Solid slab bridges Single spans

The solid slab is the simplest form of reinforced concrete bridge deck. Ease of construction resulting from the simplicity makes this the most economic type for short span structures. Solid slabs also have good distribution properties which makes them efficient at carrying concentrated movable loads such as wheel loads for highway bridges. However, above a span of around 10 m the deadweight starts to become excessive, making other forms of construction more economic.

Solid slab bridges can be simply supported on bearings or built into the abutments. Until recently, bridge engineers tended to be quite pedantic about providing for expansion and even bridges as short as 9 m span were often provided with bearings and expansion joints. However, bearings and expansion joints have proved to be among the most troublesome components of bridges. In particular, deterioration of substructures due to water leaking through expansion joints has been common especially in bridges carrying roads where de-icing salt is used.

Recently, the fashion has changed back to designing bridges that are cast integral with the abutments or bank seats (Department of Transport, 1995). Apart from the durability advantages, this can lead to saving in the deck due to the advantage of continuity. On short span bridges with relatively high abutment walls, being able to use the deck to prop the abutments can also lead to significant savings in the abutments. However, this normally depends on being able to build the deck before backfilling behind the abutments. When assumptions about construction approach such as this are made in design, it is important that they should be properly conveyed to the contractor, normally by stating them on the drawings.

A feature of the design of integral bridges which has not always been appreciated is that, because the deck is not structurally isolated from the substructure, the stress state in the deck is dependent on the soil properties. This inevitably means that the analysis is less 'accurate' than in conventional structures. Neither the normal at-rest pressure behind abutments nor the resistance to movement is ever very accurately known. It might be argued that, because of this, designs should be done for both upper and lower bounds to soil properties. In practice, this is not generally done and the design criteria used have sufficient reserve so that this does not lead to problems.

Depending on the ground conditions, span and obstacle crossed, the abutments of a single-span bridge may be separate or may be joined to form a complete box. Such box type structures have the advantage that they can be built without piles even in very poor ground, as the bearing pressure is low. Since the box structure is likely to be lighter than the displaced fill, the net bearing pressure is often negative. This can lead to problems in made ground as the embankment either side of the box may settle much more than the box, leading to problems with vertical alignment and damage to the surfacing or rails over the bridge.

RC slab bridges are normally cast in situ. An exception is very short span shallower structures (typically up to some 6m span and 3.6m clear height) which can be most economically precast effectively complete as box culverts, leaving only parapets and, where required, wing walls to cast in situ. This form of construction is most commonly used for conveying watercourses under embankments but can be used for footway and cycletracks.

In situ construction is very convenient for greenfield sites and for crossing routes that can be diverted. It is less convenient for crossing under or over live routes. For the latter, spanning formwork can be used if there is sufficient headroom. However, in many cases beam bridges are more convenient and the precast beams will normally be pre-stressed. RC box type structures can, however, be installed under live traffic. They can be pushed under embankments. The issues are considered by Allenby and Ropkins (2004). A reasonable amount of fill over the box is needed to do this under live traffic. The box structure is cast adjacent to its final position and then jacked into position with antidrag ropes preventing the foundations below and the fill above moving with it. If there is not much fill depth, it becomes impractical to push the box whilst keeping a road or rail route over the top still. A similar approach can, however, be used with the box cast in advance and then jacked into place in open cut over a relatively short possession.

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Responses

  • vihtori
    What is solid slab bridge?
    6 months ago

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