introductiontobridges07-08-090507223401-phpapp02.ppt

MAB1053
Bridge Engineering
Introduction
Prof. Dr. Azlan Abdul Rahman
Faculty of Civil Engineering, UTM,

Course Objectives
Upon completion of the course, participants will be able to:
 Identify types, structural forms & design process of
common concrete bridges.
 Perform basic calculations of bridge live loading based on
BS5400, BD37/01 & JKR Specification.
 Use LUSAS finite element program for basic analysis of
concrete bridge deck.
 Perform calculations for the basic design of prestressed
concrete bridge beam, bridge abutment and pier.

CONTENT WEEK/DATE LECTURER CONTENT WEEK/DATE LECTURER
1. INTRODUCTION
- Introduction to Bridges
- Choice of Bridge Decks
W1 (12/7/07)
W2 (19/7/07)
Ir. Dr. Wahid
Prof. Azlan
4.0 METHODS FOR
BRIDGE DECK
ANALYSIS
- Introduction to Bridge Deck
Analysis
- Orthotropic Plate Theory
- Grillage Analysis
- Finite Element Analysis
- LUSAS Software
- Worked Example
W8 (30/8/07)
W9 (6/9/07)
W10 (13/9/07)
W11 (20/9/07)
W12(27/9/07)
Dr. Redzuan
Dr. Redzuan
1st Ramadhan
Dr. Redzuan
Dr. Redzuan
2.0 BRIDGE
SUB-STRUCTURE
- Bridge Abutments & Piers
- Bridge Bearings & Joints
- General Bridge Loading
W3 (26/7/07)
W4 (2/8/07)
W5 (9/8/07)
Prof. Azlan
Prof. Azlan
Prof. Azlan
5.0 PRESTRESSED
CONCRETE BRIDGE
- Fundamentals of Prestressed
Concrete
- Design of Post-Tensioned
Concrete Beam
- Composite Prestressed
Concrete
W13 (4/10/07)
W14(11/10/07)
W15(18/10/07)
Ir. Dr. Wahid
Ir. Dr. Wahid
Ir. Dr. Wahid
3.0 BRIDGE LOADING
- General Bridge Loading
- Live Loads to BS5400 &
BD37/01 & JKR Live
Loads
- Examples
W6 (16/8/07)
W7 (23/8/07)
Prof. Azlan
Sem. Break
Lecture Time Table

Introduction to Bridges
MAB1053
Bridge Engineering

What is a bridge?
 A bridge is a structure that spans a divide
such as:
• A stream/river/ravine/valley
• Railroad track/roadway/waterway
 The traffic that uses a bridge
may include:
• Pedestrian or cycle traffic
• Vehicular or rail traffic
• Water/gas pipes
• A combination of all the above

Function of A Bridge
 A bridge has to carry a service (which
may be highway or railway traffic, a
footpath, public utilities, etc.) over an
obstacle (which may be another road or
railway, a river, a valley, etc.) and to
transfer the loads from the service to the
foundations at ground level.

Classification of Bridges
 According to functions : aqueduct, viaduct, highway,
pedestrian etc.
 According to materials of construction : reinforced concrete,
prestressed concrete, steel, composite, timber etc.
 According to form of superstructure : slab, beam, truss, arch,
suspension, cable-stayed etc.
 According to interspan relation : simple, continuous,
cantilever.
 According to the position of the bridge floor relative to the
superstructure : deck, through, half-through etc.
 According to method of construction : pin-connected,
riveted, welded etc.

Classification of Bridges
 According to road level relative to highest flood
level : high-level, submersible etc.
 According to method of clearance for navigation :
movable-bascule, movable-swing, transporter
 According to span : short, medium, long, right,
skew, curved.
 According to degree of redundancy : determinate,
indeterminate
 According to type of service and duration of use :
permanent, temporary bridge, military

Bridges which Carry Loads
Mainly in Flexure
 By far the majority of bridges are of this type.
The loads are transferred to the bearings and
piers and hence to the ground by slabs or
beams acting in flexure, i.e. the bridges obtain
their load-carrying resistance from the ability of
the slabs and beams to resist bending
moments and shear forces.
 Only for the very shortest spans is it possible
to adopt a slab without any form of beam. This
type of bridge will thus be referred to generally
as a girder bridge.

Bridges which Carry their Loads
Mainly as Axial Forces
 This type can be further subdivided into those bridges in
which the primary axial forces are compressive (arches)
and those in which these forces are tensile (suspension
bridges and cable-stayed bridges). Such forces normally
have to be resisted by members carrying forces of the
opposite sense.
 It must not be thought that flexure is immaterial in such
structures. Certainly, in most suspension bridges, flexure
of the stiffening girder is not a primary loading in that
overstress is unlikely to cause overall failure; however, in
cable stayed bridges (particularly if the stays are widely
spaced) flexure of the girder is a primary loading.

Bridges which Carry their Loads
Mainly as Axial Forces

Basic Types of Bridges
 Girder/Beam Bridge
 Truss Bridge
 Rigid Frame Bridge
 Arch Bridge
 Cable Stayed Bridge
 Suspension Bridge

Girder/Beam Bridge
• The most common and basic type
• Typical spans : 10m to 200m

Truss Bridge
• Truss is a simple skeletal structure.
• Typical span lengths are 40m to 500m.

Forces in a Truss Bridge
In design theory, the individual members of a simple truss are
only subject to tension and compression and not bending
forces. For most part, all the beams in a truss bridge are
straight.

Arch Bridges
 Arches used a curved structure
which provides a high
resistance to bending forces.
 Both ends are fixed in the
horizontal direction (no
horizontal movement allowed
in the bearings).
 Arches can only be used
where ground is solid and
stable.
 Hingeless arch is very stiff and
suffers less deflection.
 Two-hinged arch uses hinged
bearings which allow rotation
and most commonly used for
steel arches and very
economical design.
Hinge-less Arch
Two hinged Arch

Arch Bridges
 The three-hinged arch
adds an additional hinge at
the top and suffers very
little movement in either
foundation, but
experiences more
deflection. Rarely used.
 The tied arch allows
construction even if the
ground is not solid enough
to deal with horizontal
forces.
Three-hinged Arch
Tied Arch

Forces in an Arch
 Arches are well
suited to the use of
stone because they
are subject to
compression.
 Many ancient and
well-known
examples of stone
arches still stand to
this today.

Cable Stayed
 A typical cable-stayed bridge is a continuous deck with
one or more towers erected above piers in the middle
of the span.
 Cables stretch down diagonally from the towers and
support the deck. Typical spans 110m to 480m.

Cable Stay Towers
Cable stayed bridges may be classified by the
number of spans, number and type of towers,
deck type, number and arrangement of cables.

Suspension Bridge
 A typical suspension bridge is a continuous deck with one or
more towers erected above piers in the middle of span. The
deck maybe of truss or box girder.
 Cables pass over the saddle which allows free sliding.
 At both ends large anchors are placed to hold the ends of the
cables.

Site Information
 Survey - existing ground level and site details.
 Soil investigation - at least one bore-hole for each support
position to determine safe bearing pressure, aggressive
conditions and predict settlement.
 Mining - details of old working and future seams.
 River Board – navigation requirements, maximum flood
levels and scour problems around foundations.
 Railways – frequency of trains, available track possession,
minimum headroom, position of supports and piling
techniques.
 Statutory Undertakers – diversion of existing services,
provision for future services in the deck.

Site Information
 Planning Authorities – normally concerned with aesthetic
appeal and the effect on local amenities.
 Road Geometry – details of horizontal and vertical
alignment together with the road cross-section.
 Design Standards – design live loading, visibility distances,
headroom standards and horizontal clearances.
 Time – the time for design and the phasing of construction
in relation to other work.
 Atmospheric Conditions – an aggressive environment may
involve high maintenance costs for steel construction and
special precautions in the detailed specification.

Conceptual Choice
Considerations
Initial conceptual choice should take account of:
 clearance requirements and the avoidance of
impact damage
 type & magnitude of loading
 topography and geology of the site
 possible erection methods
 local skills and materials
 future inspection and maintenance
 aesthetic and environmental aspects

Factors Affecting Conceptual
Choice
 The functional considerations that have
greatest influence on conceptual choice are:
 The clearance requirements (both vertically
and horizontally) and avoidance of impact
 The type and magnitude of the loading to be
carried
 The topography and geology of the site

Clearance Requirements
 All bridges must be designed to ensure, as far as is
possible, that they are not struck by vehicles, vessels or
trains which may pass below them. This requirement is
normally met by specifying minimum clearances.
 It must be remembered that designed values must take
into account deflections due to any loading that may
occur on the bridge structure.
 Clearance requirements may thus determine the span of
a bridge and also have a significant bearing on the
construction depth. Whilst the requirements will not
normally determine precisely the type of bridge, it may
well eliminate some possibilities.

 Typically, for example, a bridge over a major
highway would be expected to have a
minimum vertical clearance of about 5,3
metres; even this may not protect it from
accidental impact
 In addition, pier positions must be such that the
likelihood of impact from errant vehicles is
minimised, both to protect the pier and the
vehicle itself. This requirement is usually
achieved by setting the pier back a reasonable
distance from the edge of the carriageway.

 Navigation authorities specify clearances over rivers,
to allow not only for the mast height and width of
vessels below the bridge, but also for particular
requirements for piers in the waterway (or on a flood
plain) to avoid excessive flow velocity and scour of
river banks.
 In considering vertical clearance, a designer must
bear in mind the problems of attaining them. The
approach gradient for a highway bridge should not
normally exceed about 4% and a railway bridge much
less. This is of great importance when comparing
fixed with moving bridges.

Loading
 The type and magnitude of loading has a
significant bearing on the form of bridge.
Highway loading by its nature is impossible to
determine exactly, either in disposition or in
magnitude.
 A highway bridge requires a deck on which the
traffic can run and (unless the span is so short
that a simple slab is adequate to span between
abutments) the deck must be strong enough to
distribute the loading to the main girders.

Loading
 Every country has its own specification for the magnitude of
loading on highway and railway bridges. For highway bridges
most national codes have in common a uniform loading
together with a line load (or series of point loads) to represent
isolated heavy axles. In many codes, the uniform load is of
decreasing intensity as the length of bridge increases, to allow
for the reduced probability of a concentration of heavy lorries.
 Furthermore, there are rules for multiple lane loadings,
frequently assuming that not more than two lanes are fully
loaded at any one time, again based on a probabilistic
approach. Many authorities also specify checks for a single
very heavy abnormal vehicle. In many codes, the effect of
impact (dynamic magnification) of highway loads is implicitly
taken into account by the static load specification.

Loading
 Additionally, forces arising from braking or
acceleration of vehicles, centrifugal effects on
curved bridges, temperature effects and wind
have to be taken into account where relevant.
 Whilst the details of applied loads are
appropriate to the detailed design, rather than
the conceptual design of a bridge, certain
aspects enter into the concept. For example,
where heavy abnormal vehicles are specified,
the bridge will require good transverse load
distribution.

Topography & Geology of Bridge
Site
 The overall topography of the site will probably determine the line of
the road or railway. Not infrequently this may mean that bridges will
have to cross other roads, railways or rivers at a substantial angle,
resulting in skew spans. Generally, the bridge site is fixed by the
geometry of the obstacle and local terrain.
 The road may be on a curve; whilst it is possible to curve a bridge to
follow this, it is frequently expensive and structurally inefficient,
usually dictating the use of torsionally stiff girders even for short
spans. If the curve is slight, it may be preferable to construct the
bridge as a series of straight spans.
 Poor foundation conditions will favour fewer foundations and hence
longer spans. A balance has to be found between the cost of
foundations and superstructure to minimise the total cost.

Other Factors
Method of Erection
 It has long been appreciated that a designer must
consider at the design stage the method by which a
bridge will be erected. Indeed it is not infrequently the
case that such consideration should be made even at the
time of conceptual choice, since it can happen that the
superficially most attractive design is impossible to erect
in a particular location.
 For example, a design that relies on being erected in
large pieces (such as a major box girder), may be ruled
out because of the impossibility of transporting such
pieces to a remote site with inadequate access roads.

Other Factors
Local Constructional Skills and Materials
 A bridge should be suited to local technology. It is not sensible
to specify a sophisticated design if all the material and labour
has to be imported.
Future Inspection and Maintenance
 Lack of attention to future maintenance both at the conceptual
design and the detailed design stages would results in many
bridges, otherwise satisfactory, have deteriorated because of
difficulty in inspection and maintenance. It is particularly
important that in locations where access is difficult (either
physically or because it would cause disruption of services)
details which deteriorate should be avoided as far as possible.
This will be considered further in various respects, for example
whether a bridge should be a series of simple spans or should
be continuous.

Other Factors
Aesthetic and Environmental Aspects
 The appearance of bridges has in recent years become a
matter of considerable importance. Frequently, a scheme takes
a road or railway through an area of great natural beauty and it
is important that any structures are in keeping with these
surroundings and do not adversely affect them.
 For example, it is commonly accepted that a bridge is more
aesthetically pleasing with an odd number of spans than an
even number. In addition, a degree of deepening at piers can
add to the attraction.
 The 3-span structures are more attractive than the two span
ones. Hence, unless there are other contra-indications, the
conceptual choice should probably tend towards a 3-span
solution.

Detailed Design Considerations
The design development needs to make the
correct choices for:
 deck structure
 layout i.e. spans and structural arrangements
 continuous or simple construction
 proportions, i.e. span/depth ratios
 reducing fabrication labour to a minimum
 design for ease of construction

Basic Components of a Bridge
The two basic parts are:
 Substructure - includes the piers, the abutments and the
foundations.
 Superstructure - consists of the deck structure itself,
which support the direct loads due to traffic and all the
other permanent and variable leads to which the structure
is subjected.
 The connection between the substructure and the
superstructure is usually made through bearings.
However, rigid connections between the piers (and
sometimes the abutments) may be adopted, particularly
in frame bridges with tall (flexible) piers.

Substructure : Piers
Piers are of two basic types:
 Column piers - Concrete column piers may have a solid
cross-section, or a box section may be the shape chosen
for the cross-section for structural and aesthetic reasons.
 Wall piers - generally less economical and less pleasing
from an aesthetic point of view. They are very often
adopted in cases where particular conditions exist, e.g.
piers in rivers with significant hydrodynamic actions or in
bridges with tall piers where box sections are adopted.

Substructure : Abutments
 The abutments establish the connection between
the bridge superstructure and the embankments.
They are designed to support the loads due to
the superstructure which are transmitted through
the bearings and to the pressures of the soil
contained by the abutment.
 The abutments must include expansion joints, to
accommodate the displacements of the deck, i.e.
the longitudinal shortening and expansion
movements of the deck due to temperature.

Basic Types of Abutments
Two basic types of abutments may be considered:
 Wall (counterfort) abutments and Open abutments.
 Counterfort wall abutments are adopted only when the
topographic conditions and the shapes of the backfill are
such that an open abutment cannot be used. They are
generally adopted when the required height of the front wall
is above 5.0 to 8.0m. If the depth is below this order of
magnitude, counterfort walls may not be necessary and a
simple wall cantilevering from the foundation may be
adopted.
 The connection between the abutments and the backfill may
include an approach slab which ensures a smooth surface
of the pavement even after settlement of the adjacent
backfill.

Basic Types of Bridge
Abutments – Wall & Counterfort
Wall Abutment Counterfort

Basic Types of Bridge
Abutments – Open Type

Superstructure – Structural
Systems
 The longitudinal system of a bridge may be one
of the following types: beam, frame, arch, cable
stayed or suspension.
 There are three main types of bridge transverse
systems, slab, beam-slab or box girder.
 Bridge superstructures may use the beam and
plate girder, truss girder or box girder structural
systems.
 Deck systems use a reinforced concrete slab,
with or without cross-girders, or a partially
prestressed concrete slab, or an orthotropic steel
plate.

Bridge Longitudinal Structural
Systems

Bridge Deck
 The principal function of a bridge deck is to
provide support to local vertical loads (from
highway traffic, railway or pedestrians) and
transmit these loads to the primary
superstructure of the bridge.
 As a result of its function, the deck will be
continuous along the bridge span and (apart
from some railway bridges) continuous across
the span. As a result of this continuity, it will act
as a plate (isotropic or orthotropic depending
on construction) to support local patch loads.

Design Case Study
MAB1053
Bridge Engineering

Longitudinal & Transverse Sections

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