A. Meher Prasad

Embedded foundation in layered soil under dynamic excitations

Abstract The critical step in the substructure approach for the soil–structure interaction (SSI) problem is to determine the impedance functions (dynamic-stiffness coefficients) of the foundations. In the present study, a computational tool is developed to determine the impedance functions of foundation in layered soil medium. Cone frustums are used to model the foundation soil system. Cone frustums are developed based on wave propagation principles and force-equilibrium approach. The model is validated for its ability to represent the embedded foundation in layered medium by comparing the results with the rigorous analysis results. Various degrees of freedom, such as, horizontal, vertical and rocking are considered for this study.

Fatigue reliability analysis in time domain for inspection strategy of fixed offshore structures

The paper describes a methodology for estimation of reliability of fixed offshore structures with respect to fatigue and extreme stress. The failure criteria for fatigue are formulated using fracture mechanics principle and that for extreme stress using API Code. Advanced Second Moment method is used to find the reliability index. The total life of the structure is divided into a set of stationary sea-states, occurring during storms and described by directional power spectrum. The method has been illustrated through application to a typical jacket platform. Usefulness of the methodology for planning in-service inspections has been highlighted.

Development of a Simplified Damage Model for Beams Aiding Performance Based Seismic Design

Performance based design is an emerging area in earthquake engineering in which damage modeling of structural components plays an important role in achieving the performance objectives. Past researches in damage index evaluation indicated no obvious shear dependent trends on cyclic combined shear and flexure. Paper focuses in finding a simple and realistic indicator, which gives a reliable measure of the structural damage, and use it in the design stage calculations of medium and long period structures. Experiments consisting of monotonic and cyclic tests were conducted on a variety of concrete beams. Two available models for damage indices in the literature, namely Park and Ang model and the model suggested by Rao et al. (1998), were used to predict the damage indices. A simple set of relationships connecting damage index to the failure cyclic ductility ratio, shear span to depth ratio and cyclic amplitude have been derived. The above relationships have been validated on the test data generated during the present investigation. A procedure to evaluate the demand-monotonic ductility ratio at failure corresponding to a given state of damage and the shear span to depth ratio has been developed from the proposed curves.

Hysteresis Model for RC Structural Element Accounting Bi-Directional Lateral Load Interaction

Experimental investigations on cyclic behaviour of reinforced concrete (RC) structures have demonstrated that, bidirectional flexural deformation is much more, compared to unidirectional responses separately. Biaxial lateral load interaction, therefore, has a significant role on the dynamic responses of structure. The present paper projected a new formulation of the biaxial force-deformation model, accounting strength and stiffness degradation in a rather simplistic manner. Also the modelling scheme dose not requires much computational rigour. The present effort is based on yield surface approach of biaxial modelling scheme. The validity of the proposed model is appraised through available biaxial test of RC column. Compared to other existing models, present model requires simple input parameters and the prediction of hysteretic responses are quite faithful. In the RC containment structures, which are the prime concern to the safety of nuclear power plant, this model may pertain to evaluate the hysteretic responses with fair accuracy during seismic loading.Copyright

Glass Fibre Reinforced Gypsum Panels for Sustainable Construction

The tremendous housing need of India is causing the depletion of virgin building materials. In an era of scarce resources, sustainable solutions are always preferable. Glass Fibre Reinforced Gypsum (GFRG) technology is one such solution where all the structural members are constructed using hollow panels infilled with reinforced concrete (RC), as per structural design. This paper discusses the sustainability of construction using GFRG panels in terms of embodied energy and indoor thermal comfort. The case study building at IIT Madras campus was found to have lesser embodied energy compared to conventional buildings. The result is significant considering the fact that the building sector consumes 40% of the total energy in world. Also, the inside temperature was found to be lesser by 2 °C which will help in reducing the air conditioning requirement of the building and thereby the operational energy.

Seismic Design Philosophy: From Force-Based to Displacement-Based Design

The Indian subcontinent has experienced several earthquakes, and the major ones have caused significant damage to the buildings and lifeline structures. Researchers have realized the importance of developing a robust design method which can reduce the probability of occurrence of such damages. This paper discusses the currently followed seismic analysis techniques, their applicability and limitations, as well as the newly emerging displacement-based design methods. There is a need for a paradigm shift from the conventional force-based approach to the more rational displacement-based design methods to limit damages and to get uniform risk structures. Researchers are now focusing on the refinement of these new design methods to make them applicable to all types of buildings, bridges, and other structures.

Seismic Evaluation of RC Stepped Building Frames

‘Stepped building’ frames, with vertical geometric irregularity, are now increasingly encountered in modern urban construction. This chapter proposes a new method of quantifying irregularity in such building frames, accounting for dynamic characteristics (mass and stiffness). The proposed ‘regularity index’ provides a basis for assessing the degree of irregularities in a stepped building frame. This chapter also proposes a modification of the code-specified empirical formula for estimating fundamental period for regular frames, to estimate the fundamental time period of the stepped building frame. The proposed equation for fundamental time periods is expressed as a function of the regularity index. It has been validated for various types of stepped irregular frames. A new approach to determine the lateral load pattern, considering the contributions from the higher modes, is proposed that is suitable for pushover analysis of stepped buildings. Also, a modification to the displacement coefficient method is proposed, based on time history analysis of 78 stepped frames. When the newly proposed load pattern is combined with the modification of the displacement coefficient method of FEMA 356, the target displacement for the stepped building frame is found to match consistently the displacement demand given by the time history analysis.

A Practical Solution of the Random Eigenvalue Problems using Factorized Decomposition Technique

This paper presents a practical solution for probabilistic characterization of real valued eigenvalues of positive semi-definite random matrices. The method involves a novel dimension reduction technique that facilitates a lower-dimensional approximation of a high dimensional problem. The present method is basically founded on the idea of high dimensional model representation (HDMR) technique. HDMR is a multivariate representation to capture the input-output relationship of a physical system with many variables. It is a very efficient formulation of the system response, if higher-order variable correlations are weak, allowing the physical model to be captured by the first few lower-order terms. Practically for most well-defined physical systems, only relatively low order correlations of the input variables are expected to have a significant effect on the overall response. HDMR expansion utilizes this property to present an accurate hierarchical representation of the physical system. The method involves multiplicative decomposition of a multivariate eigenfunction into multiple one-dimensional eigenvalues.

Efficient Reliability Analysis Using Multipoint Response Surface Method

In the reliability analysis of complex structural system, to approximate the performance function accurately, the most common approach is Response Surface Method. Response surface is usually constructed around the design point, for better approximation of the performance function. Existence of multiple design points accelerates the difficulty of approximating the performance function. This paper presents i) a simplified approach for identifying the existence of multiple design points/or regions on the limit surface which have significant contributions to the failure probability and ii) a newly developed concept of multipoint response surface. Identification of multiple design points is based on a weight function, which can provide the weighting index of sampling points. Failure probability can be estimated by constructing multipoint response surface around the identified pints. In addition to the effort of identifying the region of importance, the method requires a small number of exact or numerical evaluations of the performance function at selected inputs. Numerical examples show the accuracy and efficiency of the proposed approach.Copyright

Adaptive Finite Element Analysis of Kinematically Non-Linear Elasto-Plastic Problems in Two Dimensions

ABSTRACT The need for today’s industrial problems is efficiency, and accuracy in analysis and design. The characteristics of these types of problems extend considerably beyond the linear elastic limit, necessitating the use of both kinematic and material nonlinearities. This paper investigates adaptive finite element analysis of kinematically and materially nonlinear problems in two dimensions and the implementation issues in High Performance Computing (HPC) platform under the distributed computing environment for industrial applications.