Arch bridges derive their strength from the fact that vertical loads on the arch generate compressive forces in the arch ring, which is constructed of materials well able to withstand these forces.
The compressive forces in the arch ring result in inclined thrusts at the abutments, and it is essential that arch abutments are well founded or buttressed to resist the vertical and horizontal components of these thrusts. If the supports spread apart the arch falls down. The Romans knew all about this.
Traditionally, arch bridges were constructed of stone, brick or mass concrete since these materials are very strong in compression and the arch could be configured so that tensile stresses did not develop.
Modern concrete arch bridges utilise prestressing or reinforcing to resist the tensile stresses which can develop in slender arch rings.
The shape attracted the attention of many of the early pioneers of concrete construction. In 1930, Freyssinet was responsible for a spectacular arched bridge at Plougastel in France and three years later, Swiss engineer, Robert Maillart created the famously elegant Schwandbach bridge in which slender cross-walls tie the arch to the horizontally curved roadway.
REINFORCED SLAB BRIDGE
For short spans, a solid reinforced concrete slab, generally cast in-situ rather than precast, is the simplest design. It is also cost-effective, since the flat, level soffit means that falsework and formwork are also simple. Reinforcement, too, is uncomplicated. With larger spans, the reinforced slab has to be thicker to carry the extra stresses under load. This extra weight of the slab itself then becomes a problem, which can be solved in one of two ways. The first is to use prestressing techniques and the second is to reduce the deadweight of the slab by including ‘voids’, often expanded polystyrene cylinders. Up to about 25m span, such voided slabs are more economical than prestressed slabs.
BEAM AND SLAB BRIDGES
Beam and slab bridges are probably the most common form of concrete bridge in the UK today, thanks to the success of standard precast prestressed concrete beams developed originally by the Prestressed Concrete Development Group (Cement & Concrete Association) supplemented later by alternative designs by others, culminating in the Y-beam introduced by the Prestressed Concrete Association in the late 1980s.
They have the virtue of simplicity, economy, wide availability of the standard sections, and speed of erection.
The precast beams are placed on the supporting piers or abutments, usually on rubber bearings which are maintenance free. An in-situ reinforced concrete deck slab is then cast on permanent shuttering which spans between the beams.
The precast beams can be joined together at the supports to form continuous beams which are structurally more efficient. However, this is not normally done because the costs involved are not justified by the increased efficiency.
Simply supported concrete beams and slab bridges are now giving way to integral bridges which offer the advantages of less cost and lower maintenance due to the elimination of expansion joints and bearings.
BOX GIRDER BRIDGE
For spans greater than around 45 metres, prestressed concrete box girders are the most common method of concrete bridge construction. The main spans are hollow and the shape of the ‘box’ will vary from bridge to bridge and along the span, being deeper in cross-section at the abutments and piers and shallower at midspan.
Techniques of construction vary according to the actual design and situation of the bridge, there being three main types:
i.e. incrementally launched
As the name suggests, the incrementally launched technique creates the bridge section by section, pushing the structure outwards from the abutment towards the pier. The practical limit on span for the technique is around 75m.
The span-by-span method is used for multi-span viaducts, where the individual span can be up to 60m.
These bridges are usually constructed in-situ with the falsework moved forward span by span, but can be built of precast sections, put together as single spans and dropped into place, span by span.
In the early 1950s, the German engineer Ulrich Finsterwalder developed a way of erecting prestressed concrete cantilevers segment by segment with each additional unit being prestressed to those already in position. This avoids the need for falsework and the system has since been developed.
Whether created in-situ or using precast segments, the balanced cantilever is one of the most dramatic ways of building a bridge. Work starts with the construction of the abutments and piers. Then, from each pier, the bridge is constructed in both directions simultaneously. In this way, each pier remains stable – hence ‘balanced’ – until finally the individual structural elements meet and are connected together. In every case, the segments are progressively tied back to the piers by means of prestressing tendons or bars threaded through each unit.
One of the difficulties in designing any structure is deciding where to put the joints. These are necessary to allow movement as the structure expands under the heat of the summer sun and contracts during the cold of winter.
Expansion joints in bridges are notoriously prone to leakage. Water laden with road salts can then reach the tops of the piers and the abutments, and this can result in corrosion of all reinforcement. The expansive effects of rust can split concrete apart.
In addition, expansion joints and bearings are an additional cost so more and more bridges are being built without either. Such structures, called ‘integral bridges’, can be constructed with all types of concrete deck. They are constructed with their decks connected directly to the supporting piers and abutments and with no provision in the form of bearings or expansion joints for thermal movement. Thermal movement of the deck is accommodated by flexure of the supporting piers and horizontal movements of the abutments, with elastic compression of the surrounding soil.
Already used for lengths up to 60m, the integral bridge is becoming increasingly popular as engineers and designers find other ways of dealing with thermal movement.
For really large spans, one solution is the cable-stayed bridge. As typified by the Dee Crossing where all elements are concrete, the design consists of supporting towers carrying cables which support the bridge from both sides of the tower.
Most cable-stayed bridges are built using a form of cantilever construction which can be either in-situ or precast.
Concrete plays an important part in the construction of a suspension bridge. There will be massive foundations, usually embedded in the ground, that support the weight and cable anchorages. There will also be the abutments, again probably in mass concrete, providing the vital strength and ability to resist the enormous forces, and in addition, the slender superstructures carrying the upper ends of the supporting cables are also generally made from reinforced concrete.