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CONTENTS
Volume 6, Number 4, December 2019
 

Abstract
This study investigated experimentally the response of thick reinforced concrete specimens having hollow cores with ‎critical parameters. The investigation includes testing of twelve specimens that are solid and hollow-core slab models. ‎Each specimen consists of two pieces, the piece dimensions are (1.2 m) length, (0.3 m) width and (20 cm) thickness tested ‎under both monotonic and repeated loading. The test program is carried out to study the effects of load type, core ‎diameters, core shape, number of cores, and steel fiber existence. Load versus deflection at mid span, failure modes, and ‎crack patterns were obtained during the test. The test results showed that core shape and core number has remarkable ‎influenced on cracking pattern, ultimate load, and failure mode. Also, when considering repeated loading protocol, the ‎ultimate load capacity, load at yielding, and ductility is reduced.

Key Words
monotonic load; repeated load; reinforced concrete; hollow-core slab

Address
Ibrahim N. Najm, Raid A. Daud and Adel A. Al-Azzawi: Civil Engineering Department, College of Engineering, Al-Nahrain University, Baghdad

Abstract
Fatigue cracks are inevitable in circumstances in which the cyclic loading exists. Therefore, many of mechanical components are in a risk of being in exposure to fatigue cracks. On the other hand, renewing the facilities or infrastructures is not always possible. Therefore, retrofitting the structures by means of the available methods, such as crack arrest methods is logical and in some cases inevitable. In this regard, this paper considers three popular crack arrest methods (e.g., drilling stop-hole, steel welded patch, and carbon fiber reinforced (CFRP) patch), which have been compared by using extended finite element method (XFEM). In addition, effects in terms of the width and thickness of patches and the configuration of drilling stop holes have been evaluated. Test results indicated that among the considered methods, CFRP patches were the most effective means for arresting cracks. Besides, in the case of arresting by means of drilling stop holes, drilling two holes next to the crack-tip was more effective than blunting the crack-tip by drilling one hole. In other words, the results indicated that the use of symmetric welded metal patches could lead to a 21% increase in fatigue life, as compared to symmetric stop holes. Symmetric CFRP patches enhanced the fatigue life of cracked specimen up to 77%, as compared to drilling symmetric stop holes. In addition, in all cases, symmetric configurations were far better than asymmetric ones.

Key Words
fatigue crack; CFRP and steel patch; crack arrest; stop-hole; retrofit

Address
Mohammad Karamloo, Moosa Mazloom and Ali Ghasemi: Department of Civil Engineering, Shahid Rajaee Teacher Training University, Lavizan, Tehran, Iran

Abstract
Performance assessment of pavements proves useful, in terms of handling the ride quality, controlling the travel time of vehicles and adequate maintenance of pavements. Roughness profiles provide a good measure of the deteriorating condition of the pavement. For the accurate estimates of pavement roughness from dynamic vehicle responses, vehicle parameters should be known accurately. Information on vehicle parameters is uncertain, due to the wear and tear over time. Hence, condition monitoring of pavement requires the identification of pavement roughness along with vehicle parameters. The present study proposes a scheme which estimates the roughness profile of the pavement with the use of accurate estimates of vehicle parameters computed in parallel. Pavement model used in this study is a two-layer Euler–Bernoulli beam resting on a nonlinear Pasternak foundation. The asphalt topping of the pavement in the top layer is modeled as viscoelastic, and the base course bottom layer is modeled as elastic. The viscoelastic response of the top layer is modeled with the help of the Burgers model. The vehicle model considered in this study is a half car model, fitted with accelerometers at specified points. The identification of the coupled system of vehicle-pavement interaction employs a coupled scheme of an unbiased minimum variance estimator and an optimization scheme. The partitioning of observed noisy quantities to be used in the two schemes is investigated in detail before the analysis. The unbiased minimum variance estimator (MVE) make use of a linear state-space formulation including roughness, to overcome the linearization difficulties as in conventional nonlinear filters. MVE gives estimates for the unknown input and fed into the optimization scheme to yield estimates of vehicle parameters. The issue of ill-posedness of the problem is dealt with by introducing a regularization equivalent term in the objective function, specifically where a large number of parameters are to be estimated. Effect of different objective functions is also studied. The outcome of this research is an overall measure of pavement condition.

Key Words
pavement roughness; condition assessment; minimum variance estimator; optimization

Address
O.A. Shereena and B.N. Rao: Department of Civil Engineering, Indian Institute of Technology Madras, Chennai 600036, Tamil Nadu, India

Abstract
Two seismic response data from the CSMIP strong motion instrumentation of Pacoima dam are selected: San Fernando earthquake (Jan 13, 2001; ML=4.3) and Newhall earthquake (Sept. 1, 2011; ML=4.2), for the identification of the dam system. To consider the spatially nonuniform input ground motion along the dam abutment, the subspace identification technique with multiple-input and multiple-output is used to extract the dynamic behavior of the dam-reservoir interaction system. It is observed that the dam-reservoir interaction is significant from the identification of San Fernando earthquake data. The influence of added mass (from the reservoir) during strong ground motion will create a tuned-mass damper phenomenon on the dam body. The fundamental frequency of the dam will be tuned to two different frequencies but with the same mode shapes. As for the small earthquake event, the dam-reservoir interaction is insignificant.

Key Words
subspace identification; stabilization diagram; dam-reservoir interaction

Address
I-No Yu: Department of Civil Engineering, National Taiwan University, Taipei 10617, Taiwan
Shieh-Kung Huang: Department of Civil Engineering, National Taiwan University, Taipei 10617, Taiwan;
National Center for Research on Earthquake Engineering, NARLabs, Taipei, Taiwan
Kenneth J. Loh: Department of Structural Engineering, University of California-San Diego, La Jolla, CA 92093, USA
Chin-Hsiung Loh: Department of Civil Engineering, National Taiwan University, Taipei 10617, Taiwan;
Department of Structural Engineering, University of California-San Diego, La Jolla, CA 92093, USA



Abstract
This study aims to provide useful information on the fatigue assessment of a top-tensioned riser (TTR) subjected to vortex-induced vibration (VIV) by performing parametric study. The effects of principal design parameters, i.e., riser diameter, wall thickness, water depth (related to riser length), top tension, current velocity, and shear rate (or shear profile of current) are investigated. To prepare the base model of TTR for parametric studies, three (3) riser modelling techniques in the OrcaFlex were investigated and validated against a reference model by Knardahl (2012). The selected riser model was used to perform parametric studies to investigate the effects of design parameters on the VIV fatigue damage of TTR. From the obtained comparison results of VIV analysis, it was demonstrated that a model with a single line model ending at the lower flex joint (LFJ) and pinned connection with finite rotation stiffness to simulate the LFJ properties at the bottom end of the line model produced acceptable prediction. Moreover, it was suitable for VIV analysis purposes. Findings from parametric studies showed that VIV fatigue damage increased with increasing current velocity, riser outer diameter and water depth, and decreased with increasing shear rate and top tension of riser. With regard to the effects of wall thickness, it was not significant to VIV fatigue damage of TTR. The detailed outcomes were documented with parametric study results.

Key Words
vortex-induced vibration (VIV); fatigue; top-tensioned riser (TTR); parametric study; offshore

Address
Do Kyun Kim: Marine Offshore and Subsea Technology (MOST) Group, School of Engineering,
Newcastle University, NE1 7RU Newcastle upon Tyne, UK;
Graduate Institute of Ferrous Technology, POSTECH, 37673 Pohang, Republic of Korea
Eileen Wee Chin Wong and Mala Konda Reddy Lekkala: Ocean and Ship Technology Research Group (Department of Civil and Environmental Engineering),
Universiti Teknologi PETRONAS, 32610 Seri Iskandar, Perak, Malaysia



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