Sampson C. Huang

The performance of Tall buildings during the 21 September 1999 Chi-Chi earthquake, Taiwan

The 21 September 1999 Chi-Chi earthquake caused serious damage to and collapse of many tall buildings. The adversely affected buildings were virtually all of reinforced concrete construction with the lateral system being composed of moment resisting frames. All seriously damaged and/or collapsed buildings inspected by our team exhibited a set of identifiable and clearly preventable structural design and/or construction defects. This paper presents case studies of several tall building complexes that were the most heavily damaged by this earthquake. The reader should keep in mind, however, that the great majority of tall buildings in Taiwan were not significantly damaged by the Chi-Chi earthquake. Copyright

Geotechnical and geological effects of the 21 September 1999 Chi‐Chi earthquake, Taiwan

When the Chi-Chi earthquake ripped through the pitch dark of the very early morning hours of 21 September 1999, a large area was affected by various geotechnical and geological hazards in addition to the strong ground motion in central Taiwan. The epicenter of the earthquake was located in the central mountains of the island and rupture occurred on the Chelungpu fault. This is a North–South trending thrust fault with a dip of about 30 degrees to the east according to a 1999 US Geological Survey report authored by M. G. Bonilla. Taiwanese authorities had not believed this fault to be an active one because of the lack of evidence of fault activity during the Holocene age (during the last 11 000 years). The earthquake hazards, other than strong ground shaking, included surface fault rupture, landsliding and liquefaction. Copyright

Calibration of strength reduction factors for concrete masonry

Abstract Despite the variation in structural reliability which results from a combination of the current non-probabilistically derived load and strength reduction factors. the number of structural failures involving concrete members is quite small. This suggests that it would be reasonable to identify the mean values of reliability for the different limit states according to present code formulations and to calibrate the new strength reduction factors to match these mean levels of reliability. Such work has been done in concrete and a set of revised strength reduction factors has been developed. In this study we will develop a set of strength reduction factors which are calibrated to the strength reduction factors given elsewhere. This set of strength reduction factors is developed by considering the relative uncertainty in failure modes and loading states between the two materials, concrete and concrete masonry, which are involved in the calibration.

Design practice for tall buildings in Taiwan

This paper presents a review of the Taiwanese building codes and their relevance to the performance of tall buildings during the 21 September 1999 Chi-Chi earthquake. The 1982 edition of the Taiwanese code as supplemented in 1991 is discussed in more detail, since very few of the buildings subjected to this earthquake were designed in accordance with the more recent 1997 edition of the code. The recommended design lateral forces and procedures in Taiwanese codes appear to be similar to, and sometimes more conservative than, their United States counterparts. However, the construction practice as observed in our evaluation of damaged buildings exhibited a general disregard for the long-established seismic design and detailing principles. It seems that lack of construction supervision and inspection as well as adverse utilization of some loop-holes in the building code significantly contributed to the poor design and construction practice that resulted in some of the most extraordinary tall building failures ever observed. Copyright

Structural design using a wind tunnel test program and a risk analysis

Abstract A wind study procedure is presented that is compatible with the level of risk criteria used in ANSI A58.1 (1982). This procedure uses wind tunnel results and a risk analysis to select design wind forces. The uncertainty analysis requires the estimation of the mean, coefficient of variation and probability density function for all random variables.

Essential Facility Benefits from New Base Isolation Technology

A common problem that is encountered in the design of low-rise base-isolated buildings is the resistance to uplift. Rubber isolators have low tension stiffness and strength and traditional spherical friction pendulum (FP) isolators operate based on contact and thus, have no tension resistance. As a result, most base-isolated buildings require a substantial number of braced frames bays to limit tension demands on the isolators. An innovative new tension-restraint FP isolator has been developed that allows tension and shear to be transferred through the isolator simultaneously. This paper describes the design of an 82,000 square-foot 2-story essential facility that utilizes both tension-restraint FP isolators and traditional spherical FP isolators. A performance-based design approach was utilized for this project in order to satisfy the enhanced seismic performance objectives dictated by the nature of the mission critical functions served by the building. A three-dimensional computer model of the building was developed utilizing nonlinear elements for the FP isolators. Time history analyses were performed using seven sets of ground motion records for each level of seismic hazard and the sensitivity of the building response to several modeling parameters was investigated. The use of the tension-restraint FP isolators was beneficial to the project because it minimized the number of the braces required in the superstructure and thus provided a cost-effective means for meeting the seismic performance objectives of the essential facility. The reduction in braces also afforded considerable architectural flexibility and minimized the impact on the functional layout of the building.

Innovative Hi-Tech Solution for a Concrete Residential High-Rise Tower

This paper describes the innovative seismic retrofit of the Webb Tower located on the campus of the University of Southern California in Los Angeles. The existing 14-story residential structure was constructed circa 1972 using light-weight concrete. The lateral system for the building consists of perimeter non-ductile post-tensioned concrete moment frames supported on isolated spread footings. A preliminary evaluation of the building revealed several seismic deficiencies, including the following: (1) non-ductile detailing, (2) excessive building deflection and (3) joint shear overstress. After considering several alternate retrofit schemes, a unique retrofit scheme was developed utilizing a single bay of buckling-restrained steel braces in combination with a reinforced concrete beamcolumn frame on each side of the building. The selection of this system provides several advantages for this project over other retrofit schemes, including: (1) provides partial views from the existing windows, (2) requires significantly less demolition than an interior scheme, (3) more cost-effective than an interior solution, (4) reduces building drift and demands on the non-ductile moment frames, and (5) buckling-restrained braces do not exhibit strength degrading and aesthetically unacceptable post-buckling deformations. A three-dimensional nonlinear computer model was developed capable of capturing the nonlinear behavior of the buckling-restrained braces, the existing concrete beams and columns with consideration for the post-tensioning effect, and the new concrete beams and columns. The seismic performance of the retrofitted building was investigated by time history analyses using seven sets of ground motion records.

The significance of the 21 September 1999 Chi-Chi earthquake, Taiwan, for tall buildings

Shortly after the 21 September 1999 earthquake, the Los Angeles Tall Buildings Structural Design Council (LATBSDC) became concerned about the casualty and damage reports from Taiwan. The most alarming reports were the reports of damage to, and collapse of, many tall buildings resulting in much injury and death. It was our belief that Taiwans building code was based on the same model building codes used in California and most of the seismically active regions of the United States. It was also believed that the construction practices were similar to those in the western United States. In addition, the type of faulting on the causative fault is similar to the thrust faults that underlie southern California, and the magnitude of the Taiwan earthquake is similar to the size that is postulated for some of the thrust faults in southern California. The LATBSDC dispatched an earthquake investigation team to make a reconnaissance of the strongly affected area in central Taiwan. The main purpose of the investigation was to confirm the reports of damage and collapse of tall buildings and to determine the reasons for the high susceptibility of tall buildings in this earthquake event. It was discovered that although Taiwan has a building code that has seismic provisions that are similar to those used in the United States, there is poor application and enforcement of the code. Also, construction practices in Taiwan may create situations that make tall buildings more vulnerable to strong ground shaking. Copyright