The improved n-method for the calculation of stresses in service
M. Taliano, Politecnico di Torino, Italy
This paper reports the main results of a parametric analysis which is based on the calculation of stresses on concrete and steel of r.c. rectangular sections subjected to bending under quasi-permanent and characteristic loads. Two calculation methods are used: a general method and a simplified one, namely the n-method. The main parameters that influence the calculation of stresses are then discussed and the differences that are produced by the n-method using the classical values found in the literature compared with the general method are evaluated. By varying only some of the involved parameters, appropriate new values of the modular ratio, called 'improved modular ratios', are determined and two equations for the assessment of the improved modular ratios are proposed, one for each considered load combination. In this way, applying the 'improved' n-method, it is possible to calculate the stresses on concrete and steel with good accuracy.
Engineering the construction of the Stonecutters Bridge concrete backspans
G. Morgenthal, Principal Engineer, Maunsell AECOM, Shatin, Hong Kong R. Sham, Executive Director, Maunsell AECOM, Shatin, Hong Kong K. Yamane, Engineering Manager, Maeda-Hitachi-Yokogawa-Hsin Chong Joint Venture
One of the most challenging aspects in the construction of Stonecutters Bridge, Hong Kong, was the erection of the concrete backspan structures. At a height of about 70 m a geometrically complex grillage deck was to be constructed. The superstructure cannot support its own weight before the stay cables are installed; it was thus constructed on a unique falsework system required until stressing of the stays. The paper describes the development of the construction procedures, the construction engineering and the design of the temporary works. The work involved the accurate stage-by-stage modelling of the entire construction process to assess the structural adequacy throughout and to facilitate geometry control. The different strands of work were highly interactive and optimised erection methodologies, developed together with the contractor, led to a successful implementation of the project.
Fatigue life of short-span reinforced concrete railway bridges
M. Pimentel, Laboratory for Concrete Technology and Structural Behaviour (LABEST), University of Porto, Portugal E. Brühwiler, School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland J. Figueiras, Laboratory for Concrete Technology and Structural Behaviour (LABEST), University of Porto, Portugal
In this paper a parametric study on the fatigue life of short-span reinforced concrete railway bridges is presented. Only reinforcement steel fatigue damage is considered. The objective of this work is to assess the factors that may have a decisive influence on reinforcement fatigue life, to evaluate the consequences of the increasing traffic loads and to identify the research needs so as to allow more accurate fatigue examinations of existing bridges under more demanding traffic conditions. It is concluded that reinforcement fatigue life is mainly governed by the existence of heavy traffic (freight trains) and that it is highly sensitive both to the axle loads increase and to the accuracy in the reinforcement stress range calculation. It is also shown that the current analysis procedures may suffice for economic design of new bridges, whereas in the case of existing bridges more elaborate analysis procedures may be necessary to prove fatigue safety.
A significant, but fairly unnoticed, use of structural concrete is as weight coating to ensure sufficient negative buoyancy, as well as mechanical protection, of submarine pipelines, primarily for the transportation of oil and gas. After a brief introduction to marine pipeline technology the paper describes the design, application and testing of concrete coatings. Finally, the enhancement of pipeline installation stresses due to the greater flexibility of the uncoated construction joints is discussed.
Post-heating bond behaviour between lightweight fibrous concrete and steel
R.H. Haddad, Dept. of Civil Engineering, Jordan University of Science and Technology, Irbid, Jordan Z.G. Al-Kofahi, Dept. of Civil Engineering, Jordan University of Science and Technology, Irbid, Jordan
The post-heating bond behaviour between fibre reinforced lightweight aggregate concrete (LWAC) and 20 mm-diameter steel bars was experimentally investigated using a modified type of pullout specimen. These were cast using volcanic tuff aggregate concrete with and without hooked steel, brass-coated steel, and a mixture of both types of fibres at a volumetric fraction of 2%, before being water cured for 28 days and subjected to high temperatures, ranging from 300 to 700 C. Standard cubes were also cast, cured and heat-treated under similar conditions before being tested to evaluate compressive and splitting strengths. Duplicate specimens, from various mixtures, were cured in water for a similar period and used as controls. The post-heating cracking pattern and extent were evaluated, and residual strengths and bond behaviour determined and described before and after heating then cooling. The results showed marked reductions in residual bond, compressive and splitting strengths of plain and fibre reinforced LWAC being exposed to high temperatures. Pullout specimens prepared with fibres attained higher residual bond strength and bond ductility after exposure to high temperatures than those of plain ones. The contribution of hooked steel fibres (at volumetric fractions of 1 or 2%) to salvaging bond was over the entire temperature spectrum, while that of brass-coated steel fibres was limited to the lower range of that spectrum (from 300 to 500 C). The findings of the present study confirmed the benefit of using lightweight aggregate in reinforced concrete structures as it contributes to maintaining higher post-heating residual bond strength than that of conventional concrete.
Analysis and cost optimisation of prestressed concrete joists in beam-block floors
Vanessa Cristina de Castilho, Universidade Federal de Uberlandia, Brazil Maria Cristina Vidigal de Lima, Universidade Federal de Uberlandia, Brazil
The overall production costs of precast concrete elements are important to quantify. Analyses of the constructive stages of these elements, however, may more realistically lead to a function representing production, transport, assembly and casting costs. This article investigates the cost minimisation problems of joists in beam-block floors using a heuristic method known as genetic algorithm (GA) and a conventional method known as generalised reduced gradient (GRG2). The continuous variables considered in the problem were prestressed reinforcements in the three levels, distance between reinforcements, concrete layer strength, distance between joists, concrete layer thickness and degree of redistribution of the continuous slabs' negative moments. Based on the results, the heuristic method was found to be effective in identifying the solution for precast element cost minimisation problems. The performance of the cost function vis-à-vis market alterations was evaluated considering the influence of concrete, reinforcement and labour prices.
