Analyses of hollow core floors subjected to shear and torsion
K. Lundgren, Chalmers University of Technology, Göteborg, Sweden H. Broo, Chalmers University of Technology, Göteborg, Sweden B. Engström, Chalmers University of Technology, Göteborg, Sweden
Hollow core units are commonly subjected to shear and torsion, for example when placed in floors with openings or skew ends. Present design codes give rough estimations for how the torsional moment can be estimated. The aim of this work was to increase the understanding of torsion in hollow core floors, and to develop a modelling strategy suited to model complete hollow core floors subjected to shear and torsion, using the non-linear finite element method. In a simplified global model, the cross-section of each hollow core unit was represented by one beam element, and the neighbouring hollow core units were coupled by means of slave nodes in the corners, allowing compression but not tension. Comparisons with test results showed that the simplified global model can, with reasonable accuracy, describe the real behaviour of hollow core floors. Furthermore, the simplified global model was used together with solid elements in a part of a hollow core unit, to enable modelling of a shear and torsion failure. Good agreement with test results was obtained concerning failure mode, crack pattern, maximum load, and displacements. Thus, the modelling technique used appears to describe the actual situation in a good way.
Direct design of hollow reinforced concrete beams. Part II: experimental investigation
A. S. Alnauimi, Sultan Qaboos University, Sultanate of Oman P. Bhatt, University of Glasgow, UK
Tests were conducted on eight reinforced concrete hollow beams subjected to combined load of bending, shear and torsion. The beams were designed using the direct design method that was discussed in Part I. All beams had an overall cross-section dimension of 300 300 mm with a wall thickness of 50 mm. The overall length of the beam was 3800 mm. The two main variables in the series were the ratio in the web of the maximum elastic shear stress due to twisting moment to elastic shear stress due to shear force which varied between 0.59 and 6.84, and the ratio of the maximum twisting moment to the bending moment which varied between 0.19 and 2.62. The beams were experimentally tested in the University of Glasgow, Scotland, UK. Good agreement was found between the design and experimental failure loads. All beams failed near the design loads and had undergone ductile behaviour until failure. The results indicate that the direct design method can be successfully used to design reinforced concrete box beams for the combined effect of bending, shear and torsion loads.
Spatial variability of concrete deterioration and repair strategies
Y. Li, Delft University of Technology, The Netherlands T. Vrouwenvelder, Delft University of Technology, The Netherlands G. H. Wijnants, Netherlands Organisation for Applied Scientific Research, Delft, The Netherlands J. Walraven, Delft University of Technology, The Netherlands
This paper presents an improved and more realistic approach to evaluate the deterioration process and optimise the repair strategy of concrete structures. It is based on the commonly used probabilistic-based reliability analysis methods, but takes into account the spatial variability of concrete properties that has great impact on the design and maintenance decisions of structures. The developed approach is exemplified by the concrete bridge 'Wilpsedijk' in the Netherlands to show the service lifetime prediction based on spatial variability of concrete deterioration including initiation and propagation period. With respect to an established repair criterion, available repair options and the corresponding repair costs, the optimal lifetime repair strategy is determined. In comparison with most of the studies that neglect the variables with random spatial variability, the approach reflects the actual situation more realistically and can produce useful information as the proportion or percentage of the surface area that shows concrete deterioration during the whole period of time. It enables the planning of different repair and maintenance strategies for the structure from a practical point of view.
Direct design of hollow reinforced concrete beams. Part I: design procedure
A. S. Alnauimi, Sultan Qaboos University, Sultanate of Oman P. Bhatt, University of Glasgow, UK
This paper, Part I, presents a general 'direct design procedure' for the design of reinforced concrete hollow beams subjected to a combined load of bending, shear and torsion. Elastic stress field in conjunction with Nielsen's twodimensional yield criterion for reinforced concrete subjected to in-plane forces were used in the design of reinforcement. This procedure is based on and satisfies the lower bound theorem of the classical theory of plasticity. The main features of this procedure are the precluding of the use of empirical equations as is the case with existing codes of practice and in addition to satisfying the requirements of the theory of plasticity it reduces the ductility demand assumed by this theory. Comparison between the direct design procedure and the truss analogy showed that the direct design procedure leads to steel requirements close to those of the truss analogy when the angle of inclination of the struts is 458. The direct design procedure produced much less reinforcement than the American Concrete Institute and the British Standards Institute codes.
Modelling effect of corrosion on bond strength of plain bar reinforcement
T. Pregartner, Engineering Office Eligehausen & Asmus, Stuttgart, Germany J. Cairns, School of the Built Environment, Heriot-Watt University, Edinburgh, UK J. Ozbolt, Institute for Construction Materials, University of Stuttgart, Germany
In the present paper a simple approach to model the effects of corrosion on bond between plain bar reinforcement and concrete with the finite element method (FE method) is shown. With the combination of a non-linear FE-solver and a discrete bond model it is sufficient to simulate corrosion by radial expansion of a concrete cylinder. The bond model used accounts for the effects of confinement and stresses in the vicinity of the reinforcement bar. Hence the effects of corrosion could be investigated with this simple approach. The model is calibrated on pull-out tests with RILEM specimen. Calculations conducted on beam end specimen containing plain surface bars show a plausible agreement with test results and with results known from literature.
Studies on strength and permeability characteristics of blended cements in lowand mediumstrength concretes
R. Vedalakshmi, Corrosion Science and Engineering Division, Central Electrochemical Research Institute, Karaikudi S. Srinivasan, Corrosion Science and Engineering Division, Central Electrochemical Research Institute, Karaikudi K. Ganesh Babu, Department of Ocean Engineering, Indian Institute of Technology, Madras, India
In the present investigation the strength and permeability characteristics of blended cements such as Portland pozzolana cement and Portland slag cement were evaluated in 20, 30 and 40 MPa concretes. The cements were produced by intergrinding mineral admixtures such as fly ash and slag with clinker and gypsum in the plant. The compressive strength over a period of 1 year and water permeability at the end of 7, 28 and 90 days were evaluated and the results were compared with ordinary Portland cement. Thermo-gravimetric analysis was also carried out to assess the pozzolanic reaction of blended cements. The results show that blended cement concretes have lower strength than Portland cement concrete at all ages in all of the grades of concrete studied. Porosity in terms of water absorption of blended cement concretes is less than Portland cement concretes in 20 MPa concrete but was not significantly different in 30 and 40 MPa concretes. Thermo-gravimetric and differential thermal analysis reveals that there is a reduction in Ca(OH)2 content in blended concretes indicating the consumption of hydroxide in pozzolanic reaction.
