Suite Of Earthquake Research Articles

Seismic retrofit of low-rise structures using rotational viscoelastic dampers

In this research, the efficiency of a rotational viscoelastic seismic energy dissipation device is evaluated in a theoretical framework. Unlike conventional dampers which block the bays, the main advantage of this damper lies within its vertical installation scheme that provides open space in the bay and architectural flexibility. The proposed damper consists of a steel column with viscoelastic hinges at both ends that dissipate the seismic energy. Conventional viscoelastic dampers usually act under translational deformation, whereas it acts under rotational deformations in the considered scheme. The theoretical foundation is laid for the proposed damper and is verified by comparing the results with the analytical model of the damper. The developed device is proven to be versatile in terms of stiffness and energy dissipation which can be easily adjusted. The application and efficiency of this damper is validated by retrofitting a soft-first-story structure to fulfill the enhanced performance objective. The performance of the structure under a suite of earthquakes is compared before and after the seismic retrofit in terms of maximum interstory drift ratio and residual displacement.

Centrifuge Modeling of the Seismic Behavior of Soft Clay Slopes

This paper presents the experimental results and analysis from two centrifuge experiments that simulated the seismic response of a gentle slope in soft clay. The two models consisted of a three-degree and a six-degree slope in soft clay, respectively, which are representative of typical slopes found on marine seabeds on the continental margins. The models were built in a laminar container in order to reproduce infinite slope boundary conditions. In-flight characterization investigations consisting of T-bar tests and air hammer tests were performed to obtain undrained shear strength profiles and shear wave velocities at various depths, respectively. A suite of earthquakes was applied, including sinusoidal waves and scaled real motions, in order to observe the response of the models in terms of the propagation of shear waves and the generation of lateral displacements at various depths in the slopes. The results showed that the model preparation approach ensures the repeatability of the experiments, enabling the evaluation of the impact of the slope angle on the seismic response of the gentle slopes studied. On average, the permanent displacements measured at the surface of the six-degree slope were three times greater than those measured at the top of the three-degree slope. In these slopes, nonlinear effects were observed in terms of the peak ground acceleration (PGA) that depended both on the slope angle and the intensity of shaking.

Clutching inerters enhanced tuned mass dampers for structural response mitigation under impulsive and seismic excitations

Inerters, a type of two-terminal inertial elements, have significantly enhanced the performance of conventional control devices. The clutching inerters are especially appealing since they take energy away from the structural system without transferring back. In this study, a clutching inerter enhanced tuned mass damper (TMDCI) is proposed. First, the formation of the TMDCI is described based on a tuned mass damper inerter and two clutching flywheels. Then, the equations of motion are derived for the TMDCI on a single-degree-of-freedom (SDOF) primary structure. Subsequently, the modal properties of the TMDCI system are revealed by examining the steady-state responses under harmonic ground excitations. Afterward, performance analyses are conducted for the TMDCI on SDOF and three-DOF steel structures when subjected to initial velocities. The frequency robustness and parametric sensitivity of the TMDCI are evaluated on the SDOF structure, whereas practical concerns regarding the hosting location and stiffness demand of the TMDCI are addressed on the three-DOF structure. Finally, the seismic performance of the TMDCI is investigated on the three-DOF structure under a suite of earthquakes. The results show that the TMDCI exhibits a large damping effect which can reduce responses in both the structure and device and maintain strong robustness against frequency uncertainties. This study demonstrates the great potential of TMDCIs as an effective and affordable control strategy for the seismic protection of structures.

Cyber‐physical approach to the optimization of semiactive structural control under multiple earthquake ground motions

AbstractThis paper presents a cyber‐physical approach to optimize the semiactive control of a base‐isolated structure under a suite of earthquakes. The approach uses numerical search algorithms to guide the exploration of the design space and real‐time hybrid simulation (RTHS) to evaluate candidate designs, creating a framework for real‐time hybrid optimization (RHTO). By supplanting traditional numerical analysis (i.e., finite element methods) with RTHS, structural components that are difficult to model can be represented accurately while still capturing global structural performance. The efficiency of RTHO is improved for multiple design excitations with the creation of a multiinterval particle swarm optimization (MI‐PSO) algorithm. As a proof‐of‐concept, RTHO is applied to improve the seismic performance of a base‐isolated structure with supplemental control. The proposed RTHO framework with MI‐PSO is a versatile technique for multivariate optimization under multiple excitations. It is well suited for the accurate and rapid evaluation of structures with nonlinear experimental substructures, in particular, those that do not undergo permanent damage such as structural control devices. The RTHO framework integrates popular optimization algorithms with advanced experimental methods, creating an exciting new cyber‐physical approach to design.

Synthetic lava brings eruption into the lab

In early May, Kilauea, a volcano on the southeast corner of Hawaii’s Big Island, began erupting, and a suite of earthquakes, the largest with a 6.9 magnitude, shook the island. The quakes caused fissures and cracks to open in the ground, and lava has been pouring down the flanks of the volcano ever since. The exact path the lava has taken has been as unpredictable as it has been destructive. As C&EN went to press, the encroaching lava had destroyed 668 homes and had flowed over at least 26 sq km, reaching the sea. Monitoring the damage from the red-hot rock is hazardous, to say the least. But that is what a number of scientists from the U.S. Geological Survey are doing, both on the summit and on the lower East Rift Zone, with help from drones. Although this on-site data collection is invaluable to scientists seeking new information about

Damage controlled optimum seismic design of reinforced concrete framed structures

In this paper, an innovative procedure is proposed for the seismic design of reinforced concrete frame structures. The main contribution of the proposed procedure is to minimize the construction cost, considering the uniform damage distribution over the height of structure due to earthquake excitations. As such, this procedure is structured in the framework of an optimization problem, and the initial construction cost is chosen as the objective function. The aim of uniform damage distribution is reached through a design constraint in the optimization problem. Since this aim requires defining allowable degree of damage, a damage pattern based on the concept of global collapse mechanism is presented. To show the efficiency of the proposed procedure, the uniform damage-based optimum seismic design is compared with two other seismic design procedures, which are the strength-based optimum seismic design and the damage-based optimum seismic design. By using the three different seismic design methods, three reinforced concrete frames including six-, nine-, and twelve-story with three bays are designed optimally under a same artificial earthquake. Then, to show the effects of the uniform damage distribution, all three optimized frames are used for seismic damage analysis under a suite of earthquake records. The results show that the uniform damage-based optimum seismic design method renders a design that will suffer less damage under severe earthquakes.

A microseismicity study of the Queen Charlotte Islands region

Nineteen land and three ocean-bottom seismographs were operated in the Queen Charlotte Islands region for periods of up to 9 weeks and 5 days, respectively, during the summer of 1983. Three hundred and seventeen seismic events were detected. One hundred and nine earthquakes ranging in size from magnitude −0.5 to 5.1 were well recorded at three or more stations and could be accurately located. Of these, 84 lie on or close to the Queen Charlotte Fault, most within the rupture zone of the great 1949 earthquake (MS = 8.1). The seismic gap, between the rupture zones of the 1949 event and the 1970 earthquake (MS = 7.4) that occurred just south of the Queen Charlotte Islands, exhibited little activity. Eighteen earthquakes, the largest with ML = 3.8, were located east of the fault on northern Graham Island or in adjacent Hecate Strait. Focal depths were generally less than 20 km, and none could be associated with known faults. Composite focal mechanism solutions were obtained for four suites of earthquakes along the Queen Charlotte Fault and for a group east of the fault zone on northern Graham Island. In all cases the solutions indicate thrust mechanisms with the predominant orientation of pressure axes northeast–southwest. The presence of thrust faulting close to the Queen Charlotte Fault suggests that the microseismicity is not occurring on the main transcurrent fault but on subsidiary faults that are moving due to the regional stress regime. Thrust faulting on northern Graham Island can best be interpreted as reflecting the stress field from a locked Pacific and North American boundary.