CHAPTER I INTRODUCTION 1.1 Introduction Bridges had been constructed since thousands of years as a path to cross over an obstacle. Bridges are constructed using various types of materials. The development of modern materials and construction techniques has resulted in the emergence of a new generation of structures remarkably flexible, low in damping and light in weight. Such structures, as well as various novel types of rigid structures, exhibit an increased susceptibility to the action of the wind. Accordingly, it has become necessary to develop tools enabling the designer to estimate wind effects with higher degree of refinement than was previously required. It is the task of the engineer to ensure that the performance of the structures subjected to the action of wind will be adequate during their anticipated life from the standpoint at both structural safety and serviceability. To achieve this, the designer needs information regarding the wind environment; the relation between environments and the forces induces on the structure and the behaviour of the structure under the action of these forces. 2 Information on the wind environment needed includes elements derived from meteorology, micrometeorology and climatologic. As shown in Figure 1.1, the bridge deck structure immersed in a steady-state wind load, U, is subjected to aerodynamic forces including drag (along wind) forces, D, which act in the direction of the mean flow, and lift (across-wind) forces L, which act perpendicularly to that direction and also torsional moments, M. The aerodynamics forces are dependant on time. Thus, the methods of the structural dynamics have to be employed to determine the response. Figure 1.1: Wind Components At A Point Along The Bridge Axis The typical method to obtain the wind effects data on the structures is acquired through conducting the wind tunnel tests or design method from Codes of Practice. Since the power of personal computers and engineering workstations is rapidly increasing, it is now feasible to employ advanced analysis techniques for direct design use. Advanced analysis indicates any method that can sufficiently capture the limit state strength and stability of a structural system and its individual members so that separate member capacity checks encompassed by the specification equations are not required. This is because advanced analysis accounts for material and geometric non-linearity directly. It is expected that the use of advanced analysis will simplify the design process considerably. 3 Bridges were developed longer and slender, however, it was not until the dramatic total collapse of the Tacoma Narrows Bridge in 1940 that the possible susceptibility of this form of long flexible structure to the dynamic effect of wind action was first investigated. It was realised that such bridges were also liable to oscillations of a discrete character, which were not subject to the rules of static wind design. Engineers are designing the bridge to withstand the wind forces with structures that have inherent stiffness achieved through engineering techniques rather than depending upon dead weight to stabilise the structure. The following factors are important that must be considered in order to establish a sufficient structural system to resist wind forces: i) Strength and stability of structural system ii) Resonance of building oscillations with vibrations iii) Effects of buffeting that may increase the magnitudes of wind velocities iv) Fatigue in structural members and connections caused by fluctuating wind loads and traffic loads v) Excessive deflection In current wind resistant design practice, the dynamic wind loads are generally represented in terms of equivalent static wind loads expressed as the mean static wind loads multiplied by the gust response factor (GRF). The gust response factor, or gust-loading factor, was originally introduced by Davenport (1967) and is defined as the ratio of the maximum expected wind load or response to its corresponding mean value. The gust response factors are generally different for each response components and may vary in a wide range, depending on the structure system, the wind load characteristics, and the influence functions related to the response components. 4 1.2 Background Knowledge of foundations in wind engineering was developed from diverging area of pre-existing fields of sciences. The practical qualitative knowledge and exploitation of wind effects can be traced back to late seventeenth century. The Tay Bridge disaster in 1877 demonstrated the need for more detailed study of wind forces on the structures. The disaster marked the beginning of wind loading research in United Kingdom for direct application to engineering design. In 1940, Tacoma Narrows Bridge collapsed due to torsional oscillations. Followed by the collapse, Farquharson and Vincent had developed routine model for testing techniques using wind tunnel for ensuring stability of suspension bridge, which are widely use until today. Forces on the building shapes can be determined and hence solved the inappropriate data problem in the Codes of Practice. Poncelet (1839) had introduced the phenomenon of fatigue of metal caused by a repeated cycle of stress. The fatigue analysis has played a fundamental role in the design of all structures subjected to repeated loads, especially metal bridges. The ratio of live load to dead load is much higher for a railroad bridge than for a similarly sized highway structure. This can lead to serviceability issues such as fatigue and deflection control governing designs rather than strength. Static wind pressures are those that cause a bridge to deflect or deform. Dynamic wind movements affect long span flexible bridge and prone to oscillate in number of modes. The most common parameter affecting the design serviceability of a steel bridge is the deflection. 5 Figure 1.2: Deformation of Tacoma Narrows Bridge Due to Wind Loads 1.3 Objectives of the Study The study was carried out by constructing 3-dimensional modelling and undergoes linear elastic analysis on a section of a suspension railway bridge. The study is aiming to: i) Obtain the stress distribution on the bridge section due to wind load. ii) Identify the stress concentration regions iii) Verify the serviceability of the bridge under sever wind loads 6 1.4 Significance of the Study The monorail suspension bridge, which is a flexible structure and built on an exposed site with subjected to severe wind condition, is expected prone to problems of wind effects. The live loads generated from monorail aggravate the effects of wind loads and thus emergence of consequential fatigue failure on the steel suspension bridge section. Therefore, effects from these combinations of loads are important to be acquired. The study is aimed to assess the distribution of stress on bridge section that subjected to wind pressure, which can represent the actual behaviour of the bridge section under the wind loadings. The stress concentration regions identified in the study are the potential fatigue failure zones where additional or concentrated observation and inspection are going to be conducted to prevent failure resulted from fluctuating stresses induced by repetitive wind and live loads. Discovery of high stress range areas or high stress concentration regions through the finite elements analysis reduced the costs of monitoring by the reason of well planning or risk based inspection can be conducted on the structure. 1.5 Scope of the Study The study was conducted on a section of steel suspension monorail bridge using the finite element software, MSC.NATRAN for Windows Version 4.5. The model of bridge section generated three-dimensionally. The study is limited to static wind load with linear geometric and material properties. The combination of loads is considered under serviceability limit states. 7 1.6 Problem Statement Suspension bridges are prone to several of wind-induced problems by the reason of exposed to severe wind conditions. The serviceability of the suspension bridge is in doubt under service loads combination of wind load and monorail live load. The wind effect on bridge section was assessed by using the designing Code of Practice. Although, the value given by the conventional method was proved to be overestimated, it is assumed as the factor of safety for neglecting the dynamic wind effects. Alternatively, wind effects on the bridge structure can be obtained precisely by undergoing wind tunnel test. However, the wind tunnel tests are time and cost consuming. Therefore, simulation of the static aeroelastic test of wind tunnel test has made using finite elements analysis, which is lesser in cost and time consuming, to obtain deflections and stresses. 1.7 Research Methodology The study was conducted on a monorail suspension bridge section. The modelling and analysis of wind loads on the bridge section was conducted using finite element software, MSC.NASTRAN for Windows Version 4.5. Service loads combinations are applied to the three dimensional model in order to perform wind analysis. The steps taken in studying the wind loads on the bridge structure can be summarised into several major steps as follow: 8 1.7.1 Literature Review The analysis and design of the bridge structures under various wind loading conditions were studied. The behaviour of the wind and its effects acting on the structures were identified. The use of the finite element analysis and concepts in applications to wind design and analysis was studied. 1.7.2 Verification of Finite Element Software The finite element analysis application software used in the study is MSC.NASTRAN for Windows Version 4.5. The ability of software in conducting the wind flow pressure is studied before carried out the actual modelling process. Several similar simple models were constructed to increase the familiarity of the software features in modelling and analysis. Analyses were carried out on models published in the journal to verify the ability of the software to conduct the analysis. 1.7.3 Developing Control Models There was no any physical laboratory experimental works conducted on the models generated. Therefore, the acceptability of the results is very important. Thus, a few simple control models have been developed to verify the suitability of finite element analysis (FEA) model of wind induced bridge analysis. The results are compared to several studies done by other researchers. 9 1.7.4 Analysis and Results Analysis of bridge section model was conducted using load combinations as stated in BS 5400:Part 2 (1978) for railway bridge under serviceability limit states. The loads considered in the analysis are dead loads, live loads and wind loads. Results obtained were checked with limits of design of steel girder stiffened suspension bridge as stated in BS 5400 to resolve the serviceability of the bridge.