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CONTENTS
Volume 37, Number 2, January25 2011
 

Abstract
The logarithmic decrement method has been long used to estimate damping ratios in systems with only one modal component such as linear single degree of freedom (SDOF) mechanical systems. This paper presents an application of a methodology that uses joint time-frequency distribution (JTFD) as input, instead of the raw signal, to systems with several vibration modes. A most important feature of the present approach is that it can be applied to a system with time-varying damping ratio. Initially the precision and robustness of the method is determined using a synthetic model with multiple harmonic components, one of them displaying a time-varying damping ratio, subsequently the results obtained from experiments with a reduced model are presented. A comparison is made between the results obtained with this methodology and those using the classical technique of Least Squares Complex Exponential Method (LSCE) in order to highlight the advantages of the former, such as, good precision, robustness and excellent performance in extreme cases, e.g., when very low frequency components and time varying damping ratio are present.

Key Words
damping; modal damping; joint time-frequency distribution (JTFD), least squares complex exponential method (LSCE).

Address
H. Bucher: Department of Civil Engineering, COPPE/Federal University of Rio de Janeiro, CP 68506, CEP 21945-970, Rio de Janeiro, RJ, Brazil
C. Magluta: Department of Civil Engineering, COPPE/Federal University of Rio de Janeiro, CP 68506, CEP 21945-970, Rio de Janeiro, RJ, Brazil
W.J. Mansur: Department of Civil Engineering, COPPE/Federal University of Rio de Janeiro, CP 68506, CEP 21945-970, Rio de Janeiro, RJ, Brazil

Abstract
The aim of this research is a comprehensive review and evaluation of beam theories resting on elastic foundations that used to model mode-I delamination in multidirectional laminated composite by DCB specimen. A compliance based approach is used to calculate critical strain energy release rate (SERR). Two well-known beam theories, i.e. Euler-Bernoulli (EB) and Timoshenko beams (TB), on Winkler and Pasternak elastic foundations (WEF and PEF) are considered. In each case, a closed-form solution is presented for compliance versus crack length, effective material properties and geometrical dimensions. Effective flexural modulus (Efx) and out-of-plane extensional stiffness (Ez) are used in all models instead of transversely isotropic assumption in composite laminates. Eventually, the analytical solutions are compared with experimental results available in the literature for unidirectional ([0]6) and antisymmetric angle-ply ([+-30]5, and [+-45]5) lay-ups. TB on WEF is a simple model that predicts more accurate results for compliance and SERR in unidirectional laminates in comparison to other models. TB on PEF, in accordance with Williams (1989) assumptions, is too stiff for unidirectional DCB specimens, whereas in angle-ply DCB specimens it gives more reliable results. That it shows the effects of transverse shear deformation and root rotation on SERR value in composite DCB specimens.

Key Words
delamination; compliance; strain energy release rate; beam theory; elastic foundation.

Address
Mahmood Mehrdad Shokrieh: Composite Research Laboratory, Center of Excellence in Solid Mechanics and Dynamics, Mechanical Engineering Department, Iran University of Science and Technology, Tehran 16846-13114, Iran
Mohammad Heidari-Rarani: Composite Research Laboratory, Center of Excellence in Solid Mechanics and Dynamics, Mechanical Engineering Department, Iran University of Science and Technology, Tehran 16846-13114, Iran

Abstract
The results of experimental and numerical investigations on reinforced concrete beams, with different longitudinal rebars affected by corrosive processes are presented in this paper. Different diameters and/or different distributions of longitudinal rebars were employed keeping constant the total section in each analyzed case, (maintaining a constant stirrup diameter and distribution). The rebars were subjected to accelerated corrosion in the experimental study. Electrochemical monitoring of the process, periodic measuring of the cover cracking and gravimetry of the rebars were performed through the test. Some building recommendations are obtained in order to be considered by designers of concrete structures. The numerical simulation was carried out through the application of the Finite Element Method (FEM), employing plane models, and using linear-elastic material model. The cracking process was associated with the evolution of the tensile stresses that were originated. This numerical methodology allows the monitoring of the mechanical behavior until the beginning of the cracking.

Key Words
corrosion; concrete; cracking; numerical simulation; experimental study.

Address
Nestor F. Ortega: Engineering Department, Universidad Nacional del Sur, Av. Alem 1253, (8000) Bahia Blanca, Argentina
Irene E. Rivas: Engineering Faculty, Universidad Nacional del Centro de la Prov, Buenos Aires, Avda, Del Valle 5737, (7400) Olavarria, Argentina
Raquel R. Aveldano: Engineering Department, Universidad Nacional del Sur, Av. Alem 1253, (8000) Bahia Blanca, Argentina
Maria H. Peralta: Engineering Faculty, Universidad Nacional del Centro de la Prov, Buenos Aires, Avda, Del Valle 5737, (7400) Olavarria, Argentina

Abstract
An economic, structurally effective and practically applicable strengthening technique was developed for reinforced concrete (RC) framed buildings. The idea of the technique is to convert the existing hollow brick infill wall into a load carrying system acting as a cast-in-place RC infill wall by bonding relatively thin high strength precast concrete PC panels to the plastered hollow brick infill. For this purpose, a total of eight one-third scale, one bay, one story frames were tested under reversed-cyclic lateral loads. Test frames were designed and constructed with common deficiencies observed in practice. Four different panel types were used for strengthening. Test results showed that both strength and stiffness of the frames were significantly improved by the introduction of PC panels. Experimental results were compared with the analytical approaches suggested by the authors.

Key Words
strengthening; reinforced concrete; hollow brick infill wall; precast concrete panels; lap splice.

Address
Mehmet Baran: Department of Civil Engineering, Kirikkale University, 71450, Kirikkale, Turkey
Melih Susoy: OM Engineering Services, Inc., Washington, Orlando, USA
Tugrul Tankut: Department of Civil Engineering, Middle East Technical University, 06531, Ankara, Turkey

Abstract
Concrete is a heterogeneous material exhibiting quasi-brittle behaviour. While homogenization of concrete is commonly accepted in general engineering applications, a detailed description of the material heterogeneity using a mesoscale model becomes desirable and even necessary for problems where drastic spatial and time variation of the stress and strain is involved, for example in the analysis of local damages under impact, shock or blast load. A mesoscale model can also assist in an investigation into the underlying mechanisms affecting the bulk material behaviour under various stress conditions. Extending from existing mesoscale model studies, where use is often made of specialized codes with limited capability in the material description and numerical solutions, this paper presents a mesoscale computational model developed under a general-purpose finite element environment. The aim is to facilitate the utilization of sophisticated material descriptions (e.g., pressure and rate dependency) and advanced numerical solvers to suit a broad range of applications, including high impulsive dynamic analysis. The whole procedure encompasses a module for the generation of concrete mesoscale structure; a process for the generation of the FE mesh, considering two alternative schemes for the interface transition zone (ITZ); and the nonlinear analysis of the mesoscale FE model with an explicit time integration approach. The development of the model and various associated computational considerations are discussed in this paper (Part 1). Further numerical studies using the mesoscale model for both quasistatic and dynamic loadings will be presented in the companion paper (Part 2).

Key Words
concrete; multi-phase material; material heterogeneity; mesoscale model; nonlinear analysis; explicit time integration.

Address
Zhenguo Tu: IKM Ocean Design As, Vassbotnen 1, 4313 Sandnes, Norway
Yong Lu: Institute for Infrastructure and Environment, Joint Research Institute for Civil and Environmental Engineering, School of Engineering, The University of Edinburgh, EH9 3JL, UK

Abstract
As a brittle and heterogeneous material, concrete behaves differently under different stress conditions and its bulk strength is loading rate dependent. To a large extent, the varying behavioural properties of concrete can be explained by the mechanical failure processes at a mesoscopic level. The development of a computational mesoscale model in a general finite element environment, as presented in the preceding companion paper (Part 1), makes it possible to investigate into the underlying mechanisms governing the bulk-scale behaviour of concrete under a variety of loading conditions and to characterise the variation in quantitative terms. In this paper, we first present a series of parametric studies on the behaviour of concrete material under quasi-static compression and tension conditions. The loading-face friction effect, the possible influences of the non-homogeneity within the mortar and ITZ phases, and the effect of randomness of coarse aggregates are examined. The mesoscale model is then applied to analyze the dynamic behaviour of concrete under high rate loading conditions. The potential contribution of the mesoscopic heterogeneity towards the generally recognized rate enhancement of the material compressive strength is discussed.

Key Words
concrete mesoscopic heterogeneity; mesoscale model; nonlinear FE analysis; quasi-static load; dynamic load; mesoscopic failure mechanism.

Address
Yong Lu: 1Institute for Infrastructure and Environment, Joint Research Institute for Civil and Environmental Engineering, School of Engineering, The University of Edinburgh, EH9 3JL, UK
Zhenguo Tu: IKM Ocean Design As, Vassbotnen 1, 4313 Sandnes, Norway

Abstract
The seismic provisions of the current edition (2005) of the National Building Code of Canada (NBCC) differ significantly from the earlier edition. The current seismic provisions are based on the uniform hazard spectra corresponding to 2% probability of exceedance in 50 years, as opposed to the seismic hazard level with 10% probablity of exeedance in 50 years used in the earlier edition. Moreover, the current code is presented in an objective-based format where the design is performed based on an acceptable solution. In the light of these changes, an assessment of the expected performance of the buildings designed according to the requirements of the current edition of NBCC would be very useful. In this paper, the seismic performance of a set of six, twelve, and eighteen story buildings of regular geometry and with concrete moment resisting frames, designed for Vancouver western Canada, has been evaluated. Although the effects of non-structural elements are not considered in the design, the nonstructural elements connected to the lateral load resisting systems affect the seismic performance of a building. To simulate the non-structural elements, infill panels are included in some frame models. Spectrum compatible artificial ground motion records and scaled actual accelerograms have been used for evaluating the dynamic response. The performance has been evaluated for each building under various levels of seismic hazard with different probabilities of exceedance. From the study it has been observed that, although all the buildings achieved the life-safety performance as assumed in the design provisions of the building code, their performance characteristics are found to be non-uniform.

Key Words
seismic hazard; uniform hazard spectra; seismic performance; concrete moment resisting frames; pushover analysis; time history analysis.

Address
Omar El Kafrawy: El Kafrawy Consulting Co., Cairo, Egypt
Ashutosh Bagchi: Department of Building, Civil and Environmental Engineering, Concordia University, EV 6.111 1455 de Maisonneuve Blvd. West, Montreal, Quebec, H3G 1M8, Canada
Jag Humar: Department of Civil and Environmental Engineering, Carleton University, Ottawa, Canada

Abstract


Key Words
assumed-strain solid-shell SHB6; six-node prism; shear and thickness locking; hourglass.

Address
Farid Abed-Meraim: LEM3, CNRS, Arts et Metiers ParisTech, 57078 Metz Cedex 3, France
Alain Combescure: LaMCoS, UMR CNRS 5259, INSA de Lyon, 69621 Villeurbanne Cedex, France


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