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Bridge Engineering

Why are precast concrete piers seldom used in seismic region?

The use of precast concrete elements enhances faster construction when compared with cast-in-situ method. Moreover, it enhances high quality of piers because of stringent control at fabrication yards. The environmental impact is reduced especially for bridges constructed near waterways. In particular, for emergency repair of bridges owing to bridge collapse by earthquake and vehicular collision, fast construction of damaged bridge is of utmost importance to reduce the economic cost of bridge users.

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What are the differences among Warren Truss, Howe Truss and Pratt Truss?

A truss is a simple structure whose members are subject to axial compression and tension only and but not bending moment. The most common truss types are Warren truss, Pratt truss and Howe truss.

Warren truss contains a series of isosceles triangles or equilateral triangles. To increase the span length of the truss bridge, verticals are added for Warren Truss.

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What are the characteristics of Vierendeel girder?

The Vierendeel girder design is sometimes adopted in the design of footbridges. In traditional truss design, triangular shape of truss is normally used because the shape cannot be changed without altering the length of its members. By applying loads only to the joints of trusses, the members of truss are only subjected to a uniform tensile or compressive stress across their cross sections because their lines of action pass through a common hinged joint.

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When should engineers consider using truss with K-bracing?

In the arrangement of triangulated framework in truss structures, it is more economical to design longer members as ties while shorter ones as struts (e.g. Pratt truss). As such, the tension forces are taken up by longer steel members whose load carrying capacities are unrelated to their lengths. However, the compression forces are reacted by shorter members which possess higher buckling capabilities than longer steel members.

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Which bridge parapet is better, steel parapet or aluminum parapet?

Steel parapets normal requires painting or pre-treatment with hot-dip galvanizing as they are prone to corrosion and they are normally the cheapest choice for normal containment level of vehicles.

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What is the purpose of installation of shear keys in bridge abutment?

In small and medium sized bridges, shear keys are often designed in bridge abutments to provide transverse support to the bridge superstructure under lateral loads. They are not intended to carry vertical loads and they have important applications in resisting seismic loads.

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What is the mechanism of scouring at obstructions (e.g. bridge piers) in rivers?

When the water flow in river is deflected by obstructions like bridge piers, scouring would occur arising from the formation of vortexes. The mechanism of formation of vortices is as follows: the flow hits the bridge piers and tends to move downwards. When the flow reaches the seabed, it would move in a direction opposite to its original flow direction before hitting the bridge piers.

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What are the effects of bridge piers across a stream?

The presence of bridge piers across a stream causes constricted flow in the openings because of the decrease of width of stream owing to the presence of the piers. Moreover, it creates the following problems from hydraulic point of view:

(i) Local scouring at the piers and bed erosion may take place. To avoid the damage to the foundation of piers, some protective layers of stone or concrete apron could be provided around the piers.

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In bridge columns, why are stirrups be placed around the vertical reinforcement?

In uniaxial compression test of concrete, upon reaching the ultimate load failure of concrete occurs where major cracks line up in the vertical direction and the concrete cube would be split up. The development of vertical cracks involves the expansion of concrete in lateral directions. In case the concrete is confined in lateral directions, it was observed that the formation of vertical cracks would be hindered as indicated in past experiments. As a result, the concrete strength is increased with also a rise in failure strain.

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What is the purpose of providing a barrier around the bridge piers?

Accidental collision of heavy vehicles such as tractor-trailer with bridge piers is not uncommon around the world. The consequence of such collision is catastrophic which may involve the collapse of bridges and loss of human lives. As such, suitable provisions are made to protect bridge piers against these accidental collisions.

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