seismic Resistance According to Japan Structural Consultants Association (JSCA) performance-based design practices in Japan, the structural design grades for seismic resistance are grouped into three categories: Standard Grade, High Grade, and Special Grade. While minimum code requirements stipulate Standard Grade, through numerous discussions during the schematic design stage, it became apparent that Swatch Group wished to target an onerous Special Grade structure with additional requirements of: 1) main structural elements to remain elastic under Level 2 (500-year return period) seismic event, and 2) life safety under Level 3 (1,000-year return period) seismic event. In order to achieve such targets with economic feasibility, it was determined that implementation of a damping system to reduce the seismic response would be essential. Subsequently, the design team commenced a thorough exploration of damping systems that would best fit the structure without imposing upon the architectural intent. At the onset of design, a pendulum-type mass damper system was proposed. While an interesting concept, we had to convince ourselves that such a system would be appropriate and effective for this building. Armed with a plethora of damping devices available in Japan, the design team began investigating the effectiveness and pros and cons of various systems. To achieve a Special Grade structure with additional stringent design targets, it is common practice that a base-isolated scheme would be the system of choice. However, since the plot of land in the Ginza district is one of the most costly in the world, allocating a +1.0-meter- (3.3-foot-) wide strip of clearance around the perimeter of the building to absorb the base-isolation movement was deemed inappropriate. Installation of hysteretic or viscous damping devices within the allowable three core frames was studied, but resulted in mediocre effectiveness due to the bending deformation shape of the slender core frames. Preliminary studies indicated that a mass-damper-type system — although dependent upon the mobilized mass amount and tuning of the system — had potential to be extremely effective for this slender building Evolution of the Mass-Damper Scheme The key point in using a mass damper system to deal with large seismic forces, unlike the types of systems used to control low-energy wind responses, is that a tremendous amount of mass must be mobilized in order to be effective. Characteristically, the mass damper begins exhibiting practical effectiveness at around five-percent mobilization of the building mass. As the mobilized mass is further increased, the system can be more appropriately categorized as a mid-level isolation system, and finally, when mobilized mass is 100 percent of the building mass, it becomes a base-isolated structure. >>> Discuss this article in the Architecture Forum...
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