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Bridge Insight : New Development in Long Span Arch Bridges
An arch bridge is a bridge whose main structure is composed of arches or reinforced arches. Arch bridges are very economical and advantageous if the horizontal reaction force cause by the loading of vertical loads is effectively applied and if they are properly designed to reduce sectional forces of members.
Arch bridges have been widely used around the world because of their unique aesthetics, and are used for long-span bridges after suspension and cable-stayed bridges.
Fig. Arch bridge (Sydney Harbour Bridge)
The structural properties of arches vary depending on the shape of the arches and the number of hingers. In general, arches become stronger as the number of hinges decreases; however, it greatly impacts settlement.
The main components of arch bridges are:
Deck arch bridge
Hangers and Spandrels
Arch bridges are considered one of the oldest types of bridges. For example, the Mycenaean bridge, which was built around 1600 BC, was considered the oldest Corbel arch bridge to ever be built. in this content, however, only modern arch bridges and their components will be reviewed.
Fig. The Mycenaean bridge
For arch bridges, the arch serves as the main structural member that carries the loads directly to the abutments or supports as compressive forces. Furthermore, bending moments and shear forces are small compared to girder bridges, making it a more economical choice. There are three main types of arch bridges being built nowadays: deck arch bridges, through arch bridges, and tied-arch or bowstring arch bridges. In deck arch bridges, the deck lies on top of the arch rib. The deck can be supported by spandrels made of a number of vertical columns (open-spandrel deck arch bridge) or by solid spandrel walls (closed-spandrel deck arch bridge). Furthermore, when the top of the arch rib directly supports the deck, it is also known as a cathedral arch bridge. In through arch bridges, the deck lies below the arch structure and is supported by vertical hangers. Furthermore, the arch rib is supported by the abutments/supports at each end. Lastly, in tied-arch bridges, the ends of the arch rib are connected by ties where the deck serves as the primary support. In structural terms, one end of the support of a tied-arch bridge serves as a hinged whereas the other end of the support serves as a roller, allowing longitudinal movement.
Fig. Deck and bridge
Fig.Through arch bridge
Fig. Tied-arch bridge
There are various components that compromises arch bridges, but the main components are the following:
The arch rib is the main structural member of the arch and is responsible of carrying the different loads generated in the structure. The arch ribs are commonly built using reinforced concrete or steel, but new innovative materials have been used for the arch ribs, including concrete-filled steel tubular, high-performance concrete, steel concrete composites, etc. The arch rib can be built as a truss, a box girder, a plate girder, or as a hollow section, depending on its usage. For long span bridges, the arch rib is composed of truss sections in order to counter the traffic loadings caused by the various vehicles crossing the bridge. The arch rib doesn’t only represent the main load-bearing element of an arch bridge, but also represent the most aesthetic component of the bridge.
Fig. Ramstore Arch bridge on Qabanbay Batyr Ave
The abutments/supports are one of the most important components of arch bridges because most of the loads carried by the arch rib are transmitted into the abutments. Therefore, the abutments must be heavy and large enough to carry the horizontal thrust from the arch. Reinforced concrete and steel are common materials used for the abutments, but new materials like cellular reinforced concrete and mass concrete are used in order to reduce costs. For example, in cellular reinforced concrete abutments, the cellular portion of the abutment is filled with soil in order to adjust the necessary weight of the abutment.
Fig. Abutment of the Blackfriars Railway bridge
Depending on the type of arch bridge, the deck can be supported by spandrels on top of the arch rib of suspended by vertical hangers. For deck arch bridges, solid spandrel walls can be placed on top of the arch rib to support the bridge deck. The fill on the walls are mostly made out of masonry or concrete. However, modern bridges use a different approach by adding vertical columns made out of steel or concrete that directly support the deck of the bridge.
For through arch bridges, the deck of the bridge is suspended by hangers, which are loaded in tension. The hangers can be designed as I-sections, circular hollow sections, or cables depending on the conditions to which the arch bridge is situated. Furthermore, recent studies on hanger arrangement optimization have shown that sparse hanger systems provide advantages such as better mechanical performance.
Fig. Spandrels (vertical columns)
Arch bridges are subdivided into deck bridges, half-through bridges, and through bridges depending on the location of the road. Moreover, they are further divided depending on the type of arch rib, such as solid arch ribs, braced arch ribs, and spandrel braced arch ribs.
There are other types of arch bridges such as the Langer bridge, Lohse bridge, Nielsen Bridge, and Tied arch bridge.
The arch rib rigidity is greater than that of the stiffened girder, so the rib mainly resists axial forces and bending moments. Furthermore, only axial forces are generated in the stiffened girder.
Fig. Tied arch
The rigidity of the stiffened girder is larger than that of the arch rib. Therefore, the stiffened girder resists axial forces and bending moments, and only axial forces are generated in the arch rib.
Fig. Langer arch
A structure in which the arch rib and the stiffening girder are connected with two elements with flexural stiffness at both ends, and are connected by vertical members that connect the arch rib and stiffening girder using hingers at both ends of the vertical members.
Fig. Lohse arch
An arch type that uses cables as stayed struts to enhance the aesthetics of the bridge instead of using vertical members with flexural strength.
Fig. Nielsen arch
An example of the structural analysis model for Arch Bridge design using midas Civil is shown in the figure below:
Fig. Steel bridge model