B.V. Fell

Full-Scale Dynamic Testing of a Sliding Seismically Isolated Unibody House

Shaking table tests were conducted on a new low cost sliding seismic isolation system aimed at significantly improving the seismic performance of low-rise lightweight residential construction. A two-story, full-scale seismically isolated wood frame house was tested dynamically under multiple ground motions on a shake table. Two different sliding isolation bearings were evaluated, one with flat and another with concave sliding surfaces, both with high-density polyethylene sliders on galvanized steel surfaces with a coefficient of friction of approximately 0.18. Each isolation system was subjected to seven severe recorded earthquake ground motions, which produced peak isolator displacements of up to 41 cm. The maximum induced inertial shear force on the superstructure was on the order of 0.4 g, yet the house remained practically damage-free with story drift ratios less than 0.1%. The study successfully (1) provides a proof-of-concept for design, construction, and behavior of a light-frame house with low-cost high friction sliding seismic isolation, (2) confirms several design assumptions regarding isolation behavior and maximum isolation displacement, and (3) provides data to validate computational models and develop design guidelines for the isolated superstructure.

Large Scale Tests and Micromechanics-Based Models to Characterize Ultra Low Cycle Fatigue in Welded Structural Details

Fracture and fatigue-induced failure in welded structural details is an important limitstate in earthquake resistant design. Despite its significance, fundamental, physics-based models to simulate Ultra Low Cycle Fatigue (ULCF) in base and weld metals are not readily available, and many of the popular approaches predict ULCF in an empirical manner without considering the complex interactions of stress and strain histories responsible for it. While convenient, these empirical methods may not be reliably transferable to untested details or connections. In this paper, newly developed physicsbased models that aim to simulate ULCF at a continuum level (and apply them through finite element analyses) are introduced. Preliminary results from experiments on six column base plate specimens are presented. These tests, part of a NEESR project, seek to validate these physics-based models. From a practical standpoint, these experiments provide important insights into modes and hierarchies of failure of column base plate details, especially fracture originating in the welds and heat affected zone. The parameters considered include variations in cyclic loading histories and in weld details similar to configurations commonly used in engineering practice.

Loss of Pressure Boundary through Buckling-Induced Fracture in the Ciudad Nezahualcóyotl Pipeline

AbstractA study is presented to investigate the fatigue and local buckling-induced fracture incident of a buried continuous pipeline during the 1985 Mexico City earthquake. Assuming pipeline wrinkling is caused by longitudinal wave propagation, peak ground strains are estimated from recorded velocity time histories near the site of the fractured pipeline and critical buckling strains are calculated from experimentally based empirical relationships. A physics-based model of the postbuckling deformation of the pipeline provides estimated deformation demands on the pipe, whereas a micromechanical fatigue-fracture model is used to determine the potential for fracture considering the estimated axial deformations from the recorded ground motion, plastic strain demands, and stress state at the critical fracture location. The ground motion at the Ciudad Nezahualcoyotl site in Mexico is assumed to have approximately seven cycles of ground deformation large enough to exceed the critical buckling strain of the membe...

Effect of Abutment Shear Keys on the Seismic Response of Bridges

Seismic analysis results of typical post-tensioned concrete bridges are presented considering the nonlinear behavior of transverse abutment shear. Current Caltrans and AASHTO bridge design criteria present overly general assumptions for the boundary conditions provided by abutment shear keys. This work uses a suite of ground acceleration time histories to compare the code specified assumptions to a more rigorous abutment shear key model that includes the lateral gap between the superstructure and the keys as well as the non-linear material response of the key itself. With the exception of the abutment shear key elements, the bridges are modeled using linear-elastic frame elements, as is typical in practice. Results of a sensitivity analysis are presented for typical parameters, including: record variability, bridge skew, bridge length, and bent heights. The analysis results precede a discussion which demonstrates the necessity for experimental work on this topic to verify the adequacy of the existing design code.

Experimental and Analytical Investigations of Net Section Fracture in Brace-Gusset Plate Connections

In recent years, braced-frame construction has gained considerable popularity for lateral load resisting systems in regions of high seismic activity. Concentrically Braced Frames (CBFs) have been one of the more prominent systems in this classification, relying on the inelastic cyclic buckling and yielding to resist seismic loads and dissipate energy. During tensile cycles, the brace places large demands on the brace-gusset plate, and in turn the gusset plate-beam or column, connections. While failures in this region have not been observed in previous earthquakes, studies suggest that net-section fracture may be a potential mode of failure in these connections. This paper focuses on investigating the inelastic seismic response of typical slotted net section connections of hollow structural sections (HSS) and round pipe under earthquake type monotonic and cyclic loads. Specifically, experimental observations from a Network for Earthquake Engineering Simulation and Research (NEESR) project on nineteen large-scale bracing members in the context of net section performance are presented. In addition to providing insights into behavior, the experiments also serve to validate micromechanics-based modeling approaches that predict ductile fracture. One such approach, utilizing the Void Growth Model (VGM) is discussed and presented as an analytical and general alternative to costly experimentation.