Why do we want to build integral bridges? What are the characteristics of integral bridges?
The integral (integral (quote from The Concise Oxford Dictionary ): “Of, or necessary to the completeness of a whole”) construction method encompasses a direct, monolithic connection of individual components of a bridge: Superstructure, piers/sub-constructions and abutments.
One major incentive to build integral bridges is the ambition to create slender and transparent bridge structures through judicious design. If piers and abutments do not require bridge bearings, the sub-constructions can be simplified and designed for a more filigree appearance, since it is no longer necessary to provide storage and floor space for the jacks. And it allows for a more slender design of piers, because their accessibility does not have to be guaranteed.
Aside from more flexibility in the planning process, leaving out bearings and joints has other advantages, in particular with regard to building maintenance:
• sturdy and sustainable construction, due to the elimination of technology susceptible to failure.
• Low maintenance, easy building inspection.
• Lower construction cost, due to savings on bearings and employment of simpler building components (e.g. walls vs. hollow piers).
There is a differentiation between the integral and the semi-integral construction method.
Integral construction method: Design of the load bearing structure as a whole; direct, “integral” connection between superstructure and sub-constructions, and abutments respectively.
Semi-integral construction method: The bridge components are structurally separated; abutments are for instance separated by bearings and joints.
Integral piers and pier walls
Integral bridges feature a force-fit, monolithic connection of the sub-constructions, abutments and piers to the superstructure. Together they form rigid frames without bearings and joints, which would separate the components from each other. Modern materials, such as high-strength concrete can be used to build filigree pier walls. On account of eliminating the bearings it is not necessary to provide access to them for the purpose of inspection, maintenance, and replacement. Not having to provide accessibility and floor space for the jacks significantly simplifies the construction of piers. The pier (walls) can thus be pre-fabricated and then mounted. In addition to this, the maintenance and up-keeping effort is much lower. Therefore costs can be reduced.
Integral abutments can be constructed in a simpler design, which makes them less prone to damages. Leaving the joints out also leads to a significant reduction of maintenance efforts. Because there are no more joints to be crossed, this construction reduces noise. It has thus proved to be of benefit also in urban areas.
Integral bridges feature some special characteristics, which require particular attention during their design.
The structural behaviour of an integral bridge is characterised by its structural properties, which change over time (e.g. adaptation of stiffness and form due to creep and shrinkage) in combination with its interaction with external conditions, such as the subsoil. Depending on the nature of the bridge, specific conditions and parameters must be calculated by simulations.
Interaction of load bearing structure – subsoil
The behaviour of the bridge as an overall system is largely dependent on its stiffness distribution. In most cases this also includes the impact of the subsoil stiffness on foundation elements. Elastic end-restraints in the subsoil must therefore be simulated. And their impact on the system must be taken into account for the entire cutting force evaluation and the dimensioning of components.
Construction states that influence the temporal course of stress conditions (e.g. creep and shrinkage) must be analysed in calculations and combined with the scheduled impacts. This also includes construction states resulting from manufacturing in sections (e.g. bay-wise manufacturing of the superstructure).
Frequently, interim construction states must also be considered, such as changeable bearing conditions and planned geometrical changes in shape or position (e.g. moving or pulling the superstructure).
The impact of changeable concrete characteristics must be considered in the cutting force evaluation and the dimensioning of components. On the one hand, temporarily changing characteristics (creep and shrinkage of concrete) impact the construction. On the other hand, changes due to independent reactions must also be taken into account, such as the cracking of concrete (transition to Phase II) and the associated loss of stiffness that generally results in the redistribution of cutting forces.
An essential feature of integral bridges is a clear, direct transfer of forces, particularly of horizontal loads, such as braking forces on railway bridges. Horizontal loads generally induce stiffness-dependent reactions in the sub-constructions.
This also applies to direct impacts, such as brake forces. By means of inclined slabs, or arch shafts these can directly be transferred into the subsoil, effectively without deformation. However, indirect impacts are also design relevant, such as constraints under conditions of restrained deformation due to temperature effects for example, or shortening as a consequence of creep and shrinkage.
The design and analysis of integral bridges is often a more challenging task for the structural engineer; and it generally entails higher planning efforts.
Thorough planning in terms of the structural analysis, as well as the structural implementation, and the preparation of construction details is therefore vital for a successful and on-time completion of an integral bridge.