Seismic Risks of Typical Double Fabs in Taiwan’s Hi-tech Industry

This study investigates the potential seismic risks of double fab structures uniquely seen in Taiwan’s semi-conductor industry. A typical eight-story double fab structure is considered as the object to explore the seismic vulnerability of such structural systems via computer simulation under realistic earthquake ground motion. As expected, excessive storydrifts are found to be focused on the clean room levels where most of the columns and shear walls have been removed to comply with the manufacturing process. As a consequence, the floor acceleration are amplified and the secondary (P∆)effects further deteriorate the earthquake-resisting capability of the fab. As an effort to relieve the soft-story problem, a seismic retrofit strategy by introducing metallic yielding dampers for energy dissipation has been proposed in this study. The metallic yielding damper proves to be effective in enhancing the seismic performance of double fab structures. INTRODUCTION The hi-tech industry in Taiwan has been suffering from unexpectedly great seismic hazard over the past few years despite the earthquake intensities have been considered moderate with PGA=0.10g~0.15g (Lee and Loh, 2000;Brain et al., 1999;SEMICON Taiwan, 2001). The seismic loss is mainly attributed to damage or dislocation of the process tools(e.g. the vertical diffusion furnaces with slender quartz tubing, the ion implanter with fragile supporting insulators, and the photolithography stepper, et al.)and the raised floor, with a subsequent operation interruption. Moreover, pounding of the main fab with its adjacent support building and the accompanying failure of the extension joint in between has been observed in typical wafer fabrication plants. Seismic protection has now become a critical issue in not only the hi-tech industry but also the industrial insurance business who has experienced lessons for underestimating the seismic risks. Since the 1999 Chi-Chi earthquake, insurance companies have been increasingly raising the premium and/or pay-by-client percentage of the claim-of-loss while mandate the clients to conduct rigorous seismic retrofit programs as a prerequisite in the contract. Among various types of wafer foundry plants, the double fabs have shown to suffer much greater PDF wurde mit pdfFactory-Prüfversion erstellt. www.context-gmbh.de damage to manufacturing tools and subsequent economic loss than standard fabs of equivalent scale, as observed in the past events. Common structural layout for standard fabs is like this: the first two stories of the fab are generally reinforced concrete structures with heavy shear walls and closely spaced columns(center-to-center column spacing of 3.6m or 4.8m). Moreover, the deep and stiff waffle slab with holes on the surface panels is built for ventilation purpose and high rigidity demands. Framing above the waffle slab are long span steel mega trusses supported by braced steel frames at the periphery of the plant to reserve a considerably column-free space for manufacturing process. In response to a rapid growth of product demands or a strategic business planning of the incorporation, double fab structures are uniquely seen in Taiwan’s semi-conductor industry restricted by the shortage of land resources of Taiwan. Unlike the standard fab (Fig.1(a)) whose cleanroom supports only a relatively light roof level, double fab structures (Figure 1(b)) contain two cleanrooms where most of the columns have been removed to comply with the manufacturing process, which must carry considerable gravity load and seismic lateral load transmitted from the stories above. As the plan area of today’s hi-tech fab has been increasingly wider, it is insufficient to brace only the peripheral frames to maintain rigidity and strength, and soft (and likely weak)stories are inevitably formed at the cleanroom levels. As a result, the double fab structures become seismically vulnerable once encountered with severe earthquakes. Excessive storydrifts are to be focused on the cleanroom levels during the earthquakes. As a consequence, the floor accelerations are amplified and the secondary (P∆)effects further deteriorate the earthquake-resisting capability of the fab and exaggerate the seismic hazards. Even if the earthquake is moderate enough to be structurally harmless, the floor acceleration could be amplified and damage the delicate manufacturing tools due to a soft-story configuration. If not prohibited, special care has to be exerted on the earthquake resistant design of double fab structural systems. In reviewing the structural design reports of some existing double fabs, it is found that some design concepts or considerations have been misled or overlooked. The major controversies include, (1) Completely following the ductility design suggested by the code The code specification is adequate for uniform buildings and appeals for minimum requirement only. With two soft stories, the double fab structures are in no way uniform and the designers should be extremely careful in applying the code–specified formula. With extremely stiff waffle slabs and soft stories, the desired strong-column-weak-beam condition does not exist, and logically no plastic hinges will be formed at the beams so that the structure will not behave in a ductile manner at all. Therefore, the reduction factor of the seismic base shear based on the ductility design concept should not be fully accounted for. In other words, by applying the code suggested formula for the design base shear would underestimate the actual seismic loading to a large extent. (2) Perform static analysis only The structure is irregular vertically due to the existence of two soft stories. In such circumstances, dynamic analysis is mandatory by the code. Moreover, the torsional response that might cause fatal damage to the structure can only be reflected via time history analyses. (3) Fail to check the ultimate strength of the story shears, in particular for the soft stories This design check is also mandatory by the code regardless of the type and height of the structure. PDF wurde mit pdfFactory-Prüfversion erstellt. www.context-gmbh.de As the fab structure is often less than 50 m in height, no peer review is mandatory by law. Unfortunately, as the above concerns in the design stage were not aware of and corrected, the seismic risks of double fab structures would be unthinkably higher than one might expect. Serious measures on seismic retrofit of the existing double fab structures are necessitated to eliminate the potential catastrophe in the next “big shock” while mitigating seismic loss in occasional moderate events. Figure 1(a) Standard fab Figure 1(b) Double fab Modern earthquake protection techniques discard the traditional idea of economy-based (ductility) design for a performance-based design concept emphasizing the maintenance of structural integrity even under exceptionally severe earthquakes. Advances in the development and practical implementation of passive energy dissipation devices for earthquake protection and retrofit of structures have been achieved in recent years (Housner, et al., 1997;Soong and Dargush, 1997;Soong and Spencer, 2002). The control forces imparted from the passive control devices are developed in response to the vibration of the structure without external power supply. Among numerous energy dissipation devices available the most widely adopted are the metallic yielding damper, VE damper and fluid damper, et al. The first is displacement-dependent while the others are velocity-dependent. Both types of dampers are functionally competitive. However, in view of long term usage and reliability, the metallic yielding dampers (working by transverse bending) are preferred for cleanroom