Structures degrade with time requiring attention to maintain good appearance, efficient operation and safety to users and passers-by. Maintenance also allows issues to be fixed early before they become costly and, in some cases, unsafe.
Degradation of the common structural material can result from a range of issues including1:
- Concrete – chlorides (from de-icing salts and marine environments), carbonation, poor aggregates, sulphates, low concrete cover, low cement content2
- Stonework – air pollution, acid attack, frost
- Steelwork – various types of corrosion
- Timber – water, dry rot, insect attack
- Brickwork – frost, thermal movements
In addition, structures move with time changing their dimensions.
Regular cleaning can help to minimise degradation, and regular planned inspections and appropriate testing can assess the extent of degradation. A maintenance plan for minor repairs, major repairs and, if needed, replacement can then be prepared to keep the structure in good order; or when immediate concerns are spotted, reactive maintenance may be necessary.
For either planned or reactive maintenance, safe access to the structures will be required. As such, the designer needs to make allowance for such access in the design of the structure.
In Britain, Regulation 9 of CDM 2015 states that:
(2) When preparing or modifying a design the designer must take into account the general principles of prevention and any pre-construction information to eliminate, so far as is reasonably practicable, foreseeable risks to the health or safety of any person—
(a) carrying out or liable to be affected by construction work;
(b) maintaining or cleaning a structure; or
(c) using a structure designed as a workplace.
(3) If it is not possible to eliminate these risks, the designer must, so far as is reasonably practicable—
(a) take steps to reduce or, if that is not possible, control the risks through the subsequent design process;
(b) provide information about those risks to the principal designer; and
(c) ensure appropriate information is included in the health and safety file.
This indicates that the designer should follow the Eliminate-Reduce-Inform-Control (ERIC) framework in designing for maintenance. The ERIC framework is discussed in more detail in an article by John Carpenter in the Structural Engineer.
2. How to address maintenance in design
The suggested approach follows the ERIC framework, and is to:
- Eliminate the need for maintenance as part of the design (although highways and rail clients are likely to require regular inspections of their assets); if this is not possible
- Design the structure so that future maintenance is reduced
- Design the structure so that when maintenance is required it can be carried out safely
- Provide relevant information on any significant residual maintenance risks that may remain; and, if necessary
- Design in any control measures that may help the contractors carrying out the maintenance (eg access for inspection or cleaning), but the majority of the control measures will typically be implemented by these contractors
Examples of these approaches include:
||Eliminate need for maintenance
- Use an integral bridge – this will remove the need for bearings and expansion joints, both of which require inspection and replacement
- Design secondary elements, such as cladding, based on the same design life as the primary structure – avoiding the need for replacement over the life of the structure
||Reduce future maintenance
- Use longer life paint or galvanising – to reduce the amount of maintenance activity
- Select cladding fixings with long design lives – to minimise the need for replacement
- Provide adequate falls to roofs – to prevent a build-up of water or debris
|Maintenance carried out safely
- Design in permanent connections and ties – to allow scaffolding or other work platforms to be connected
- Design bridge bearings with access galleries in front of the bearing shelves to allow for inspection and replacement – so that inspectors can see the bearings clearly and contractors can jack the deck to replace them
- Provide access galleries on complex cladding over escalator barrels – to allow safe access for inspection
- Provision of platforms in ventilation shafts – to allow safe access for inspection
- Design lining walls to incorporate access hatches – to allow visual inspections (including hatches which allow for endoscope use where space is limited)
- Where cladding needs to be replaced, design panels to minimise weight – to allow safe removal for inspection or replacement
- Ensure that structures are inspectable – inspection is a key source of lifetime information and enables degradation to be identified before it is too late
- Provide information on areas of a structure that have only be designed for light loads – routes to plantrooms may be subject to heavy loads if replacement plant is required
- Use digital technology to store information on assets – the use of bar codes or similar allows immediate access to asset data during any inspections
- Drawings to be prepared which clearly indicate allowable floor live loads – also winch points and crane beams with allowable loads indicated
- Design to facilitate inspection and cleaning – particularly in high-risk areas
- Requirements for inspection and maintenance of multi-storey car parks should be clearly set out – a life-care plan provides a mechanism for this as can Operations & Maintenance manuals
- Minimise the number of structural interventions through remote sensing – use of structural health monitoring, drone access, etc.
Addressing maintenance in design will require liaison with other designers; for example, to understand what the requirements for generators and other heavy plant are. Such requirements may generate a demand for overhead lifting gear that needs to be supported off the structure.
Some clients will require designers to prepare an Access and Maintenance strategy to demonstrate that the structure as designed can be maintained safely.
3. Design issues for particular types of structures
Bridges are exposed to a relatively severe environment. The major issues with bridges relate to leaking or damaged movement joints, ageing bearings, blocked or broken drainage, damaged waterproofing and corrosion of reinforcement or other metallic inserts.
In the UK, General Inspections are required at two-year and Principal Inspections at six-year intervals. These inspections are vital to identify potential problems at an early stage when they can be addressed with relatively minor works required. However, inspection can only be carried out effectively if inspectors can work in a safe and reasonably comfortable environment. Inspection galleries and walkways need to be provided where possible to avoid the need for specialist access equipment.
Indeed, CIRIA Report 155 notes that: When establishing the layout of a scheme, designers should consider how inspections can be economically carried out, utilising, as far as possible, readily available plant and equipment. The cost of traffic delays due to temporary lane or road closures and the cost of associated traffic management measures should be taken into account. For some structures, it may be appropriate to incorporate permanent access facilities, such as abutment galleries, span cradles, or simply cast-in stainless steel fixings.
Bridges on the M6 Toll Road provide a good example of the use of abutment galleries, whilst the Dartford Crossing incorporates access walkways between piers below deck.
3.2 Multi-storey building structures
Typical building structures are exposed to less severe environments than bridges (unless they are near the coast). The typical issues that structural engineers will need to address include:
- Can the building be inspected easily and safely?
- Can inspection be undertaken by modern technologies such as drones or borescopes to reduce the need for human access or the removal of cladding?
- What methods and equipment are suitable for cleaning (including clearing gutters), inspection and other maintenance activities (common practice includes provision of gantries to allow routine maintenance such as window cleaning to be undertaken safely)?
- Can the need for overly complex equipment that may be required to replace cladding, and which may get misplaced, be avoided? If not, can lifting points for cradle gantries be incorporated in the roof?
- Are there safe routes for removing and replacing heavy plant in plant rooms (this is also an important issue in the rail industry where stations, shafts and portals have to be designed to allow the replacement of large pierces of plant such as fans)?
3.3 Multi-storey car park structures
Multi-storey car parks are exposed to environments much closer to those of bridges than buildings but, historically, have been designed to guidance applicable to buildings. Failures in some older car parks have observed as a result of concrete degradation, reinforcement corrosion and failure of car park barriers. Extensive repair works have been observed on older car parks.
Guidance is now available both for the design of new car parks and for the development of life-care plans based on regular inspection, assessment, maintenance of management (for both new and existing car parks).
Waterproof membranes, lifts, electrical and data systems may well have to be replaced during the life of the car park. Consideration of this should be included in the design. The use of two forms of waterproofing may well mitigate the likelihood of leaks (this is a requirement on TFL underground stations and tunnels). Reinjection tubes provide a means of addressing leaks without significant structural intervention. Cladding and/or edge protection may well be damaged by accidental vehicle impact. Ease of replacement should be considered in the design.
4. Guidance/ further reading
- Carpenter, J: Designing for safer concrete structures, The Concrete Centre, CCIP-043, November 2011
- Crossrail: Healthy by Design
- Gilbertson, A: CDM 2015 – Workplace 'in-use' guidance for designers, CIRIA Report C765, Second Edition, 2015
- Highways England: Designing health and safety into maintenance, GD304, version 2, March 2020
- Iddon, J and Carpenter, J: Safe access for maintenance and repair – Guidance for designers, CIRIA, Report C686, 2009
- Institution of Civil Engineers: Recommendations for the Inspection, Maintenance and Management of Car Park Structures, Second edition, 2018
- Institution of Structural Engineers: Design recommendations for multi-storey and underground car parks, Fourth edition, March 2011
- Ove Arup & Partners and Gilbertson, A: CDM 2015 – Construction work sector guidance for designers, CIRIA, Report C755, Fourth edition, 2015
- Ray, S S; Barr, J and Clark, L A: Bridges – design for improved buildability, CIRIA, Report 155, 1996
Michael Webster and Donal Coughlan
Safety, Health and Wellbeing Panel