Bridge Engineering Design

BRIDGE ENGINNERING

Literature Review for Engineering Design of Bridges www.rowland-adewumi.com

1.0

INTRODUCTION

1.1

Bridge structure is designed to provide continuous passage over an obstacle. Bridges

commonly carry highways, railways, deep valley and other transportation routes. Bridges may also carry water, support power cables or house telecommunications lines. Some special types of bridges are defined according to their functions. An overpass allows one transportation routes, such as highway or railway line, to cross over another without traffic interference between the two routes. The overpass elevates one route to provide clearance to traffic on the lower. The design of bridge structures has become intricate with the danger over from the conventional girder slab bridges to the complex interchanges requiring curved units or cable stayed or suspended units. The analysis of such structures having different forms and shapes requires ingenuity of a high order as research way lag behind practical possibilities.

www.rowland-adewumi.com

1

How then can we, builders of bridges, calculate and design those daring structures to safely support the loads railways trains or heavy vehicles and to withstand the often unpredictable forces of wind and water. The emphasis on theory and too-little consideration for structural detailing and on site realities have resulted in bridge collapses in the not too distant past. Above all, this research work, has been directed towards an in depth study of structural analysis and design of 120 (one hundred and twenty) meters span bridge as a contribution to knowledge. 1.2

AIMS AND OBJECTIVES OF BRIDGE CONS TRUCTION The project aims at the following: 1.

To construct a bridge to facilitate easy movement between the two communities and overcome obstacles posed by nature/physical geography.

2.

To boost the socio-economic life of the communities in neighbourhood To reduce traffic congestion occasioned by bad terrain that serves as obstacles to easy movement.

3.

To cope with growing pedestrian and vehicular traffic.

The choice of bridge is crucial to the achievement of the aims. Consequently, the reinforced concrete bridge is adopted in order to realize the aim of this project. The emphatically analytical method is employed in the determination of load, analysis and sizing of structure with a view of arriving at set objectives. The objectives of this project are as follows: 1.

To design a reinforced concrete bridge to link Oyo and Iseyin.

2.

To use the empirical design methods in arriving at safe, economical and functional structure.

3.

To achieve ethically pleasing structure.

www.rowland-adewumi.com

2

CHAPTER TWO 2.0

LITERATURE REVIEW

2.1

HIS TORICAL BACKGROUND The earliest bridges were simple structure created by spanning a gap with timber or

rope. Designs became more complex as builders developed new construction methods and discovered better materials. The stone arch was first advance in bridge design. It was used by the ancient Greeks, Etruscans and Chinese. The Romans perfected arch design, using arches to build massive stone bridges through out the Roman Empire. Stone arch construction remained the premier bridge design until the introduction of the stream locomotive in the early 19th century. Between 1830 and 1880, as railroad building expanded throughout the world, bridge design and construction also evolved to carry these heavy vehicles over new obstacles. Designers experimented with a wide variety of bridge types and materials to meet the demand for greater heights, spans and strength. Locomotives were heavier and moved faster than anything before requiring stronger bridges. The basic bridge, a simple beam over a span was strengthened by adding support piers underneath and by reinforcing the structure with elaborate scaffolding called a truss. During the period expansion, iron trusses replaced stone arches as the preferred design for large bridges. In 1855 British Investors Sir Henry Bessemer developed a practical process for converting cast iron into steel. This process increased the availability of steel and lowered production costs considerably, the strength and lightness of steel revolutionized bridge building. In the late 19th Century and the first half of the 20th Century, many large scale steel suspension bridges were constructed over major waterways. Also in the late 19th century, engineers began to experiment with concrete has been combined with steel girders, which are

www.rowland-adewumi.com

3

solid beams that extend across a span. When the interstate Highway system and similar road systems in other countries were constructed in the mid to late 20th century, the steel and concrete girder bridge was one of the most commonly used bridge design. The last decades of the 20th century saw a period of large - scale bridge building in Europe and Asia. Current research focuses on using computers, instrumentation, automation and new materials to improve bridge design, construction and maintenance. Rennie, John (1761 – 1821), British Civil Engineer, began his engineering career in London, where he improved the construction of mill machinery by substituting iron in many parts that were formerly made of wood. Rennie constructed the London and East India dockyards at Plymouth. He also built the Waterloo (1811 – 1817) and Southwark (1815 – 1819) bridges located in London. He designed the London Bridge, which was constructed after his death by his son Sir John Rennie and ultimately completed in 1831. Eiffel, Alexander – Gustave (1832 – 1932), the French engineer and builder most famous for the construction of the Eiffel tower in Paris. Born in Paris, his family included artisans and timber coal merchants. In 1858 the company was granted a contract to erect a railway bridge in Bordeaux. Eiffel oversaw the construction with such success that in 1866 he founded his own company and soon became for his wrought iron structures. In 1877, he erected over the Duoro River in Porto (Oporto), Portugal, a steel arch bridge 160m (525ft) in height. Soon after, he began work on his greatest project, the building of the Eiffel tower. It was completed in 1889 for the celebration of the centennial of the French Revolution (1789 – 1799). The imposing tower - constructed 7,000 tons of iron in 18,000 parts held together by 2,500,000 rivets – dominates the Paris skyline. Stephenson, Robert (1803 – 1859), British Civil Engineer, known for the construction of several notable bridges. The son of George Stephenson, he was born in Willington Quay, near Newcastle and educated at Newcastle. Stephenson built several famous bridges

www.rowland-adewumi.com

4

including the following: the Victoria Bridge at Berwick – Upon – Tweed, England; the Britannia Bridge, a tubular – girder structure over M enai Strait at Bangor; Wale, two bridges across the Nile at Damietta, Egypt and the Victoria, which spans the Saint Lawrence at the M ontreal, Canada. Telford, Thomas (1757 -1834), British Civil Engineer, who pioneered techniques in the construction of Canals, roads and bridges. Born in Westerkirk, Scotland, he was apprenticed to a stonemason, a trade which he practiced in Edinburgh before moving to London in 1782.

2.1.1

NOTABLE BRIDGE FAILURE Notable Bridge failures include: 1.

The collapse of the Firth of Tay Bridge in Scotland in 1879.

2.

The collapse of the Quebec Bridge in Scotland while under construction in1907.

3.

The collapse of the Tacoma Narrows Bridge, nicknamed Galloping Gertie in Washington State in 1940.

2.2

TYPES OF BRIDGES Bridge designs differ in the way they support loads. These loads include the weight of

the bridges themselves, the weight of the material used to build the bridge, the weight and stresses of the vehicles crossing the bridge. There are basically eight common bridge designs:1.

Beam

2.

Cantilever

3.

Arch

4.

Truss

www.rowland-adewumi.com

5

5.

Suspension

6.

Cable Stayed

7.

M ovable

8.

Floating Bridges

Combination bridges may incorporate two or more of the above designs into a bridge. Each design differs in appearance, construction methods and materials used and overall expense. Some designs are better for long span. Beam bridges typically span the shortest distances. 2.2.1

BEAM BRIDGES Beam bridges represent the simplest of all bridge design. A beam bridge consists of a

right horizontal member called a beam that is supported at both ends either by a natural land structure, such as the banks of a river or by vertical posts called piers. Beam bridges are the most commonly used bridges in highway construction. Single – piece, rolled – steel beams can support spans of 15 to 30m (50 to 100ft). Heavier, reinforced beams and girders are used for longer spans. 2.2.2

CANTILEVER BRIDGES Cantilever Bridges are more complex versions of the beam bridge design. In a

cantilever design, a tower is built on each side of the obstacle to be crossed and the bridge is built outward or cantilevered from each tower. The towers support the entire load of the cantilevered arms. The arms are spaced so that a small suspended span can be inserted between them. The cantilevered arms support the suspended span and the downward force of the span is absorbed by the towers. Cantilever bridges are self – supporting during construction. They are not often used as temporary supports would be difficult.

www.rowland-adewumi.com

6

2.2.3

ARCH BRIDGES Arch bridges are characterized by their stability. In an arch the force of the load is

carried outward from the top to the ends of the arch, where abutments keep the arch from spreading apart. Arch bridges have been constructed of stone, brick, timber, cast iron, steel and reinforced concrete. Steel and concrete arches are particularly well suited for bridging reveries or chasms with steep solid walls.

2.2.4

TRUS S BRIDGES Truss Bridges utilize strong, rigid frameworks that support these bridges over a span.

Trusses are created by fastening beams together in a triangular configuration. The truss framework distributes load of the bridge so that each beam shares a portion of the load. Beam, cantilever and arch bridges may be constructed of trusses. Truss bridges can carry heavy loads and are relatively lightweight. They are also inexpensive to build.

www.rowland-adewumi.com

7

2.2.5

S US PENS ION BRIDGE Suspension bridges consist of two large or main cables that are hung (suspended)

from towers. The main cables of a suspension bridge drape over two blocks known as anchorages. The road way is suspended from the main cables that hang down from the cables. In some cases, diagonal cables run from the towers to the roadway and add rigidity to the

A classic suspension bridge in New York City

structure. The main cables support the weight of the bridge and transfer the load to the anchorages and the towers. Suspension bridges are used for the longest spans.

www.rowland-adewumi.com

8

2.2.6

CABLE – S TAYED BRIDGES Cable – Stayed bridges represent a variation of the suspended bridge. Cable – Stayed

bridges have tall towers like suspension bridges but in a cable – stayed bridge, the roadway is attached directly to the towers by a series of diagonal cables. A cable – stayed bridge is constructed in much the same way as a suspension bridge is but without the main cables.

Cable – Stayed designs are used for intermediate length spans. Advantages a cable – stayed has over a standard suspension bridge include speed of construction and lower cost since anchorage are not necessary, there are no massive cables as with suspension bridges and making cable repairs or replacements is simpler. The Pont de Normandie (Normandy Bridge) over the Eine River near La Harre in France opened I 1995 with a span length of 856m (2,808ft). 2.2.7

MOVABLE BRIDGES M ovable bridges make up a class of bridge in which a portion of the bridge moves up

or swings out to provide additional clearance beneath the bridge. M ovable bridges are usually found over heavily travelled waterways. The three most common types of movable bridge are bascule (Draw Bridge), Vertical lift and Swing Bridge. M odern bascule bridges usually have two movable spans that rise upward, opening in the middle. A vertical lift bridge consists of a rigid deck frame held between two tall towers. The bridge opens by hoisting the entire bridge

www.rowland-adewumi.com

9

roadway upward between the towers in an elevator – like fashion. Swing bridges are mounted on a central pier and open by swinging to one side, allowing ships to pass. Movable bridges are impractical or too costly to build bridges with high enough clearance for water traffic to pass underneath. Bascule bridges are used for short spans. A bascule bridge over the Black River in Lorain, Ohio has a length of 102m (333ft). Vertical lift bridges are useful for longer spans but they must be built so they can be lifted high enough for tall ships to pass underneath. Swing bridges have the advantage of not limiting the height of passing vessels but they do restrict the horizontal clearance or width of passing ships. The longest swing bridge span is that of a railroad and highway bridge crossing the M ississippi River at Fort M adison, Iowa. The bridge has a span of 166m (545ft).

2.2.8

FLOATING BRIDGES Floating bridges are formed by fastening together sealed floating containers called

pontoons and placing roadbed on top of them. A pontoon typically contains many compartments so that if a leak occurs in one compartment, the pontoon will not sink. Some floating bridges are constructed using boats or other floating devices rather than the pontoons. Floating bridges were originally developed and are most widely used as temporary structures for military operations. For everyday use, floating bridges are popular for deep water, bad riverbed conditions or other conditions that make it difficult to construct traditional bridge piers and foundations. A concrete pontoon bridge carries a highway across Lake Washington, near Seattle, Washington State in the U S. It consists of 25 floating sections bolted together and anchored in place and a span that can be opened to permit the passage of large ships. The floating section of the bridge is 2.3km (1 mile) long.

www.rowland-adewumi.com

10

2.3

METHODS OF BRIDGE D ES IGN Economy in bridge deck design can be achieved if lateral load distribution

characteristics can be accurately predicted. Different methods have been developed to solve the problem viz: 1. The grillage analogy method. 2. The Orthotropic method. 3. The Articulated method. 4. The Finite element method. These refined methods are by now well established for analysis of load distribution in bridges of various types. Water, bad river or bridge piers and foundations.

2.3.1

ORTHOTROPIC PLATE THEORY One of the earlier distribution co-efficient methods is due to Guyon and M assonate.

This method is based upon the analysis of orthotropic plates in which the loads are represented by a harmonic series. Only the first term of the series is used to obtain the coefficient which are given either in a graphical form or in a tabular form from the characterizing parameters and Ø are used as a basis of the method. 2.3.2

Calculation of Characterizing Parameters The first step in analysis by most of the simplified methods is to calculate. The values of the relevant characterizing parameter torsional parameter & flexural

parameter Ø.

= (Dxy + Dy x +D1 + D2)/ (2(DxDy )0.5

www.rowland-adewumi.com

11

= b/2 (Dx/Dy )0.5 Where X – Direction = the longitudinal direction, i.e. the direction of traffic flow. Y – Direction = the transverse direction (perpendicular to the longitudinal direction) Dx = the longitudinal flexural rigidity Dy = the transverse flexural rigidity Dxy = the longitudinal torsional rigidity D1 = the longitudinal coupling rigidity (which is the contribution of transverse flexural rigidity to longitudinal torsional rigidity through Poisson’s ratio) per unit width. D2 = the transverse coupling rigidity per unit length.

Additional Bridge Forces We have so far touched on the two biggest forces in bridge design. There are dozens of other forces that also must be taken into consideration when designing a bridge. These forces are usually specific to a particular location or bridge design. Torsion, which is a rotational or twisting force, is one which has been effectively eliminated in all but the largest suspension bridges. The natural shape of the arch and the additional truss structure of the beam bridge have eliminated the destructive effects of torsion on these bridges. Suspension bridges, however, because of the very fact that they are suspended (hanging from a pair of cables), are somewhat more susceptible to torsion, especially in high winds. All suspension bridges have deck-stiffening trusses which, as in the case of beam bridges, effectively eliminate the effects of torsion; but in suspension bridges of extreme length, the deck truss alone is not enough. Wind-tunnel tests are generally conducted on models to determine the bridge' s resistance to torsional movements. Aerodynamic truss structures, diagonal suspender cables, and an exaggerated ratio between the depth of the stiffening truss to the length of the span are some of the methods employed to mitigate the effects of torsion. Resonance (a vibration in something caused by an external force that is in harmony with the natural vibration of the original thing) is a force which, unchecked, can be fatal to a bridge. Resonant vibrations will travel through a bridge in the form of waves. A very famous example of resonance waves destroying a bridge is the Tacoma Narrows bridge, which fell apart in 1940 in a 40-mph (64-kph) wind. Close examination of the situation suggested that the bridge' s deck-stiffening truss was insufficient for the span, but that alone was not the www.rowland-adewumi.com

12

cause of the bridge' s demise. The wind that day was at just the right speed, and hitting the bridge at just the right angle, to start it vibrating. Continued winds increased the vibrations until the waves grew so large and violent that they broke the bridge apart. When an army marches across a bridge, the soldiers are often told to "break step." This is to avoid the possibility that their rhythmic marching will start resonating throughout the bridge. An army that is large enough and marching at the right cadence could start a bridge swaying and undulating until it broke apart. In order to mitigate the resonance effect in a bridge, it is important to build dampeners into the bridge design in order to interrupt the resonant waves. Interrupting them is an effective way to prevent the growth of the waves regardless of the duration or source of the vibrations. Dampening techniques generally involve inertia. If a bridge has, for example, a solid roadway, then a resonant wave can easily travel the length of the bridge. If a bridge roadway is made up of different sections that have overlapping plates, then the movement of one section is transferred to another via the plates, which, since they are overlapping, create a certain amount of friction. The trick is to create enough friction to change the frequency of the resonant wave. Changing the frequency prevents the wave from building. Changing the wave effectively creates two different waves, neither of which can build off the other into a destructive force. The force of nature, specifically weather, is by far the hardest to combat. Rain, ice, wind and salt can each bring down a bridge on its own, and in combination they most certainly will. Bridge designers have learned their craft by studying the failures of the past. Iron has replaced wood and steel has replaced iron. Pre-stressed concrete is used in many highway bridges. Each new material or design technique builds off the lessons of the past. Torsion, resonance and aerodynamics (after several spectacular collapses) have been addressed in better designs. The problems of weather, however, have yet to be completely conquered. Cases of weather-related failure far outnumber those of design-related failures. This can only suggest that we have yet to come up with an effective solution. To this day, there is no specific construction material nor bridge design that will eliminate or even mitigate these forces. The only deterrent is preventive maintenance.

www.rowland-adewumi.com

13