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Since its commissioning in 2004, the UC San Diego Large High-Performance Outdoor Shake Table (LHPOST) has enabled the seismic testing of large structural, geostructural and soil-foundation-structural systems, with its ability to accurately reproduce far- and near-field ground motions. Thirty-four (34) landmark projects were conducted on the LHPOST as a national shared-use equipment facility part of the National Science Foundation (NSF) Network for Earthquake Engineering Simulation (NEES) and currently Natural Hazards Engineering Research Infrastructure (NHERI) programs, and an ISO/IEC Standard 17025:2005 accredited facility. The tallest structures ever tested on a shake table were conducted on the LHPOST, free from height restrictions. Experiments using the LHPOST generate essential knowledge that has greatly advanced seismic design practice and response predictive capabilities for structural, geostructural, and non-structural systems, leading to improved earthquake safety in the community overall. Indeed, the ability to test full-size structures has made it possible to physically validate the seismic performance of various systems that previously could only be studied at reduced scale or with computer models. However, the LHPOST's limitation of 1-DOF (uni-directional) input motion prevented the investigation of important aspects of the seismic response of 3-D structural systems. The LHPOST was originally conceived as a six degrees-of-freedom (6-DOF) shake table but built as a single degree-of-freedom (1-DOF) system due to budget limitations. The LHPOST is currently being upgraded to 6-DOF capabilities. The 6-DOF upgraded LHPOST (LHPOST6) will create a unique, large-scale, high-performance, experimental research facility that will enable research for the advancement of the science, technology, and practice in earthquake engineering. Testing of infrastructure at large scale under realistic multi-DOF seismic excitation is essential to fully understand the seismic response behavior of civil infrastructure systems. The upgraded 6-DOF capabilities will enable the development, calibration, and validation of predictive high-fidelity mathematical/computational models, and verifying effective methods for earthquake disaster mitigation and prevention. Research conducted using the LHPOST6 will improve design codes and construction standards and develop accurate decision-making tools necessary to build and maintain sustainable and disaster-resilient communities. Moreover, it will support the advancement of new and innovative materials, manufacturing methods, detailing, earthquake protective systems, seismic retrofit methods, and construction methods. This paper will provide a brief overview of the 1-DOF LHPOST and the impact of some past landmark projects. It will also describe the upgrade to 6-DOF and the new seismic research and testing that the LHPOST6 facility will enable.

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The most common method used to determine the crack initiation life of a component containing a stress raiser in the low cycle fatigue regime is to calculate the maximum strain and then to use a strain-life curve. General practice is to base fatigue life estimates on the stabilized strain amplitude and to neglect the effects of transient behavior due to cyclic hardening or softening and ratcheting. For certain structures in which the accumulation of plastic strains may be significant, a separate check may be performed to ensure that these strains remain below a specified level. An objective of this research is to understand the notch tip local strain ratcheting and shakedown through finite element analyses and physical experiments. Towards planning a set of notched flat coupon experiments, this study performed analyses of various notched coupons under force-controlled cyclic loading. A question that will be addressed, what is the notch tip failure mechanism under a force-controlled load cycle with a non-zero mean force? Smooth specimens under such a force-controlled load cycle normally results in strain ratcheting. It is investigated whether notch tip strain responds in a similar manner under a force controlled loading cycle. The analysis results show that the strain ratcheting rate at the notch tip depends on the sharpness of the notch. In case of semi-circular and blunt elliptical notches shakedown of strain ratcheting within 25 cycles is observed, whereas for the sharp elliptical notch strain ratcheting doesn’t shakedown after 300 cycles. A novel observation made from the analysis results is that the mean stress at the notch tip gradually decreases with inelastic cycle while the stress amplitude remains unchanged. These result and future experimental plan on notch specimens are presented in this article.