What is a Bridge?: A Complete Guide To Engineers

What is a bridge? This is a common question for Engineers and Engineering Students. A bridge is a structure constructed to cross a physical barrier (like a water body, valley, road, or railway) without closing the way underneath.

Table of Contents

Types of Bridges

Several types of bridges can be discussed based on various classifications, the following are different types of bridges according to various classification:

1. Classification according to the material used.

Based on the material used there are Timber bridges, Steel bridges, Reinforced Concrete Bridges, and Composite bridges.

2. Classification according to usage.

Based on the anticipated use of the bridge, the bridge may be a pedestrian-type bridge, Highway bridge, or Railway bridge.

3. Classification according to span.

The bridge may be a small-span bridge (up to 15m), a medium-span bridge (15m to 150 m), a large-span bridge (50m to 150m), or an extra-large large (above 150m).

4. Classification according to the form of construction.

Based on their construction type, bridges are slab girder, truss, arch, suspension, and cable-stayed.

5. Classification of the bridge according to the structural arrangement.

The classification of bridge types can also be according to the location of the main structure elements relative to the surface on the use travels as follows;-

• The main structure below the deck line (e.g Arch bridge),
• The main structure above the deck line (e.g Suspension bridges, Cable-stayed bridges),
• The main structure coincides with the deck line (e.g. Girder bridges).

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Selection of Bridge

One of the engineering challenges is to design and construct a structure that is economical both for the initial construction costs and anticipated maintenance costs. The structure must also be structurally able to safely transmit the applied loads to the subsoil and sufficiently serve the purpose of crossing the gap without compromising the aesthetic requirements.

Any designer (Engineer) needs to carefully select a suitable structure by considering site conditions, e.g., geology location, size of crossing, availability of construction material, construction method, and so on.

And then come up with an economical structure. In each type of bridge, suggestions have been on the suitability for a professional selection.

The following aspects will lead to the choice of bridge type

• Economy: the availability of resources for the project can prompt the choice of the type of bridge/ drainage structure to be used.
• Availability of materials: The available material can influence the type of bridge
• Site condition: The site condition, such as geological conditions, type of subsoil, catchment areas, length of the waterway, location of the crossing, and estimated discharge of the structure will influence the choice of type of bridge.
• Availability of construction method: The available technology, expertise, and construction means can restrict the type of bridge structure that can be used.
• Utility: The principal use of the structure, as well as the class of the road on which the bridge is constructed, can necessitate the use of a certain type of bridge. The road may be classified according to the traffic volume, which employs expected loading, connectivity, and economic viability. Also, a bridge used by pedestrians or bicycles can restrict the type of bridge.
• Time crisis: The urgent circumstance for the structure’s requirement can lead to the selection of a certain type of bridge.

Bridge Location

The bridge location should be selected to suit the particular obstacle, such as a river crossing. The initial cost of bridge works and the minimization of total costs, including river training works and the maintenance measures necessary to reduce erosion, should be considered.

For highway and railroad crossings, provisions should be for possible future works such as road widening.

The summarized characteristics of an ideal bridge across a river are:

1. A narrow channel with firm banks (has well-defined banks).
2. Suitable high banks above high flood level on each side.
3. Steady river flow without serious whirls and without obstacles that will cause water currents to change direction.
4. Economical approaches, free from obstacles such as hills, frequent drainage crossing build-up areas, or troublesome land acquisition.
5. Be in alignment with the road to be connected.
6. Favorable geotechnical soil conditions.

Please note: Considering all the mentioned above, the reconnaissance survey should be done.

Bridge width & bridge cross-section

Under the draft road manual, the carriageway should be at least 1.0 m wider than the width of the approach road carriageway.

An appropriate cross-section for the bridge shall be determined based on the following criteria:

• Road safety considerations.
• Class and cross-section of the approach road.
• Volume of both pedestrian/ cyclic traffic and vehicle traffic.
• Speed of vehicle traffic.
• Length of the bridge

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Carriageway of the bridge

The carriageway of the bridge can be determined according to the road class of the road (AADT) measured in the passenger car units (pcu).

The values set in the table below may be used as a guideline for choosing the cross-section.

(Table above: Standard drainage structures manual-part 1 for small span bridge by the Ministry of Works, the Government of The United Republic of Tanzania)

Components of Reinforced Concrete Bridge

A bridge can a broadly subdivided into two parts, such as:

1. The superstructure

The superstructure bears the load passing over the bridge and transits the same together with the other forces caused by the moving loads to the sub-structure.

The superstructure of the bridge also termed decking consists of the following main components;

• Deck slab including structural system of longitudinal beams, cross beams provided for supporting deck slab.
• Curbs, footpath, handrail.
• Wearing coat.

2. The substructure

The sub-structure of the bridges consists of the following main components:

• Abutments, wing walls, and piers together with their foundations may be of shallow type (isolated footing, combined footing, strip footing, or raft) or deep type (piles, wells, or caissons).
• The bearing is provided above the abutment and piers on which the superstructure rests. The bearings transmit the load received from the superstructure to the substructure and also permit a small magnitude of movements (due to temperature variations, deflection, or sinking of supports of the superstructure) without any damage to the bridges.

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Design principles of various members

Structures and structural members should be designed to have strengths at all sections at least equal to the structural effects of design loads and forces that occur during construction and use, as determined by the relevant design method.

Structures and structural elements should also meet all other requirements of the relevant codes (i.e. BS 5400 part 2).

The design should always follow the principles of mechanics, recognized design methods, and sound engineering practice.

In particular, adequate consideration should be given to the effects of continuity on the distribution of bending moments and shears due to monolithic construction.

Reasonable factors of safety must be taken in designing the members. The ratio of the strength of the section of the member at which it becomes unserviceable (may be due to failure or because of excessive deflection or cracking or some other similar reason) to actual forces (maybe bending moment, shear, the axial force of torsional moment) to which the section is subjected is termed as Factor of Safety.

Methods of analysis

Various methods have been adopted in various codes of practice in different countries for bridge design.

Some of the known methods of design are briefly explained subsequently.

• Working Stress Method.

This method is also known as the Modular Ratio Method or the ‘Elastic Stress Method’. In this method, the moment and force acting on a structure are computed from the actual values of service loads, but the stresses in concrete and reinforcing steel are restricted to only a fraction of their true strengths to provide an adequate safety factor.

A greater factor of safety is adopted in concrete because, in the event of failure, crushing of concrete takes place unexpectedly and explosively since it is a brittle material.

Whereas steel yields gradually and hence gives a warning.

NOTE1: In this elastic approach, the design is based upon a set of theories available in many books because of the basic assumption that the reinforced concrete is homogeneous. All the forces, such as bending moments, shear forces, torsion moments, and axial loads, can be easily computed assuming the material behaves perfectly elastic.

Since reinforced concrete is not a perfectly elastic material, the methods of analysis do not give real behavior of the structure, highly stressed parts of the structure start deforming unproportionately to the loading and distribution of moments is also not the same as for a perfectly elastic material.

Concrete members never have uniform moments of inertial due to variable cracking along their length and varying areas of reinforcement

NOTE 2: Because of these reasons (as given in note 1) Working stress design moment is unable to give any real guide to the safety of a structure. There is absolutely no guarantee that a structure is designed by the elastic method.

• Limit state methods

In this design method based on the limit state concept, the structure shall be designed to withstand safely all loads liable to act on it throughout its life. It shall also satisfy serviceability requirements such as preventing excessive deflection, excessive cracking, and excessive vibrations.

The acceptable limit for the safety (ultimate) and serviceability requirement before failure occurs is called the ‘Limit State’.

The design aims to achieve acceptable probabilities that the structure will not become unfit for its intended use. To ensure this, various partial factors of safety are employed in the limit state design.

In this concept, design loads are obtained by multiplying a partial factor of safety for loads with characteristics loads, and similarly, the design strength of materials is obtained by dividing the characteristics strength with respective partial factors of safety for materials.

NOTE1: This approach is more logical and rational and has the merits of simplicity. It is based on the probability approach.

By using this method both the strength and serviceability requirements of structures are satisfied

NOTE 2: This approach is grouped into two main groups as follows:

a. Ultimate limit state

This is the state whereby the structure will become unfit to sustain any increase in load, including;

• Strength limits (generally yielding and rapture)
• Overall loss of equilibrium or stability (overturning)
• Elastic or plastic deformation
• Fatigue or brittle fracture
• Structural integrity (including accidental damage)

b. Serviceability limit

This is the state whereby the structure loses its strength to carry loads but does not
collapsed, thus requiring remedial action. This may be caused by:

• Excessive deformation due to deflection
• Durability of the structure
• Excess vibration
• Corrosion of steel
• Cracking of steel

Partial Safety factor

These are factors used in all designs to allow variability of load, material, and workmanship that can not be assessed with absolute certainty.

These factors of safety are to be applied to the nominal loads and strength properties of material taking into account the probability of loads being exceeded and assessed design strength not being reached.

Static System

One prerequisite for the bridge designer is to know the load-supporting condition that has to be applied in the structure and how the superstructure acts when carrying loads; this is termed the static system of the structure.

A wide range of static systems may apply in the design. Commonly used static systems are:

• Supported System: A simply supported static system means that the superstructure is freely supported at the end of each span. This is the most common type. The simplest form consists of one span and two end supports. For wider gaps, several simply supported spans are used with joints at the intermediate supports.
• Continuous Static System: A continuous static system has a superstructure, with extends over one or more intermediate supports with no joints over the intermediate support.
• Cantilever with Suspended Spans: This type of static system consists of continuous superstructures over piers with a suspended span supported on cantilevers.
• Arch Systems: Arch static systems have arched superstructures, sometimes hinged at both supports, in the middle of the span length, or at both supports and in the middle of the span. Arches can also be cantilevered at the supports with no hinges.

Source: (Bridge Management System for Tanzania: Handbook for Bridge ( 2010))

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Loading on Bridges

Loading: The force acting on a bridge due to its weight, imposed live load, or constrained deformation.

Loads can be classified according to their variation in time, and space, or according to their dynamic nature.

Loads acting on the bridge can be classified into two main types: Permanent loads and Transient loads.

1. Permanent load

This is the load considered to be acting at all times. It can be classified into two types: dead load and superimposed load.

• Dead load:

Weight of the material and parts of the structure that are structural elements, but excluding superimposed materials such as road surfacing, rail track ballast, parapet, main, ducts miscellaneous furniture, etc (BS5400-2, (2006))

The design dead load (Qk), is obtained by multiplying the nominal dead load with a partial factor (γfl) the partial factor should be applied to all parts of the dead load for various combinations of actions as shown in action in the table below:

(BS5400-2, (2006))

• Superimposed dead load

Weight of all materials forming loads on a structure that are not structural elements. These
Include permanent fixtures on the bridge such as parapets, guardrail-wearing surfaces (bitumen,
Concrete or gravel). Table below:

(BS5400-2, (2006))

2. Transient loads

This is all loads other than permanent loads including: Live load, Temperature, and wind load.

• Live load:

These area loads are due to vehicle or pedestrian traffic. They can either be static or dynamic
depending on the type of load.

During design, these loads are converted to a simplified format to ease the design process.

Table above: Live load

• Temperature

Daily and seasonal fluctuations in shade air temperature, solar radiation, re-radiation, etc., cause the following:

a) Changes in the effective temperature of a bridge superstructure which, in turn, govern its movement.

The effective temperature is a theoretical temperature derived by weighting and adding temperatures at various levels within the superstructure.

The weighting is the ratio of the cross-section area at the various levels to the total area of the cross-section of the superstructure.

Over some time, there will be a minimum, a maximum, and a range of effective bridge temperatures, resulting in loads and/ or load effects within the superstructure due to:

1) Restraint of associated expansion or contraction by the form of construction (e.g. portal frame, arch flexible pier, elastomeric bearings) referred to as temperature restraint; and

2) Friction at roller or sliding bearings where the structure’s form permits associated expansion-contraction is referred to as frictional bearing restraint.

b) Differences in temperature between the top surface and other levels in the superstructure. These are referred to as temperature differences and they result in loads and/or load effects within the superstructure.

3. Wind loads

The wind pressure on a bridge depends on the geographical location, the terrain of the surrounding area, the fetch of terrains upwind of the site location, the local topography, the height of the bridge above ground, and the horizontal dimensions and cross-section of the bridge or element under consideration.

The maximum pressures are due to gusts that cause local and transient fluctuations in the mean wind pressure.

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What is a Bridge?: Wrapping Up

As discussed above, a bridge is a structure that can span physical obstacles without blocking the way underneath.

The main function is to cross the barrier, whatever that may be (people, vehicles, trains, pipelines).

Bridges are classified into different types and materials based on their purpose and the terrain they cross.

Different types of bridges include beam bridges, arch bridges, suspension bridges, and cable-stayed bridges as mentioned.

Each type has its unique engineering principles and aesthetic qualities.

Is there a specific type of bridge or a particular aspect of bridges that you’re interested in learning more about?

We hope this article helped you learn more about: What is a Bridge?: A Complete Guide To Engineers. You may also want to learn The Complete list of BSs in Civil Engineering Subjects, Building Materials: Types, Properties, and Uses all Engineers Should Know, and What is the Function of Buildings

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