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Bridge Engineering. Introduction S.S.Bapat Civil Engineering Department Walchand College of Engineering, Sangli. Introduction to Bridges. What is a bridge?. A bridge is a structure that spans a divide such as: A stream/river/ravine*/valley** Railroad track/roadway/waterway
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Bridge Engineering Introduction S.S.Bapat Civil Engineering Department Walchand College of Engineering, Sangli
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 *A deep narrow steep-sided valley (especially one formed by running water) **A long depression in the surface of the land that usually contains a river
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.
Roman Arch Bridge History of Bridge Development 100 B.C. Romans 2,104 years ago 700 A.D. Asia 1,304 years ago Clapper Bridge • Tree trunk • Stone • Arch design evenly distributes stresses • Natural concrete made from mud and straw Great Stone Bridge in China • Low bridge • Shallow arch • Allows boats and water to pass through
History of Bridge Development 1900 Truss Bridges 2000 • Mechanics of Design • Wood 1920 Suspension Bridges • Use of steel in suspending cables • Prestressed Concrete • Steel
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 • 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 very short 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). • It must not be thought that flexure is immaterial in such structures. In most suspension bridges, flexure of the stiffening girder is not a primary loading. However, in cable stayed bridges (particularly if the stays are widely spaced) flexure of the girder is a primary loading.
Basic Types of Bridges • Girder/Beam Bridge • Truss Bridge • Rigid Frame Bridge • Arch Bridge • Cable Stayed Bridge • Suspension Bridge • Floating Bridges
Types of Bridges • Basic Types: • Truss Bridge • Beam Bridge • Arch Bridge • Suspension Bridge • Floating Bridge Floating Truss Beam Arch Suspension The type of bridge used depends on the obstacle. The main feature that controls the bridge type is the size of the obstacle.
Girder/Beam Bridge • The most common and basic type • Typical spans : 10m to 200m
Types of Bridges Beam Bridge Consists of a horizontal beam supported at each end by piers. The weight of the beam pushes straight down on the piers. The farther apart its piers, the weaker the beam becomes. This is why beam bridges rarely span more than 250 feet.
Types of Bridges Beam Bridge Forces When something pushes down on the beam, the beam bends. Its top edge is pushed together, and its bottom edge is pulled apart.
Truss Bridge • Truss is a simple skeletal structure. • Typical span lengths are 40m to 500m.
Truss Bridge All beams in a truss bridge are straight. Trusses are comprised of many small beams that together can support a large amount of weight and span great distances.
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
Types of Bridges Arch Bridges The arch has great natural strength. Thousands of years ago, Romans built arches out of stone. Today, most arch bridges are made of steel or concrete, and they can span up to 250 m.
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 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.
a brief history • it was first applied in the 1800’s in England (incorporated with suspension bridges), many of which had failed due to insufficient resistance to wind pressure (but also failure of the designer, and failure to understand the mechanics of such a bridge.)
…continued • But in the 1900’s, several factors contributed to successful implementations: • Better methods of structural analysis of statically indeterminate structures (via computers) • Development of steel decks • High strength steel, new methods of fabrication and erection
Deck, Tower are made of reinforced concrete. A box girder supports the deck so as to reduce buckling of the deck from high compressions, twisting or torsion, and distribute among the stays non-uniform loads.
Cables are made of high-strength steel, usually encased in a plastic or steel covering that is filled with grout , a fine grained form of concrete, for protection against corrosion.
classifications radial : cables connect evenly throughout the deck, but all converge on the top of the pier harp : cables are parallel, and evenly spaced along the deck and the pier fan : a combination of radial and harp types star-shaped : cables are connected to two opposite points on the pier
pros construction method is simple (cantilever method) appropriate for our span, although they are typically built for larger spans …however, elements of the cable-stayed bridge may be employed simple to design (as opposed to the suspension bridge) cons may require building pier, or at least a tower on either side of the site more susceptible to damage by wind forces similarly, it is weak in the sense of torsion and twisting although cheaper than suspension bridges, can be more expensive for short spans (as opposed to truss bridges)
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.
Types of Bridges Suspension Bridges This kind of bridges can span 600 to 2000 m -- more than any other type of bridge! Most suspension bridges have a truss system beneath the roadway to resist bending and twisting.
Types of Bridges Floating Bridge • Pontoon bridges are supported by floating pontoons with sufficient buoyancy to support the bridge and dynamic loads. • While pontoon bridges are usually temporary structures, some are used for long periods of time. • Permanent floating bridges are useful for traversing features lacking strong bedrock for traditional piers. • Such bridges can require a section that is elevated, or can be raised or removed, to allow ships to pass.
