10. Durability, Inspection and Maintenance
The durability of the Main Crossing is of paramount importance considering the aggressive marine climate, large capital investment required for construction and the great cost and difficulty that can be associated with extending the life of, or replacing, such a major structure if it deteriorates to an unacceptable level once in use. The durability issues which have become apparent with the existing Forth bridges underline this point.
Requirements for durable structures make recognition of the fact that durability is not an absolute property of a material but can be affected by both design and construction factors. Definitions of design life require that the design criteria are achieved, not that materials or components remain in the same condition unchanged for the design period and imply maintenance and some repair for its achievement. Thus an assessment of durability would require that the processes of deterioration be examined on the one hand and the means of protection (by durability design) and mitigation (by maintenance) are assessed on the other hand in order to ensure that the design life can be achieved with a reasonable degree of confidence. The design life of the structure will be 120 years. For the major structural elements this is usually interpreted to mean the design life without replacement, for other secondary elements, systems and components where replacement is feasible a shorter service life is usually assumed.
Whilst a full durability assessment of the structure has not yet been carried out a number of principles have been established as well as a number of potential measures to ensure adequate durability:
- Specification of appropriate materials and finishes
- Provision of comprehensive facilities for the inspection and maintenance of the structure
- Design for ease of replacement of secondary elements and systems (e.g. stay cables, bearings, movement joints, deck furniture etc.)
- Use of dehumidification where appropriate to protect the interior spaces of fabricated steelwork (e.g. deck, tower anchor boxes)
- Special measures to protect the reinforcement in the outer layers of reinforcement in the intertidal and splash zones of the towers and piers to extend the life of these structures in the most aggressive of microclimates. At this stage, stainless steel reinforcement has been assumed in these areas but provision for cathodic protection is an alternative to be considered.
- Use of stainless steel guide pipes and/or facia plates in the upper tower to reduce the maintenance requirements for these high elevation and difficult to access locations
10.2 Inspection & Maintenance
A comprehensive set of facilities for inspection and maintenance of the structure will be included in the design. In addition to fixed access facilities throughout the bridge (walkways, stairs, ladders etc.), a suite of motorized access machines will be recommended which may include under-deck inspection gantries, lifts within the towers, an internal deck shuttle (for the Three Corridor Option), a stay cable inspection gantry and an access platform to be suspended from a Building Maintenance Unit at each tower top.
The overall approach is to ensure that, as far as possible, normal inspection and maintenance activities can be carried out with minimum disturbance to the traffic. At the same time easy and safe access for maintenance personnel must be provided.
10.2.1 Three Corridor Option
The access facilities for the Three Corridor Option cable stayed bridge are illustrated in Drawing FRC/C/076/S/501 in Appendix B.
The primary access for the bridge is for maintenance vehicles to drive along the walkways on either side of the bridge and park close to the work area in the same way that access is achieved to the existing road bridge. This completely avoids the need for parking on the hard shoulders of the main carriageways for routine inspection work.
To facilitate this, designated cross passages will be provided in the bridge at approximately 90m intervals. Each cross passage will consist of a secure and weatherproof hatch in the walkway to allow access to the interior of the bridge deck. Ladders and transverse walkways will be provided connecting the hatches on either side of the bridge to two additional hatches in the structural zone reserved for the stay cable anchorages. In that way personnel can access the structural zone between the motorway and the multi-modal corridor at any point on the bridge in complete safety with no disruption to traffic.
Inside the bridge deck a motorised shuttle and a pair of walkways allow for longitudinal movement of personnel and equipment.
The soffit of the bridge will be inspected and maintained by under-deck gantries. Three separate gantries would be provided thus allowing the gantries to pass the towers and piers and maintain the full length of the bridge. On the existing road bridge, in common with many other bridges, there is provision for personnel to access the gantry from the top surface of the bridge deck. This is incompatible with the anti-climb windshields and therefore access to the gantries will be only from the towers below deck level. However, in the event of mechanical failure of a gantry away from the towers access is still required in order to evacuate personnel and repair the gantry, This is solved by the operation of three independent gantries and provision will be made to move safely between the gantries allowing one gantry to "come to the rescue" of another.
Within the towers the main inspection and maintenance requirements are above deck level, to access the stay cable anchorages as well as lighting and instrumentation in or at the top of the tower. A rack and pinion lift will be provided from deck level to the tower top with a minimum capacity of 5 persons together with emergency ladders for escape in the event of mechanical failure of the lift.
10.2.2 Double Level Option
The access facilities for the Double Level Option cable stayed bridge are illustrated in Drawing FRC/C/076/D/511.
As for the Three Corridor Option the primary access for bridge maintenance is along the walkways on either side of the upper deck. Maintenance access for the lower deck will require traffic management measures to be in operation albeit that this will be relatively easy for the multi-modal corridor.
The truss structure will be dehumidified and therefore frequent internal inspection will not be required. However, some internal access may be required.
An pair of underbridge inspection gantries will be provided for the inspection and maintenance of the external parts of the truss. Upper levels to the gantry will also provide access to the stay cable anchorages and the underside of the upper deck cantilevers. Access platforms on hydraulic arms will provide access to the underside of the lower deck soffit whilst allowing the gantry to pass a pier and thus run the entire length of the bridge.
Inspection and maintenance of the central part of the underside of the upper deck will be via mobile hydraulic platforms running on the lower deck. Access within the towers will be similar to that proposed for the Three Corridor Option with lifts being provided in each tower leg.
Complementary to the physical inspection and maintenance facilities, a Wind and Structural Health Monitoring System (WASHMS) is proposed to provide real time data and also to allow investigation of the structure to be undertaken after an extreme event such as a major wind storm or an earthquake.
The real-time data can be used to assist with inspection and maintenance by immediately highlighting anomalies that could indicate a fault (for example oil-pressure out of range on hydraulic buffers) or else by tracking long term changes in bridge behavior (for example gradual increase of effective friction coefficient on bearings).
A four level system architecture is envisaged:
1. Data Collection Level
System collects data from sensors and forwards to pre-processing.
2. Data Pre Processing and Transmission
Data Acquisition Units (DAU’s) distributed through the bridge pre-process the data prior to transmission to central processor (signal conditioning and conversion of analogue data to digital). As well as the fixed DAU’s, there can also be a system of Portable Data Acquisition Systems which can be used in conjunction with removable accelerometers for specific field vibration measurements.
3. Data Processing and Analysis Level
Collection, processing, analysis, display, archiving and storage of all data by a centralized Data Processing and Control System.
4. Structural Health Evaluation Level
Analysis and interpretation of measured data, comparison with criteria for inspection and maintenance, display archive and storage of analyzed or interpreted results, production of structural health evaluation reports.
The range of sensors that can be included at the data collection level is very extensive and may include monitoring of climactic conditions external and internal to the bridge, structural displacements, accelerations and strains, direct measurement of chloride ingress into concrete structures, traffic measurements etc. as well as sensors specific to any bridge equipment that may be installed (e.g. to monitor stroke position, effective friction between sliding partners etc.). The detailed specifications of the WASHMS will be developed in consultation with Transport Scotland.