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Structural Concrete, Vol. 6, no. 4, December 2005

Discussion: The influence of the parameter φ/ρeff on crack widths (Structural Concrete, 2004, 5, No. 2, 71 - 83)

A.W. Beeby, University of Leeds, UK
C. Ålander, Ruukki Metals, Finland
J. Cairns, Heriot-Watt University, UK U. Mayer, University of Stuttgart, Germany
S. Lettow, University of Stuttgart, Germany
D. Ferretti, University of Parma, Italy
I. Iori, University of Parma, Italy
P. Gambarova, Milan University of Technology, Italy
P. Bamonte, Milan University of Technology, Italy
E. Giuriani, University of Brescia, Italy
G. Plizzari, University of Bergamo, Italy
S. Pantazopoulou, Demokritus University of Thrace, Greece
S. Tastani, Demokritus University of Thrace, Greece

Professor Beeby's 2004 paper questioned the basis of the widely cited "classical" model for estimation of crack widths in reinforced concrete elements. It provides a valuable reminder of the need to verify the predictions of theoretical models against experimental data, a task which he has performed to the benefit of our understanding on several occasions in the past. Professor Beeby will therefore not be surprised to learn that others have subjected his own ideas to similar scrutiny. In the several contributions below, members of fib Task Group 4.5 "Bond Models" raise questions over the validity of conclusions in the paper, based on a wider range of experimental data and more recent developments in theoretical modelling. (John Cairns, Convenor, fib Task Group 4.5 "Bond Models")

Structural Concrete, Vol. 6, no. 3, September 2005

Influence of the shear force and transverse reinforcement ratio on plastic rotation capacity

R. do Carmo, Instituto Superior de Engenharia de Coimbra, Coimbra, Portugal
S. M. R. Lopes, Universidade de Coimbra, Coimbra, Portugal

Ductility, particularly the plastic rotation capacity of critical regions, conditions the available degree of moment redistribution and the ability to exploit the additional resistance of hyperstatic structures. A theoretical model for calculating plastic rotation capacity, considering the influence of the main factors, is presented. Special attention has been paid to the influence of the tensile reinforcement ratio, the shear force and the confinement of compressed concrete on plastic rotation capacity. Theoretical results are compared with those obtained using an experimental programme designed to study the influence of these factors. Some extrapolations are made on the basis of the model, and conclusions are drawn.

Structural Concrete, Vol. 6, no. 4, December 2005

Performance design of reinforced concrete slabs using commercial finite element software

A. Khennane, University of Southern Queensland, Australia

A fundamental task in the design of reinforced concrete structures is to search for minimum cost through the variation and placement of the quantities of the relatively expensive steel reinforcement without jeopardising the safety of the structure. The use of nonlinear finite element software can assist greatly in achieving an economical and safe design. However, commercially available finite element software is not designed for this task as most packages have been developed to be used as verification rather than design tools. "Home-written" software can be designed to achieve this task, however it may suffer from serious drawbacks such as bugs, lack of user friendliness, lack of generality, and unproven reliability. This present study shows that if a given software package comes with a scripting interface, it can be transformed easily from a verification tool to a performance design tool. This is illustrated with the use of ABAQUS, but it can be adapted to any other software with a scripting interface.

Structural Concrete, Vol. 6, no. 3, September 2005

A study of the influence of environmental effects on the behaviour of a pre-stressed concrete viaduct

R. W. Howells, Cardiff School of Engineering, Cardiff University, UK
R. J. Lark, Cardiff School of Engineering, Cardiff University, UK
B. I. G. Barr, Cardiff School of Engineering, Cardiff University, UK

This paper addresses the influence that changing environmental effects have on the strains developed in a major pre-stressed concrete viaduct. The changes in strain, resulting from variations in temperature and relative humidity, were analysed over a period of one year. It was found that changes in temperature had a greater influence on the strain behaviour of the structure than changes in relative humidity, with strain variations of up to 75 microstrain (με) being recorded. A further study was conducted on the effect that changing temperature has on the strain behaviour of the viaduct during each of the four seasons. It was observed that the variation of temperature over the course of a week resulted in strains of up to 15 με. Finally, in both studies, a linear relationship was observed between strain and temperature, although these relationships differ slightly depending on the season, and the segment location within the span.

Structural Concrete, Vol. 6, no. 3, September 2005

Long-term monitoring of electrically isolated post-tensioning tendons

B. Elsener, Institute of Materials Chemistry and Corrosion, ETH Zurich, Switzerland

Electrically isolated tendons with plastic ducts for internal grouted post-tensioning were developed about 15 years ago. This new generation of tendons offers enhanced corrosion protection of the steel strands and the possibility to monitor the corrosion protection by simple non-destructive measurements (electrical impedance). This paper reports practical experience on quality control and long-term monitoring of two flyovers with electrically isolated tendons in Switzerland. The results of impedance measurements are rationalised on the basis of a simple model of a capacitance C (of the polymer duct) in parallel to a resistance R (leaks, defects) that both have a clear physical meaning and depend on the length of the tendon. The penetration of (chloride-containing) water at defects of the duct will lead to a decrease of the resistance R of that tendon. Thus for the first time the corrosion protection of the structurally important post-tensioned tendons can be monitored during the whole service life of the structure. 

Structural Concrete, Vol. 6, no. 3, September 2005

Influence of the shear force and transverse reinforcement ratio on plastic rotation capacity

R. do Carmo, Instituto Superior de Engenharia de Coimbra, Coimbra, Portugal
S. M. R. Lopes, Universidade de Coimbra, Coimbra, Portugal

Ductility, particularly the plastic rotation capacity of critical regions, conditions the available degree of moment redistribution and the ability to exploit the additional resistance of hyperstatic structures. A theoretical model for calculating plastic rotation capacity, considering the influence of the main factors, is presented. Special attention has been paid to the influence of the tensile reinforcement ratio, the shear force and the confinement of compressed concrete on plastic rotation capacity. Theoretical results are compared with those obtained using an experimental programme designed to study the influence of these factors. Some extrapolations are made on the basis of the model, and conclusions are drawn.

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