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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.
Lecture Time Table CONTENT WEEK/DATE LECTURER CONTENT WEEK/DATE LECTURER
- 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 1 st 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
Introduction to Bridges MAB1053 Bridge Engineering
What is a bridge?
A bridge is a structure that spans a divide such as:
The traffic that uses a bridge
Pedestrian or cycle traffic
Vehicular or rail traffic
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
General Span Types
Materials for Construction
A Typical Single Span Bridge
Basic Components of a Bridge
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.
Typical Beam/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
Bridges which Carry their Loads Mainly as Axial Forces
Basic Types of Bridges
Rigid Frame Bridge
Cable Stayed Bridge
The most common and basic type
Typical spans : 10m to 200m
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.
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
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.
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.
Cable Stay Arrangements
Cable Stayed Bridges
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.
Forces in Suspension Bridge
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.
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
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.
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.
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.
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.
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.
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.
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.
Estimated Bridge Costs
Detailed Design Considerations
The design development needs to make the correct choices for:
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.
Basic Types of Bridge Piers
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.
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
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.