An overview of seismic ground response methods over the world and their applications in Vietnam

The estimation of the impact of earthquakes on buildings and mega structures in large urban areas is of extremely importance. That is why it always gets attentions from construction planners and policy makers who are concerned about construction rules. When earthquake occurs, the vibration is transferred to sites. Although the vibration intensity is at first not too strong, the motion probably becomes stronger and lasts longer under special conditions of the local site. Two famous examples for these effects occurred in Mexico City in 1985 and in Taiwan in 1999. There are a number of approaches to this problem, such as evaluations based on seismic field observations, the microtremor method, the method using the weak motion data, the method using the strong motion data, the one-dimensional wave propagation method or the three dimensional wave propagation method with simulation etc. In this paper, we will give an overview and discuss about the advantages and the disadvantages of the methods that have been commonly applied in the world. We also present the application of these methods in studies carried out in Vietnam in general and in particular, in Hanoi city. We found that the studies for Hanoi city were mainly carried out in the western areas of Hanoi and a few positions in the urban districts. In addition, the authors only gave comments about and assessments of the shear wave velocity, and classified the ground type without a detailed map of local site effects for the entire area of Hanoi. In order to obtain a full site effects evaluation for Hanoi city, future studies should focus on the application of 1D analysis for the central area of Hanoi city and combining 1D analysis with 2D or 3D to give a better picture about the impact of local site effects. This hybrid approach is necessary in order to compare and verify the data obtained by the empirical and the analytical methods. On the other hand, many problems need to be addressed, for instance, the construction of a detailed 3D geological model for Hanoi, the calculation of the dominant periods and the amplification of the local soil conditions for the urban areas.

Simulated Lamb wave propagation method for nondestructive monitoring of matrix cracking in laminated composites

The estimation of the damping coefficient may help to improve the damage detection in composite materials. The purpose of this study was to develop the simulated Lamb wave propagation method for nondestructive monitoring of matrix cracking in laminated composites via the accurate estimation of their damping coefficient. Cross-ply composite specimens with different crack densities were fabricated and tested by the Lamb wave propagation technique. The phase velocity of the Lamb wave and the damping coefficient of the specimens were measured. The finite element models were developed at micro-scale (representative volume elements) and macro-scale (laminated specimens) levels to simulate the Lamb wave propagation in composite specimens. An optimization process was performed through the model updating procedure to achieve finite element models that were in good agreement with experiments. The phase velocity and damping coefficient, obtained from the updated FE models for two crack densities other than those used in the model updating procedure, were successfully examined by experimental results. It was also revealed that the damping coefficient and the rate of increase in the damping coefficient in terms of the crack density were higher for the composite laminates with a higher number of 90° layers. The damping of the fiber–matrix interphase and crack regions were considered in the model and shown as a significant contribution to the overall damping of the composite specimens. The proposed simulated Lamb wave propagation method can be used as a virtual lab for in-situ monitoring of laminated composites with different material properties, stacking sequences, and crack densities.