Written on 19 août 2011.
A. Ansell, Royal Institute of Technology (KTH) Stockholm, SwedenFor 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.
Written on 19 août 2011.
A.A. Abbas, Imperial College London, UKM.N. Pavlovic, Imperial College London, UKM.D. Kotsovos, National Technical Univesity of Athens, GreeceThe 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.
Written on 19 août 2011.
G. Xu, Huazhong University of Science & Technology, ChinaJ. Wei, Huazhong University of Science & Technology, ChinaT. Tan, Huazhong University of Science & Technology, ChinaH.Q. Liu, Huazhong University of Science & Technology, ChinaIn 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.
Written on 19 août 2011.
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.
Written on 19 août 2011.
E.G. Burdette, University of Tennessee, Knoxville, USAS.C. Howard, University of Tennessee, Knoxville, USA J.H. Deatherage, University of Tennessee, Knoxville, USAD.W. Goodpasture, University of Tennessee, Knoxville, USAThe 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.
Written on 19 août 2011.
E.G. Burdette, University of Tennessee, Knoxville, USAS.C. Howard, University of Tennessee, Knoxville, USA J.H. Deatherage, University of Tennessee, Knoxville, USAD.W. Goodpasture, University of Tennessee, Knoxville, USAThe 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.
Written on 19 août 2011.
A. Cladera, University of Balearic Islands, Palma de Mallorca, SpainA.R. Mari, Technical University of Catalonia, Barcelona, SpainThe 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.
Written on 19 août 2011.
Á. Cunha, University of Porto, PortugalE. Caetano, University of Porto, Portugal F. Magalhães, University of Porto, PortugalThis paper stresses the important role that output-only dynamic testing of bridges and special structures can play in the assessment of dynamic structural behaviour by presenting a set of applications recently performed at several Portuguese bridges (railway, roadway and pedestrian bridges), as well as at the new Braga Sports Stadium suspended roof, and briefly referring to the most important tools developed for that purpose at the Laboratory of Vibrations and Monitoring at the Faculty of Engineering of the University of Porto.
Written on 19 août 2011.
H. Broo, Chalmers University of Technology, Göteborg, SwedenK. Lundgren, Chalmers University of Technology, Göteborg, SwedenB. Engström, Chalmers University of Technology, Göteborg, SwedenThe present calculation methods for shear and torsion in prestressed hollow core slabs add stresses from various influences without taking into account deformations and compatibility, the softening of cracking concrete, or restraint at the boundaries; therefore, they are most likely conservative. The main purpose of this work is to establish three-dimensional finite element models, which can be used both to analyse the effect of parameters that influence the shear and torsion response and to be included in global models of complete floors. An important aspect was therefore to simplify the models to avoid time-consuming analyses. Coarse meshes with solid elements were combined with beam elements. The established models were validated by simulating a series of full-scale tests conducted on both 200 mm and 400 mm thick hollow core units subjected to various combinations of shear and torsion. In general, although very coarse meshes were used, the finite element analyses of the tests succeeded in describing the overall behaviour, crack pattern, failure mode, and maximum load, with a reasonably good agreement.
Written on 19 août 2011.
B. Briseghella, University of Architecture in Venice, ItalyT. Zordan, University of Architecture in Venice, ItalyA case study aiming to transform a simply supported pre-stressed concrete flyover into a frame structure throughout its full length, through refurbishment, is presented. Hogging moment resistance is achieved at the piers by the construction of a rigid transverse cast-in-place concrete diaphragm connected to the piers and to the beams of adjacent bays, in an attempt to fulfil the requirements of the integral abutment bridge (IAB) concept specified. All former bearings and expansion joints are eliminated resulting in an improved durability of the structure and greater comfort for the users. According to the IAB concept, hogging moment resistance is obtained by the flow of tensile forces through the concrete slab thanks to an increased ratio of reinforcement and the transmission of compressive forces directly through the concrete. Shear transmission is ensured by the installation of a convenient number of concrete-to-concrete shear studs on the webs of each of the beams forming the deck. Experimental tests were used to investigate the behaviour of shear studs. The possibility of making a simply supported deck continuous for a certain length depends on the soil-structure interaction and, consequently, on a wide number of parameters, varying from case to case, that will not be investigated in the present paper.
Written on 19 août 2011.
G. E. Thermou, Demokritus University of Thrace, Greece S. J. Pantazopoulou, Demokritus University of Thrace, Greece This paper presents the results of a recent experimental research study where metallic (high-strength steel cord) fabric jackets (MF jackets) were utilised for the seismic upgrading of substandard reinforced concrete members. The proposed intervention method and its practical application are described in detail. Specimens were cantilevers with a square cross-section, representing a typical building column at half scale. The length of the test region corresponded to half the span of a typical storey building column under lateral sway. Due to lack of adequate seismic detailing the specimens were susceptible to various modes of failure such as web shear failure, buckling of compression reinforcement or failure in the lap splice region. The as-built specimens were first damaged up to failure after being subjected to combined axial loading and cyclic lateral displacement reversals simulating seismic loading. In the next phase, specimens were retrofitted with both composite and metallic fabric jackets and then tested again under the same load history. The results of this preliminary experimental research programme show that the metallic fabric jackets performed in an excellent way compared to glass- and carbon-fibre reinforced polymer jackets, increasing substantially both the strength and the deformation capacity of the repaired members. The excellent mechanical performance of the metallic fabrics combined with many of the advantages of the synthetic wraps (easy handling, no change in member dimensions) and the intrinsic favourable properties of steel (fire resistance), underline the potential of this novel material in repair/strengthening of reinforced concrete as an alternative option for jacketing applications.
Written on 19 août 2011.
A. Nowek, Gdansk University of Technology, Gdansk, PolandP. Kaszubski, Gdansk University of Technology, Gdansk, PolandH.S. Abdelgader, Al-Fateh University, Tripoli, LibyaJ. Górski, Gdansk University of Technology, Gdansk, PolandAccording to two-stage (pre-placed aggregate) concrete technology, special grout is injected into forms or foundation trenches with aggregate, as backing, placed earlier. The fresh grout differs from the ordinary concrete mixture. Good quality two-stage grout can be prepared in high-speed mixers, for example an Ultramixer (3000 rpm). It is characterised by high fluidity, low sedimentation, good viscosity, intensive hydration and a notable increase in the cement particles' surface. However, because of the unique and complex equipment involved, two-stage concrete technology is still underutilised. The aim of the investigation presented in this work is to design a mixture composition similar in features to Ultramixer grout. For this purpose, the new grout was prepared using a normal mixer (140 rpm) and some admixtures. The experimental tests were performed with different mix proportions. On the basis of the obtained results, an optimal composition has been proposed. A comparison of the new grout features with the properties of the grout produced by the Ultramixer has also been elaborated. The new grout design method can improve two-stage concrete technology. Some preliminary two-stage concrete tests are also presented.
