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.
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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.
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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.
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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.
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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
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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.
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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:
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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.
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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.