Implementation of a Novel Inertial Mass System and Comparison to Existing Mass-Rig Systems for Shake Table Experiments

Shake table testing is one of the more effective experimental approaches used to study and evaluate seismic performance of structures. Reduced-scale models can still result in large-scale specimens where incorporating the required inertial mass effectively and safely can be challenging. This study proposes a new system of arranging the mass in the experiments that combines the realism of mass participation during earthquake excitation when supported by the shake table with laboratory practicality considerations of the mass positioned off the specimen. The characteristics and dynamic motion equations for the proposed system are described and applied to shake table experiments involving large-scale cantilevered columns. Using data from large-scale experiments to validate a numerical model, the proposed approach was numerically compared to two other testing approaches. Based on the measured performance and the validated numerical simulations, it can be concluded that the proposed inertial mass system can result in seismic performance as if the mass was placed directly on top of the specimen. Combined with the advantages of reduced setup time, incorporating safety restraints and direct measurement of inertial loads, the proposed system can be suitably used for effective shake table testing of large-scale specimens taken to non-linear near-collapse performance levels.

Effect of skew on the minimum support length requirements of single-span bridges with seat-type abutments

Skew bridges are known to be susceptible to girder unseating during earthquakes, and empirical equations for minimum support length are used in their design. To determine the degree of conservatism or un-conservatism in these equations, a rigorous model of a skewed bridge was developed in OpenSees and validated against a comprehensive dataset from shake table experiments. The validated model was then used to perform a parameter study on a series of single-span, simply supported, prototype bridges with seat-type abutments. Nonlinear response history analyses were performed that included impact and friction effects at the abutments and were repeated for both far-field and near-field motions. It was found that the additional support length required to prevent unseating is a linear function of skew angle over the range 0°–60° for both types of motions. This is contrary to common practice, which uses a quadratic function (American Association of State Highway and Transportation Officials, AASHTO) or the inverse of cosine of angle of skew (Federal Highway Administration (FHWA)). Consequently, common practice appears to be underestimating support length requirements by up to 50% at midrange skew angles of 30°–40°.

Geotechnical Seismic Isolation System Based on Sliding Mechanism Using Stone Pebble Layer: Shake-Table Experiments

Using a shake-table, the effects of several stone pebble layer parameters (the layer thickness, the fraction of pebbles, the pebble compaction, the pebble moisture, the vertical contact stress below the foundation, and the effect of repeated excitations) on layer aseismic efficiency were investigated. For each considered parameter, a model of a rigid building on an aseismic layer was exposed to four different accelerograms, with three levels of peak ground acceleration (PGA), while all other layer parameters were kept constant. For each test, the characteristic displacements and accelerations were measured. Based on the test results, the main conclusions regarding the effect of the considered parameters on the effectiveness of the adopted aseismic layer are given.