Week 1 – Introduction to Bridge Engineering

1.1. Introduction
A bridge is a large structure used as a passage for vehicles, pedestrians, trains, services and other
means to avoid obstacles or provide access to unapproachable areas, such as seas, rivers, valleys,
other bridges, roads or constructions. Bridges consist of specific structural elements, including
foundations and holding down systems, trusses, slabs, beams and girders, piers, cable structures
and arches. A Civil Engineer should be able to evaluate such elements and perform analysis and
design methods relevant to such structures as per the current codes of practice. This topic overview
is aimed at introducing students to the basic structural parts of a bridge, namely the superstructure
and substructure. Bridges are further classified based on the superstructure and other structural
characteristics. The topic overview also provides insights into the materials used in the
construction of bridges.

1.2. Learning Outcomes

Upon the completion of this week of study you will be able to:
LO1: Critically evaluate the use of common structural materials in large structures.
LO2: Evaluate a range of structural theories and methods of analysis and their relevance to large
structures, according to current codes of practice.

1.3. Types of Bridges

As bridge structures have evolved over the years from simple to contemporary and more complex
structures, it is necessary to classify them as per their superstructure. Superstructure of a bridge is
the assembly above the substructure that provides the horizontal span, as shown in Figure 1 below.
The span of a bridge is the centre-to-centre distance measured between the edge supports. The
superstructure of a bridge includes the deck, girder, truss and other elements, whereas the

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substructure includes the foundations such as cap beam, columns, piles and footing,
bolting/anchoring system and bearings.

Figure 1: An example of the superstructure and substructure arrangement on a typical girder

bridge (Lee and Sternberg, 2015).

Modern bridges can further be classified according to the following characteristics (Weiwei and
Yoda, 2017):
A. Materials of construction: Bridges can be termed according to the key material of which their
superstructures consist, i.e. concrete, steel, timber, stone, aluminium or other advanced
materials.

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B. Span length: Albeit bridge classification by span length is ambiguous, Taly (1997) proposed
to characterise them as culverts those with a length shorter than 6m, short-span bridges those
with a length between 6 and 38m, medium-span bridges those that their length varies
between 38 and 125m and long-span the bridges with a bigger length. However, as more
advanced methods and materials are employed, the limits given above are constantly
increasing.

C. Position: This criterion applies only for moveable bridges that are designed in such a way in
order to typically allow a passage for sea transport means, such as boats, vessels, cargo ships,
carriers and tankers. Bascule, whose main girders are lifted with the deck, is the most
common type of a moveable bridge, refer to the famous tower bridge in London.

D. Span support types: These bridges are characterised by the support method of their load
carrying element and they can be further classified into simply supported span, continuous
span and cantilever span bridges, as seen in Figure 2 below. Continuous span bridges are
supported by only one structural element from one edge to the other, whereas simply
supported span bridges are supported by several individual elements. Cantilever span bridges
on the other hand are supported by a specific number of cantilevers, which are anchored only
at one of their end.

Figure 2: (A) Simply supported span, (B) Continuous span, (C) Cantilever span bridges
(Weiwei and Yoda, 2017)

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E. Deck location: When the deck is located above the superstructure the bridge is characterised
as a deck bridge, while in the opposite arrangement it is called a through bridge. When the
deck is placed on the middle of the main body of the bridge, the bridge is called a half-
through bridge. Therefore, the bridges in this occasion are characterised by the location of
the deck in relation to the main body of the bridge, which carries the load.

F. Usage: Another very common way to name bridges is by their respective application, i.e.
what they are designed to carry (traffic, pedestrians, services etc). Highway, railway,
pedestrian, airport runway and other bridges belong in this category (Weiwei and Yoda,
2017). Also, double deck bridges can carry a combination of two of the above loads.

G. Geometric shape: Different geometric shapes used in bridge structures can determine their
nomenclature, such as straight bridges, the axis of which follows a straight line, skewed
bridges, which are constructed only when a straight bridge is not feasible to connect the two
points of interest, and curved bridges, which are difficult to design and construct and they
are mostly used in roads.

H. Structural form: The most common classification for bridge constructors emerges from the
structural form of the bridge. The structural form affects the design, the service life of the
bridge, and the repair and maintenance procedures that will be followed. Beam (or girder),
rigid-frame, truss (Figure 3), arch, cable-stayed and suspension bridges are examples of
bridges characterised by the structural form. A typical example of a girder bridge is shown
in Figure 1 above. The first, is the oldest and simplest structure and it is supported by beams
(from steel, concrete, wood etc) that connect the two ends of the bridge. Rigid-frame bridges
are supported on legs (columns), while truss bridges consist of elements connected together
in triangular geometries. On the other hand, arch bridges are shaped as an upward curved
arch to sustain the vertical loads (Figure 4). Cable-stayed bridges employ cables, each group
of which converges to a single point called a tower, to support the load of the bridge, as
illustrated in Figure 5. Finally, suspension bridges are suspended with the use of cables,
which are fastened by anchorages and towers, shown in Figure 6. Cable-stayed and
suspension bridges are modern designs that can support great loads and can have a long span.

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 Figure 3: Truss bridge example (Weiwei and Yoda, 2017).

Figure 4: Arch bridge principle (Weiwei and Yoda, 2017).

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 Figure 5: Cable-stayed bridge principle (Weiwei and Yoda, 2017).

Figure 6: Suspension bridge principle (Weiwei and Yoda, 2017).

1.4. Materials used in Bridge Superstructure

The materials used in the construction of bridges should be rigid and be able to withstand bending
forces and stresses applied during the carrying of loads. Therefore, they should have an elasticity
and not deform more than a certain limit under stress, and be able to return at the initial form once
the load is removed (Lee and Sternberg, 2015).
Structural steel performs very well under such conditions due to its high elasticity, especially when
used for the production of cables that support a cable-stayed or a suspension bridge. Wood logs
used to construct beam bridges are nowadays replaced by steel. Nevertheless, structural steel is an
expensive material and undergoes corrosion over time. Therefore, preventive and protective
maintenance is required when the material is used.
Concrete is made from water, fine aggregates (sand) and coarse aggregates, and is a strong
material with high resistance to compressive forces and is used in some occasions to form the
main body of a bridge (especially in arch bridges). It is a cheaper material compared to steel but,

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in order to enhance it and increase its tensile strength, steel bars should be installed within the
concrete structure. In the older times, rock was used instead of concrete, especially when culvert
and slab bridges were constructed. Culvert bridges are actually large pipes crossing under the
road, used to allow surface water to drain or channels for rivers, space for mechanical,
electrical or other services, or even to allow animals to move around. Slab bridge on the other
hand, is just a flat block of material laid across a small span (Lee and Sternberg, 2015).
Rock was also used in the construction of arch bridges, which can withstand high loads and have
a long life due to the durability and strength of rock. However, rock is not used anymore in bridge
building, and it has been replaced by reinforced concrete (refer to Figure 8) or steel in modern arch
bridges. In order to use less material, truss bridges were invented and used instead of arch bridges.
Truss bridges are composed of a series of metal triangles, which are known as trusses, that offer
great durability in stress distortion. Therefore, they can support considerable loads over large
spans. A truss consists of chords and diagonal and vertical elements, as illustrated in Figure 7.
More details about truss bridges will be presented in the topic overview of week 5.

Figure 7: An example of a truss bridge (Lee and Sternberg, 2015).

The deck of a bridge, which is the upper surface of the superstructure (refer to Figure 1), can be
made of concrete, steel, open grating or wood. According to the intended use of the bridge, the

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deck can be covered by the appropriate bed, such as asphalt for roads, pavement for pedestrians or
railroad.
Girder bridges use a girder to support the deck, which are steel beams shaped to either an I-section
(plate girder) or a box section (box girder), which are both shown in Figure 9. Box girders can
better handle torsion forces imposed by wind, traffic and earthquakes. The role of the girder is to
withstand bending forces, which are greater at the middle of the span of the bridge. The longer the
span is, the deeper the girder needs to be, while the flanges that support the girder must be of larger
cross-sectional area (Lee and Sternberg, 2015).

Figure 8: Cross section of a reinforced concrete beam (Lee and Sternberg, 2015).

Figure 9: I-beam profile (left) and steel box beam profile (right) (Lee and Sternberg,
2015).

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1.5. Conceptual Design of Bridge Superstructure
Concept bridge design is an iterative process requiring a comparative analysis (preliminary design
phase) of a variety of possible options, satisfying a set of data obtained from topographic,
geometrical, hydraulics, geotechnical and environmental investigations (Oliveira and Reis,
2019). Afterwards, the design phase can be implemented.

The structural materials, the longitudinal structural system, the transverse cross section of the deck,
the connection type between the deck and the piers and abutments and the execution method are
decided prior the conceptual design of the superstructure. For superstructure conceptual design
and execution methods please refer to Oliveira and Reis (20019), Chapters 4.4, 4.5 and 4.6. Design
of the main girders, stiffeners and bracing systems is also provided in Chapter 4.5.

1.6. Substructure and Foundations and their Materials

Substructure of a bridge is the unit that supports the superstructure and it is placed under the
bearing, as previously shown in Figure 1. It is used to transmit all the loads that act on the bridge
to the ground. Bridge substructure consists of abutments, piers, retaining walls and other
foundation systems, such as columns, piles and shafts, which could be made from, wood, stone,
concrete and/or steel (Weiwei and Yoda, 2017).
Abutments: They are vertical load-bearing structures at the ends of a bridge span used to support
its superstructure.

Retaining or Wing walls: They are the walls that retain earth and they can be built of brick, stone,
masonry, cement concrete or reinforced cement concrete.

Bridge Foundation: It is the system built under the abutments and piers and over the soil or rock
and it usually consists of columns or piles. The foundation is responsible for sending the loads that
act on the superstructure to the earth to prevent shear failure or harming of the superstructure.

Piers: They are also vertical load bearing elements, but, in contrast to the abutments, they provide
only intermediate support. Piers are usually made to rest on piles or on concrete structures called

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spread footings, as seen in Figure 10. The type of pier has to be selected according to the soil
morphology, whereas water pressure and possible earthquakes should be also considered.

Drilled Shafts: They are part of the foundation system and they are deep foundation cylindrical
solutions within the ground, filled with concrete and used to support structures with large axial
and lateral loads.

Figure 10: Types of piers and foundations (Lee and Sternberg, 2015).

1.7. Summary
The main types of bridges along with the common structural materials used in large structures
were analysed in this topic overview. Furthermore, the superstructure and substructure of a bridge
were briefly examined in terms of materials, design and construction methods.

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References
Weiwei, L. and Yoda, T. (2017) Bridge Engineering: Classifications, Design Loading, and
Analysis Methods, Elsevier Science & Technology, Oxford. Available from: ProQuest
Ebook Central.

Taly, N. (1997) Design of Modern Highway Bridges, McGraw-Hill, New York.

George C. Lee and Ernest Sternberg (2015) Bridges : Their Engineering and Planning.
Albany, NY: SUNY Press. EBSCOHost.

Oliveira, P.J.J. and Reis, A.J. (2019) Bridge Design: Concepts and Analysis. Bognor Regis:
John Wiley & Sons, Incorporated. Available from: ProQuest EBook Central.

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