Evaluation of simulated ground motions for predicting elastic response of long period structures and inelastic response of structures

The response of linear elastic and non-linear hysteretic systems having a single degree of freedom to recorded and simulated ground motions is studied. The objective is to evaluate whether the commonly used simulated motions are appropriate for predicting inelastic response of structures and elastic response of long period structures. Eight simulated motions were generated to model properties of horizontal ground motions recorded during four earthquakes. The simulated motions are sample functions of a stationary, Gaussian white noise process, multiplied by a temporal intensity function and passed through a linear single-degree-of-freedom filter. Two versions, corresponding to parabolic and ‘standard’ base line corrections (BLC), of each of the simulated and recorded accelerograms were considered. The following general conclusions are deduced. Simulated ground motions should be subjected to the standard BLC, because it results in more reliable ground velocities and displacements, which in turn would lead to more reliable predictions of response of long period structures. Furthermore, the spectral density of the underlying random process, from which the simulated motions are obtained, should be modified to be more representative of the frequency content of recorded motions, especially in the low frequency range. Such an improved model can be expected to lead to better agreement, over a broad range of periods, in the average response spectra of simulated and recorded motions, for elastic as well as inelastic systems.     

A note on photogeological mapping of the cretaceous tertiary formations in southern fringes of meghalaya

Mapping of the Cretaceous-Tertiary sedimentary formation in the southern fringes of the Khasi Hills and the adjoining districts of the Shillong Plateau through Photo-interpretation techniques has been found to be fairly accurate and reliable. Their unique photo characters and other recognition elements are briefly described in this paper.     

Effects of reservoir bottom absorption on earthquake response of concrete gravity dams

A procedure is presented to analyse the response of concrete gravity dams due to horizontal and vertical earthquake ground motion components considering dam-water interaction and partial absorption of hydrodynamic pressure waves at the reservoir bottom into the foundation medium. The effects of reservoir bottom absorption on the hydrodynamic force on a rigid dam are examined first. The harmonic response of an idealized dam cross-section is presented for a wide range of parameters characterizing the properties of the dam, the impounded water and the foundation medium. Based on these frequency response functions the effects of dam-water interaction and of reservoir bottom absorption in the response of dams due to horizontal and vertical components of ground motion are investigated.     

Hydrodynamic and foundation interaction effects in frequency response functions for concrete gravity dams

The linear response of idealized dam cross-sections to harmonic horizontal or vertical ground motion is presented for a range of the important system parameters characterizing the properties of the dam, foundation rock and impounded water. Based on these frequency response functions, the separate effects of interaction between the dam and water and interaction between the dam and foundation, and the combined effects of the two sources of interaction, on dynamic response of dams are investigated.     

Hydrodynamic effects in the dynamic response of concrete gravity dams

Hydrodynamic effects in the acceleration response of concrete gravity dams to harmonic ground motion are investigated. The effects include the presence of water, compressibility of water, interaction between the fluid and underlying foundation medium, shape of the fluid domain, and the extent of excitation applied to an infinite fluid domain under vertical ground motion.     

Effects of reservoir bottom absorption and dam-water-foundation rock interaction on frequency response functions for concrete gravity dams

The linear response of an idealized concrete gravity dam monolith to harmonic horizontal or vertical ground motion is presented for a range of the important system parameters that characterize the properties of the dam, foundation rock, impounded water and reservoir bottom materials. Based on these frequency response functions, the effects of alluvium and sediments at the reservoir bottom on the response of the dam, including its interaction with the impounded water and foundation rock, are investigated. It is shown that the partial absorption of hydrodynamic pressure waves by the reservoir bottom materials has an important effect on the dynamic response of concrete gravity dams.     

Earthquake analysis of concrete gravity dams including reservoir bottom absorption and dam-water-foundation rock interaction

The available substructure method for the earthquake analysis of concrete gravity dams, including the dynamic effects of the impounded water and the flexible foundation rock, is extended to include the effects of alluvium and sediments invariably present at the bottom of actual reservoirs. Modelled approximately by a reservoir bottom that partially absorbs incident hydrodynamic pressure waves, these effects are incorporated into the continuum solution for the hydrodynamic pressure. The dam-water-foundation rock system is idealized as a two-dimensional system and analysed under the assumption of linear behaviour. An example earthquake analysis is presented to demonstrate the results obtained from the analytical procedure. Computation times for several cases illustrate the efficiency of the analytical procedure. In particular, the additional computation time required to include reservoir bottom absorption is shown to be very small.     

Earthquake response analysis of multistorey buildings including foundation interaction

An efficient method, based on the Ritz concept, for dynamic analysis of response of multistorey buildings including foundation interaction to earthquake ground motion is presented. The system considered is a shear building on a rigid circular disc footing attached to the surface of a linearly elastic halfspace. In this method, the structural displacements are transformed to normal modes of vibration of the building on a rigid foundation. The analysis procedure is developed and numerical results are presented to demonstrate that excellent results can be obtained by considering only the first few modes of vibration. As the number of unknowns are reduced by transforming to generalized co-ordinates, the method presented is much more efficient than direct methods.     

The earthquake experience at koyna dam and stresses in concrete gravity dams

A number of questions concerning the response of concrete gravity dams to earthquakes, motivated by the structural damage caused to Koyna Dam, which has an unconventional cross section, by the December 1967 Koyna earthquake, are considered in this work. The study is not restricted to the earthquake experience at Koyna Dam, but includes consideration of a dam with a typical section and another earthquake motion having similar intensity but different peak acceleration and frequency characteristics compared to the Koyna ground motion. The earthquake response in a number of cases is analysed by the finite element method and results are presented. These results lead to a number of conclusions. Significant tensile stresses must have developed in Koyna Dam during the Koyna earthquake and similar stresses would have developed even in typical gravity dam sections. The Koyna ground motion is relatively more severe, compared to California earthquakes of similar intensity, on concrete gravity dams. The extra concrete mass near the crest of a gravity dam to support the roadway, etc. is responsible for causing a significant part of the critical tensile stresses; attention should therefore be given to developing lightweight supporting systems.     

Earthquake analysis of arch dams including dam-water-foundation rock interaction

The available substructure method and computer program for the earthquake response analysis of arch dams, including the effects of dam-water interaction, reservoir boundary absorption, and foundation rock flexibility, is extended to include the effects of dam-foundation rock interaction with inertia and damping of the foundation rock considered. Efficient techniques are developed for evaluating the foundation impedance terms, computationally the most demanding part of the procedure.