ISSN 0970-0137
Current Issue
August- September 2017
Vol. 44 No. 3


Hermite model based estimation of peak factors for tall buildings subjected to wind induced pressures

M. Keerthana and P. Harikrishna



The peak factor for non-Gaussian fluctuating pressures occurring in the flow separated regions of a rectangular tall building has been evaluated based on three translation based Hermite methods, basic Hermite Model (HM), Revised Hermite Model (RHM) and Modified Hermite Model, with parameters evaluated from Yang (MHM-Yang). The evaluated peak factors have been compared with observed values from the data and other Hermite based models of peak factors available in literature. The relative performance of the models towards evaluation of peak factors, especially in the regions of flow separation have been assessed. It has been observed that use of data driven Hermite based models could provide a better and realistic estimate of peak factor, as compared to that offered by Gaussian framework.


A study on compressive strength and corrosion behaviour of reinforcing steel in chloride contaminated fly ash based geopolymer concrete

Sathishraj Mani and Bulu Pradhan



This paper investigates the effect of alkali concentration on compressive strength and corrosion behaviour of reinforcing steel in chloride contaminated geopolymer concrete (GPC) mixes. In this work, fly ash based GPC mixes were prepared with different concentrations of NaOH solution i.e. 8 M and 10 M. Chloride was admixed to GPC mixes at the time of preparation in the form of sodium chloride. Sodium chloride concentrations used were 0%, 1.5% and 3% by mass of geopolymer solids content. Cube specimens and prismatic reinforced concrete specimens were prepared from GPC mixes for determining 7-days compressive strength and corrosion parameters respectively. Potential measurement and corrosion current density by linear polarization resistance (LPR) measurement were carried out on prismatic reinforced concrete specimens at different testing ages. From the results, it is observed that the compressive strength of GPC specimens increased with increase in molarity of NaOH solution. The potential values of steel reinforcement in chloride contaminated GPC specimens were more negative than -270 mV (SCE) for both molarity of NaOH solution, indicating greater probability of occurrence of steel reinforcement corrosion. Further, the corrosion current density of steel reinforcement increased with increase in admixed sodium chloride concentration for both concentrations of NaOH solution. There was an increase in corrosion current density with increase in molarity of NaOH solution i.e. from 8 M to 10 M at all testing ages till 180 days.


Modal identification and damage detection in structures using non-linear and non-stationary vibration response

Timir Baran Roy, Srishti Banerji, Soraj Kumar Panigrahi, Ajay Chourasia, Lucia Tirca and Ashutosh Bagchi



Vibration based method for Structural Health Monitoring (SHM) utilizes the dynamic response of a structure measured using a set of sensors to identify the modal properties and potential damage in the structure. Signal processing tools are widely used for analyzing these response signals. Change in the dynamic characteristics of a structure can provide an indication of damage. However, a direct comparison of the vibration signals at different periods of time may not be sufficient to identify the modal properties and the damages. Therefore, it is important to analyze the vibration signals to extract the modal properties and the morphologies of the changes in these response-signals and correlate them with the types, location and magnitude of structural damage. This present article presents a set of novel techniques for system identification and damage detection in structures based on their vibration signals. The modal parameters are estimated using Frequency Domain Decomposition (FDD) and the response signals are decomposed into intrinsic mode functions (IMF) using empirical mode decomposition (EMD) technique. Those IMFs are then processed with Hilbert-Huang transform (HHT) to obtain their corresponding Hilbert spectra, which allows the estimation of the time-varying instantaneous properties of those response signals. Then a marginal Hilbert spectrum (MHS)-based technique has been applied on the Hilbert spectrum coefficients to calculate associated Damage Indices (DI). The proposed method was tested using experimental tests conducted on a cantilever steel beam prototype at the CBRI laboratory, Roorkee, India, and a three-storey steel frame at Concordia University, Canada. Modal parameters were identified from system identification and the damage locations were determined by comparing the DIs of the damaged steel beam and frame with that of the corresponding baseline (undamaged) structures.


Performance of compliant liquid column damper for seismically excited structures

Soumi Bhattacharyya, Aparna (Dey) Ghosh and Biswajit Bas



The Compliant Liquid Column Damper (CLCD) is a variation of the conventional Liquid Column Damper (LCD) that is applicable for the vibration control of relatively short period structures. Along with the long period structures short period structures are also highly vulnerable to the seismic excitation, as the frequency content of earthquake excitation is generally more broad-banded and richer in high frequency as compared to that of wind excitation. In the present study the equations of motion of the SDOF structural system with attached CLCDare formulated and a time domain parametric study with real earthquake input data is carried out. For a short period model structure the sensitivity of the design damper parameters are investigated and optimum values of the same are identified. Further, an experimental study on a base-excited stiff scaled structural model with CLCDis carried out for white noise and random loading. Identification of designed structural model followed by an experimental sensitivity study of the CLCD design parameters are presented. The optimal values of design parameters are identified and the performance of the optimally designed CLCD is examined for random base excitation. The efficacy of the CLCD for vibration mitigation of short period structures is thereby validated.


Adaptive metamodel based efficient robust design optimization of offshore structure under stochastic wave loading

Gaurav Datta, Soumya Bhattacharjya and Subrata Chakraborty



The present study deals with Robust Design Optimization (RDO) of an offshore steel structure under stochastic wave loading considering parameter uncertainty. An adaptive Moving Least Squares Method (MLSM) based metamodeling strategy has been adopted in place of direct MCS to evade extensive computational time requirement. The optimization problem is formulated to minimize the weight of an offshore deck supporting structure under maximum lateral displacement constraint. The random wave load is modelled by two approaches i.e.1) a spectral representation method with stochastic weighted amplitude wave superposition technique and 2) a simplified approach considering uncertainty in load related parameters. The RDO is formulated by simultaneously optimizing the expected value and variance of the performance function. The robustness in constraint is ensured by adding suitable penalty term to attain a specified target reliability index. The results indicate that by compromising a small increment in structural weight designer can have robust and reliable design solution within affordable computational time by the proposed RDO approach.


Experimental estimation of time variant structural reliability via intelligent sampling

S. D. Sonal and C.S. Manohar



This paper presents a testing protocol developed using the idea of subset simulations, for estimating time variant reliability of dynamical systems subjected to random excitations. The developed testing protocol does not require any prior information about the mathematical model of the test structure, and hence, can be employed in a wide range of contexts. However, this protocol requires that, the metric, which is used in defining the performance function, should be measureable with the help of a set of suitable sensors. The subset simulation method is employed to sample the random excitations in such a way that the reliability estimation is carried out with reduced sampling variance and hence, with reduced test times. The main challenge in the study is to interface the code developed on the Matlab platform (which is used for sampling the random excitations) with the software controlling the actuators, so that the entire test can run in an uninterrupted manner. This method is shown to be well suited for treating linear and a class of nonlinear problems. Illustrative example consists of reliability estimation of an earthquake driven, two-story, geometrically nonlinear steel frame. This frame is tested on a reaction wall based system under excitations specified via random process models. A limited amount of direct ensemble testing is also performed to assess the results obtained using the experimental subset simulations.


Nonlinear system identification using empirical slow flow model

J. Prawin and A. Rama Mohan Rao



Most of the existing SHM damage detection methodologies are based on the changes in the system parameters estimated by the linear model fitted to the structure. However, real-life structures exhibit nonlinearity even in their healthy state due to complex joints and interfaces etc. Nonlinear identification and damage detection is a very challenging inverse engineering problem. In this paper, we present a nonlinear system identification methodology using empirical slow flow model. This nonlinear system identification technique using empirical slow flow model requires only input-output measurement and need not know about the type of nonlinearity present in the system. Numerical simulation studies have been conducted on Duffing oscillator and a beam with breathing crack to demonstrate the effectiveness of the proposed algorithm.


Dynamic hybrid simulation using an electromagnetic shaker

Mohit Verma, Mettupalayam V. Sivaselvan and J. Rajasankar



This paper discusses a controller design strategy for the dynamic hybrid simulation using an electromagnetic shaker. A two-storey shear building is adopted as the system whose response is emulated using hybrid simulation. The bottom storey is taken as the physical subsystem while the top storey is taken as the virtual subsystem. A control law is derived such that the mechanical impedance of the electromagnetic shaker matches to that of the virtual subsystem. The control law is validated by comparing the frequency responses of the virtual subsystem and the electromagnetic shaker from the physical subsystem acceleration to the force transferred. Finally, the experimental validation of the controller design strategy is carried out by the dynamic hybrid simulation of the two storey structure. The frequency response obtained from the experiment and emulated system are found to be in good agreement with each other.

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