Abstract

Radiating boundary conditions are essential in the numerical analysis of infinite domains such as dynamic SoilStructure-Interaction analysis. Local absorbing boundary conditions proposed by Lysmer and Kuhlemeyer (1969) are widely used in SSI analysis. These boundary conditions are developed based on the wave propagation in elastic materials. However, almost all soil materials exhibit viscoelastic properties. Badry and Ramancharla (2018) proposed the Absorbing Boundary Conditions for wave propagation in viscoelastic materials (VABC). The VABC conditions were developed from the original Absorbing Boundary Conditions (ABC) by including a spring in addition to the dashpot. In fact, these boundary conditions are inefficient when the waves impinge other than normal direction. In this paper, the radiating boundary conditions are modelled using VABC and ALID (Absorbing Layers by Increase in Damping) together to include the advantages of both the boundary conditions and to overcome the drawback of ABC on angle incidence. A detailed numerical analysis is carried out to verify the boundary conditions. The efficiency of the proposed method has been verified for both 1D and 2D wave propagation problems. The efficiency on angle incidence is verified using 2D plane strain model response through spatial Fourier Transformation. The study concludes that the proposed method is simple and reliable method for modelling radiating boundary conditions. Keywords: Local Absorbing Boundaries, Viscoelastic wave propagation, Rayleigh damping, Soil-Structure interaction, Absorbing Layers by Increase in Damping (ALID)

Abstract

Damage index models are used for quantification of damage in any numerical model of a building. While using any particular damage model there is high possibility that it might give similar result for two identical structures, each having different attributes. So, though currently available damage index models precisely quantify damage in the structure but fails to identify the reason or attributes/irregularities present in it. Therefore, for quantitative assessment, a unique aspect should be used which will depend on structural as well as architectural features. This paper presents new method for quantification of damage for reinforced concrete (RC) moment resisting framed (MRF) structures using the pattern of hinge formation in structure when it is subjected to a monotonic loading. A two-dimensional RC MRF structures with different building irregularities were modeled. As a primary step to estimate the damage from the pattern of hinge formation, it is necessary to understand and study the relation between damage and hinge pattern. For this purpose, energy-based damage index model is used. The relation between damage index and pattern of hinge formation is studied with the help of regression analysis. A multiple regression analysis is performed taking damage index as dependent variable and the number of hinges formed in each damage state as independent variables. The paper also discusses two different approaches for regression analysis determining the corresponding coefficients. The regression analysis resulted in an equation which can approximately quantify the damage in the structure using total number of hinges formed in each structural member.

Abstract

In last two decades, operational modal analysis is widely used for modal identification of structures which uses ambient responses of structure for determining the modal parameters and Frequency Domain Decomposition is one of the most popular method, but seismic excitations and high damping values of structures do not satisfy the assumptions of this classical method. The dynamic modal parameters derived from earthquake responses are more reliable than those from small amplitude responses since dynamic properties of civil structures are dependent on amplitude. In recent years, a new algorithm known as refined Frequency Domain Decomposition (rFDD) is developed which uses structural response to strong ground motions that can be short in duration and is applicable for structures with light to heavy damping. The refined Frequency Domain Decomposition (rFDD) algorithm was initially developed using synthetic seismic response signals as input and later real strong motion response records were used to demonstrate the effectiveness of this algorithm for modal dynamic identification of structures under real earthquake excitations. This method is well developed to counter the problems that normally arise in modal identification and in cases of heavy damping and non-stationarity of the signal, making it lengthy and tedious to process. In this paper the main feature of this method which is Integrated Power Spectral Density matrix computation is taken and applied on mild earthquakes responses of structure. The modal parameters identified are then compared with target values obtained from classical FDD method using ambient responses and interpreted, presenting a case for usage of rFDD in simplified manner in modal dynamic identification of structures. Keywords: Operational Modal analysis, mild earthquake response, refined Frequency Domain Decomposition

Abstract

An earthquake Mw7.8 occurred on 25 April 2015 in a seismically dominant area of Nepal. No major earthquakes (>Mw7.0) were recorded in the past two decades, though Nepal is one of the active seismic regions in the world. The combined effect of the main event and the aftershocks, which are temporally distributed over 40 days, has led to a huge devastation, killing thousands of people in the epi-central area. A complete seismic catalog and influencing parameters such as type of fault, epi-central/hypo-central distance, type of soil are required for a region to assess seismic hazard scenario. The seismic hazard can be represented in terms of Peak Ground Acceleration (PGA), which is the most widely used parameter in strong motion studies. The main objective of this paper is to understand the characteristics of ground motion of the 2015 Nepal earthquake. Since the available ground motion records are limited, an attempt has been made to generate ground motion records using a modified semi-empirical approach. Contour maps of PGA and intensity have also been generated. It is observed that around 22% of buildings experienced an intensity of VI on MMI scale at Bageswari mandal. A few buildings have experienced an intensity of VII in Thankot, Seuchatar, Satikhel, Mahadevathan, Indrayani, Chhaimale, and Balambu.

Abstract

Development of Force-based seismic design is historical, and every earthquake has thrown new challenges for already existing assumptions and principles, which helped in the evolution of force-based design. Although many of the assumptions were answered but some remained unaltered and unanswered. Hence, this paved way for developing new theories and philosophies and one such theory is Direct Displacement-Based seismic Design (DDBD). Indian Standard codes, IS 1893 and IS 13920, underwent a major revision in the year 2016, considering the latest developments in Force based design principles. Current work is an attempt to understand the performance comparison between latest IS 1893 & IS 13920 with DDBD method. Initially, three buildings i.e., 3, 6 and 9 storey buildings were designed using DDBD at inter-storey drift of 4% with a performance objective to achieve “No collapse”. The spectrum developed using IS code equations does not represent ideal displacement spectrum shape and hence PGA, T and CA dependent equations are used for DDBD methodology of design. PGA of 0.36g is used in the current study which represents the highest seismic zone in India. Later these buildings were subjected to lateral displacement using displacement-based pushover analysis method. The inelastic strength along with drifts achieved by DDBD are compared with the IS code compliant buildings. From the comparison, it was observed that inelastic strength achieved by 6,9 storey buildings designed according to FBD are greater than the buildings designed according to DDBD at “No Collapse”. For 3 storey structure, the strength achieved by FBD is less than DDBD. If zone is reduced from Zone V to IV or PGA is increased from 0.36g, then inelastic strength achieved by DDBD with performance objective “No Collapse” would be more than FBD requiring revision of response reduction values for different categories of buildings or PGA values in respective seismic zones. The inelastic drifts achieved by both the methods are comparable only for 3 storey structure.

Abstract

Twisting of a building about vertical axis increases the shear force demand on lateral force resisting elements and it is not desirable as increased shear force results in brittle failure. Generally, twisting is induced in a building due to eccentricity between centre of Mass (CM) and centre of Stiffness (CS) at diaghragm level. Eccentricity can be induced in a structure due to mass, strenght and stiffness arising out of various construction and design limitations. Code insists to apply design forces calculated according to equivalent static method or response spectrum method at displaced center of Mass (CM) so as to cause design eccentricity between CM and CS. The displaced center of mass resulting from design eccentricity consists of two terms static eccentricity and accidental eccentricity. Also, earthquake ground motion has the ability to induce torsion in a structure. Hence in order to account for all the eccentricities, code suggests 5% accidental eccentricity to be included in design eccentricity. In current work, an adjustment of mass eccentricity is verified. For this purpose, three one storey models are created with 5 % unidirectional mass eccentricity (CM), 5% unidirectional stiffness eccentricity (CS) and 5% unidirectional Mass-Stiffness eccentricity (CM-CS). Linear Increment Dynamic Analysis is performed by considering chamoli earthquake and edge displacements are obtained. Later, twist is calculated based on edge displacements. It is concluded that the same amount of mass and stiffness eccentricity (i.,e 5%) , the twist incurred in stiffness eccentric model is signifcantly higher whereas no twist about vertical axis is noticed in mass eccentric model unless mass eccentricity has significant impact on torsional response, the torsional response of combined CM-CS eccentric model is approximately equal to CS eccentric model. The results conclude that universal adjustment of center of mass will not provide the assumed conservativeness of safety against torsion. Apart from this, few countries guidelines like New Zealand [NZS, 2004] has revised accidental eccentricity from 5% to 10%. Hence, design eccentricity needs revision considering the statistical evaluation of various paramters inducing accidental eccentricities , without adjusting mass centre for every eccentricity.

Abstract

Modelling the Sommerfeld radiating boundary is a major challenge appearing in the study of many engineering problems involving elastic wave propagation [1]. Particularly, in the finite element analysis of Structure-Soil-Structure-Interaction (SSSI) problems, where the soil modelling has to be truncated at a finite distance. This truncation of the model at finite boundary leads to reflection of radiating elastic waves. The reflected waves from the boundary will affect the solution and may lead to instabilities in the numerical analysis. Therefore, it is necessary to provide an artificial boundary condition that will transmit the outward propagating waves with minimum or negligible reflections. To address the radiating boundary condition problem, researchers have developed various kinds of formulations over few decades, such as: a). Local Absorbing Boundary Conditions (ABC) [2], b). Absorbing Layers techniques which includes Perfectly Matched Layers [3-4], Caughey Absorbing Layer Method (CALM) [5], Absorbing Layers by Increasing Damping (ALID) [6] c). Boundary element method [7], and d). Infinite elements [8-10]. ABC corresponds to the situation where the boundary is supported on infinitesimal dash-pots oriented normal and tangential to the boundary. Though these boundary conditions are very simple, the major drawback is they are inefficient when the wave impinges the boundary in inclined direction. The key property of a PML that distinguishes it from an ordinary absorbing material is that it is designed such that the propagating waves incident upon the PML from a bulk medium do not reflect on the interface. The CALM consists of defining the absorbing layers at the boundaries of the elastic medium under consideration but requires many absorbing layers which requires much computational time. The objective of this study is to present a method for radiating boundary conditions by using modified ABC to take the effect of viscous damping combining with ALID

Abstract

Recent revision of Indian standard code on earthquake resistant design suggested certain recommendations for inclusion of effects of vertical ground acceleration for design of buildings. Clause 6.3.2.1 of IS 1893 (Part 1) suggests that for non-orthogonal frames, load combinations should include 30% of earthquake load in other directions along with the direction considered and Clause 6.3.2.1.1 suggests that for orthogonal frames, full load has to be applied. Besides, clause 6.3.3 mentions that all the buildings present in seismic zone IV and V have to consider vertical ground motion effects. This results in 73 load combinations for calculating the design forces in structural members. However, the necessity to consider the effects of vertical ground motions for regular buildings, although present in seismic zones IV and V, is not verified. To verify the above clauses for regular buildings, a case study is performed on a 5 storied regular framed RC building. Using, linear static analysis, design forces in structural members for the contradicting clauses on load combinations are compared. From the study, it is concluded that 3-dimensional ground motion effects are not necessary for every building even if the building is located in seismic zone IV or V.

Abstract

Radiating boundary condition is an important consideration in the finite element modelling of unbounded media. Absorbing layer techniques such as Perfectly Matched Layers (PML) and Absorbing Layers by Increasing Damping (ALID) becoming popular as they are efficient in absorbing outward propagating waves energy. In this study, a comparative analysis has been carried out between PML and ALID+VABC (Absorbing Boundary conditions for Viscoelastic materials) methods. The methods are analyzed using LSDYNA explicit solver and the efficiency is compared with standard solutions. The study concluded that PML requires less number of elements to model the boundary conditions when compared with ALID+VABC. But PML requires a smaller element length which increases overall computational time. Both the methods are efficient in absorbing the wave energy. However, PML requires additional implementation cost to solve the complex equations.

Abstract

This paper is concerned with the study of effect of opening in infills on RC framed structures. Reinforced concrete buildings with masonry infill walls have been widely constructed for commercial, industrial, buildings. In this paper presents the results of analytical program showing behaviour of frames with different masonry infill i.e semi-interlocked masonry & Unreinforced masonry. The effect infill opening on seismic response of frame buildings were evaluated. A nonlinear static analysis has been carried out on different model frames by using finite element analysis software, seismostruct. The results obtained show that calculation of response reduction factor of RC infilled frames with & without opening on infill.