Marshall Lew

On the Use of Design Spectrum Compatible Time Histories

To a designer of a nonlinear structure, there is nothing more attractive than a real or fictitious ground motion time history whose response spectrum matches the target design spectrum. Frequency-domain scaled, design spectrum compatible time histories (DSCTH) are widely used in analysis and design of special structures, particularly seismic-isolated buildings. Their use has been even mandated by some code provisions. At the first glance, it seems that DSCTH records furnish designers of earthquake resistant structures with a consistency and compatibility bridge between the two very different worlds of elastic and inelastic response. Closer examination, as presented in this paper, reveal however that there are significant potential problems associated with uncontrolled use of DSCTH records in seismic design. It is shown that the use of design spectrum compatible time histories can lead to exaggeration of displacement demand and energy input. This in turn can distort the expected performance of the structure when subjected to design earthquake ground motions.

Representation of Bidirectional Ground Motions for Design Spectra in Building Codes

The 2009 NEHRP Provisions modified the definition of horizontal ground motion from the geometric mean of spectral accelerations for two components to the peak response of a single lumped mass oscillator regardless of direction. These maximum-direction (MD) ground motions operate under the assumption that the dynamic properties of the structure (e.g., stiffness, strength) are identical in all directions. This assumption may be true for some in-plan symmetric structures, however, the response of most structures is dominated by modes of vibration along specific axes (e.g., longitudinal and transverse axes in a building), and often the dynamic properties (especially stiffness) along those axes are distinct. In order to achieve structural designs consistent with the collapse risk level given in the NEHRP documents, we argue that design spectra should be compatible with expected levels of ground motion along those principal response axes. The use of MD ground motions effectively assumes that the azimuth of maximum ground motion coincides with the directions of principal structural response. Because this is unlikely, design ground motions have lower probability of occurrence than intended, with significant societal costs. We recommend adjustments to make design ground motions compatible with target risk levels.

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

The 1999 earthquake disasters worldwide: how many times do we have to re‐learn the fundamentals of seismic engineering?

1999 was a one-of-a-kind year where several earthquakes hit densely populated areas in Turkey, Greece and Taiwan and caused heavy loss of life and wide-spread damage. Contrary to what a layman may perceive, the nature of damage observed is not a strong function of geographical location or socio-political environment of the affected areas, while the extent of damage may be. Basically, we see the same handful of issues and problems being responsible for most of the devastation, whether the earthquake occurs in Taiwan or Turkey, southern California, or northern Chile. This paper is an attempt to summarize the key earthquake engineering principles, learned once and forgotten or ignored many times, observance of which could result in the saving of many thousands of lives, the avoidance of untold injuries, and significant damage reduction every year. Let us hope that this time, as design professionals and as public officials, we can remember them! Copyright

Monitoring of a Los Angeles Metro Red Line Subway Deflection Due to Adjacent Deep Excavation

The Wilshire Grand Redevelopment project in downtown Los Angeles includes the demolition of a 16-story hotel built in the early 1950s and the construction of a new 73-story tower that will be the tallest building in the western United States. The construction of the basement and foundation of the new tower required excavation up to 93 feet (28 m) deep. One side of the basement excavation, with an excavation height along that side of up to 57 feet (17 m) required placement of shoring within about 6 to 10 feet (1.8 to 3.0 m) horizontally from a 400-foot-long (122 m) section of the Los Angeles Metro Red Line subway tunnel. A monitoring and contingency plan was established for the shoring itself and the subway tunnel. The shoring monitoring included periodic surveying, slope inclinometers, load cells on tie-back anchors, and strain gauges on raker braces. Deflection and vibration monitoring was performed on the interior of the Red Line tunnel utilizing automated total survey machines installed in the tunnel; the monitoring system provided real time data and automated alerts were provided when threshold readings were exceeded. Presented herein is a description of: (1) deflection and earth loading criteria for monitoring; (2) response levels utilized during monitoring; (3) data transmission methodologies; (4) communication protocols; (5) installation of instruments; (6) evaluation of monitoring data; and (7) modifications to monitoring protocols during project.

Design of a Deep Tied-Back Excavation Adjacent to the Los Angeles Metro Red Line Subway

The Wilshire Grand Redevelopment project in downtown Los Angeles includes the demolition of a 16-story hotel built in the early 1950s and the construction of a new 73-story tower that will be the tallest building in the western United States. The construction of the basement and foundation of the new tower required excavation up to 93 feet (28 m) deep. One side of the basement excavation, with an excavation height along that side of up to 57 feet (17 m) required placement of shoring within about 6 feet (1.8 m) to 10 feet (3.0 m) horizontally from a 400-foot-long (122 m) section of the Los Angeles Metro Red Line subway tunnel. The temporary shoring support system for the Wilshire Grand deep excavation consists of soldier piles spaced generally at 8 feet on center with multiple levels of tieback anchors which extended above the subway tunnels. The bottom-most level of bracing consisted of rakers supporting walers attached to the soldier beams, because a bottom-most level of tie-back anchors could not be installed due to the presence of the subway tunnels. The shoring monitoring included periodic surveying, slope inclinometers, load cells on tie-back anchors, and strain gauges on raker braces (raking struts). Deflection monitoring was also performed on the interior of the Red Line tunnel. The paper presents a description of: (1) deflection and earth loading criteria for design; (2) geometric constraints of shoring design; (3) limitations on tie-back shoring due to limited easement width; (4) comparison of deflection and load from design criteria using results obtained from geotechnical instrumentation; (5) raker preloading methodology; (6) temperature dependence on raker loading; and (7) deflection distribution as a function of distance from shoring wall. Lessons learned regarding preloading of rakers and deflection of shoring and retained earth will be described.

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

New building code requirements for the seismic design of tall buildings near active faults

The seismic provisions in the new 1997 edition of the Uniform Building Code (UBC) contain significant changes affecting the seismic resistant design of buildings. For buildings in California, the most significant change is related to the amplification of forces in areas near major active faults. This is done through the introduction of a near-source factor. This factor affects the design of buildings throughout the spectral range, but particularly affects mid-rise and high-rise buildings owing to their response to long period ground motions for which the near source effect is most pronounced. This paper provides an overview of the code change, a detailed discussion of ground motion mapping in the near field, and a discussion of the effect of the code change on tall buildings. Preliminary near-source maps are developed for the Los Angeles Basin. Studies are presented, comparing base shear values calculated based on the 1994 and 1997 editions of the Uniform Building Code.

The Right Mix

An innovative combination of deep soil mixing and spread footings will support a community hospital in California, preventing liquefaction of the deep and varied soils in the face of a strong earthquake.