A. Ansell, Royal Institute of Technology (KTH) Stockholm, Sweden
For shear capacity, the current design procedure used for concrete structures has been found to be inaccurate for some dynamic design load cases. Loads traditionally believed to be highly dynamic are only within the quasi-static range, but it is known that the combination of a high impact velocity and a hard, stiff impact will lead to shear failure. Load cases on concrete structures which can be classified as dynamic are, for example, those relating to weapons and hard impacts from steel objects. The characteristic of a dynamic load has great influence on the overall response and mode of failure of concrete structures. When structures which have been designed to fail in flexure under static loads fail in shear when loaded dynamically, the reason is probably the changing frequency content of the load. On the basis of a literature review, some important conclusions and recommendations are presented in this paper. The results will be used in further evaluation of existing design tools, aiming at accurate and reliable routines for the design of safe and cost-efficient concrete structures.
A.A. Abbas, Imperial College London, UK M.N. Pavlovic, Imperial College London, UK M.D. Kotsovos, National Technical Univesity of Athens, Greece
The design of ground-floor slabs (GFS) is largely based on concentrated patch loads (CPL) rather than uniformly distributed loads (UDL) as the former are more critical. This is true provided that the UDL is applied throughout the floor and that the slab is also uniformly supported by the soil beneath (i.e. there are no local soft spots). Clearly, such scenarios will not induce any significant bending stresses in the slab. However, if the UDL is applied only locally on the slab, as is often the case, then bending stresses will occur. A particular case is the arrangement in which two layers of UDL are applied on the floor with an unloaded aisle in between. This results in tensile stresses in the mid-aisle at the top of the slab which must be considered in the design process. A similar situation arises when two spaced CPL are applied, which is common in the case of racking-leg loads. The present article reports on the numerical research work that was carried out to study these effects. The work is based on linear finite-element analysis (LFEA) and the ensuing results are presented herein in the form of design charts.
Modelling bond strength of corroded plain bar reinforcement in concrete
G. Xu, Huazhong University of Science & Technology, China J. Wei, Huazhong University of Science & Technology, China T. Tan, Huazhong University of Science & Technology, China H.Q. Liu, Huazhong University of Science & Technology, China
In this paper, the corrosion influence on bond strength between plain bar and concrete without confinement is studied at the time of cracking and before cracking occurs, and causes of bond strength increment are analysed quantitatively. A new calculation model for corrosion pressure and corrosion depth at the time of cracking is proposed. Considering the change of bar surface and increment of corrosion pressure, a new calculation model for bond strength at the time of cracking and before cracking occurs is given. Additionally, the theoretical model is verified using different test results.
Cost optimisation of lattice-reinforced joist slabs using genetic algorithms
V.C. Castilho, Universidade Federal de Uberlândia, Brazil M.C.V. Lima, Universidade Federal de Uberlândia, Brazil
Genetic algorithms (GA), a search method inspired by Darwin's theory of evolution, offer an optimisation tool that has been used very successfully to solve a variety of engineering problems. The search process it implements starts with a set of one or more chromosomes (initial population) and, by applying selection and reproduction operators, iteratively 'evolves' the population into better ones, until a stopping criterion is reached. This article investigates lattice-reinforced joist slab cost optimisation problems using a GA with continuous variables. The problem considered concerns one-way slabs, continuous over two spans, in which only the in situ concrete characteristics and joist spacing are varied. The design variables are: concrete layer thickness, concrete layer strength, reinforcement, distance between joists and degree of redistribution of the continuous slabs' negative moments. The search for a solution includes an investigation into the use of discrete variables for data representation. To obtain results that allow for a comparative empirical analysis, these problems are also evaluated by a conventional optimisation method. The results indicate that the GA method is a viable optimisation tool for solving lattice-reinforced joist slab cost minimisation problems.
The use of prestressed concrete piles to support integral abutments
E.G. Burdette, University of Tennessee, Knoxville, USA S.C. Howard, University of Tennessee, Knoxville, USA J.H. Deatherage, University of Tennessee, Knoxville, USA D.W. Goodpasture, University of Tennessee, Knoxville, USA
The Tennessee Department of Transportation (TDOT) in the United States has become the national leader in the design and construction of jointless bridges with abutments which are integral with the bridge deck. While TDOT criteria call for limits of length of 152 m (500 ft) and 244 m (800 ft) for steel and concrete bridges, respectively, they have on more than one occasion exceeded these limits significantly. TDOT's desire to address any questions raised about the efficacy of using prestressed concrete piles to support integral abutments and perhaps to extend the limits on bridge length led to the tests reported in this paper. Four full-size abutments, 3.05 m (10 ft) wide, were built and tested in the field. The description and results of these tests are reported and discussed. Of particular interest are the testing of one pile to failure and the cyclic tests performed on one pile. The conclusions drawn were, first, that prestressed concrete piles are appropriate to use to support integral abutments and, second, that the TDOT criteria for bridge lengths are reasonable and conservative.
Shear strength in the new Eurocode 2. A step forward?
A. Cladera, University of Balearic Islands, Palma de Mallorca, Spain A.R. Mari, Technical University of Catalonia, Barcelona, Spain
The shear strength of reinforced concrete beams with stirrups has been a highly controversial matter since Ritter and Mörsh proposed the first truss models. Since then, different analytical models have been discussed, such as truss models with concrete contribution, shear/compression theories, truss models with variable angle of inclination, and compression field theories. However, some of these models were too complex to be implemented in a code of practice and they had to be simplified. As Regan has pointed out, for simpler models the problem is mostly that of the need to neglect some factors, considered secondaries. However, what is secondary in one case may be primary in another. With the release of the new Eurocode 2 (prEN 1992-1-1:2003) the controversy has been raised again. The EC-2 proposes a very simple formulation based on a truss model. However, the authors think that it is a gross oversimplification of a complex problem as it neglects important key variables. In this paper the new EC-2 shear procedure predictions are compared to empirical tests and to other simplified formulations. It is concluded that the EC-2 procedure is very easy to use by practising engineers but it presents a great scatter of results. On the one hand, it may be too conservative for slightly shear-reinforced beams or for prestressed beams. On the other, it may be slightly unconservative for heavily reinforced members.
