Introduction
Transport Infrastructure Ireland has identified that the durability of many bridges has been compromised by design decisions regarding bridge configuration and detailing Inspections and maintenance have revealed durability issues, even when materials and construction practices were adequate, often due to a design philosophy focused on minimizing initial costs This approach has frequently overlooked long-term performance and maintenance needs, leading to costly repairs To address these challenges, Transport Infrastructure Ireland is advocating for a design philosophy centered on durability, promoting a lowest whole life cost approach.
This standard supersedes DN-STR-03012-Design for Durability dated June 2016
The principal changes from the previous standard are: i) Amendments to Appendix B – Concrete Cover Requirements
This Standard aims to establish requirements that, when integrated with Transport Infrastructure Ireland’s current design standards for road structures, will enhance durability and reduce the overall life-cycle costs of new constructions.
This Standard outlines how design can enhance the durability of structures, highlighting critical aspects of structural form and details that need special attention While many practices discussed are recognized by designers as effective, their implementation has not been as common as expected Additionally, the document briefly addresses material specifications, construction practices, inspection, and maintenance related to durability, with further details available in TII Publications (Standards).
This Standard establishes minimum requirements and is not exhaustive; therefore, designers must apply their judgment and expertise to effectively address durability considerations in new structures.
The figures incorporated in this Standard are only indicative Designers should satisfy themselves as to the suitability of the suggested details to specific designs
This Standard must be immediately implemented for all national road construction and improvement projects It should also be applied to the design of ongoing schemes, unless Transport Infrastructure Ireland determines that doing so would lead to substantial extra costs or delays In such instances, Design Organisations are required to verify the applicability of this Standard for specific schemes with Transport Infrastructure Ireland.
The Design Organisation is tasked with creating designs that ensure durability in both the overall concept and intricate details Designs must either adhere to this Standard's requirements or incorporate approved alternative provisions that guarantee sufficient durability Any alternative provisions will require an approved departure from the established standards.
Serviceability refers to a structure's ability to meet all intended needs without limitations For road structures, this includes accommodating all permitted traffic, ensuring user safety through proper containment and separation of different user classes, and effective drainage Additionally, it involves providing user comfort by minimizing excessive deflections and vibrations, addressing public concerns related to structural integrity, and maintaining an acceptable appearance by preventing unsightly damage such as cracking and staining.
In structural design, it is essential to assess the ultimate limit state, which evaluates the maximum load carrying capacity Additionally, Transport Infrastructure Ireland's new structures must accommodate abnormal vehicles, although such loading events are considered infrequent and do not significantly impact the maintenance of the structure's serviceability.
Durability refers to the capacity of materials or structures to withstand various effects over time, ensuring consistent performance with regular maintenance In road structure design, the intended durability period aligns with the design life specified in Chapter 2 of this document.
Durability is influenced by the following factors: i) Design and detailing; ii) Specification of materials used in construction; iii) Quality of construction
Effective control of items (ii) and (iii) relies on established standards and procedures In contrast, ensuring the durability of structural designs often receives insufficient focus, with considerations limited to factors like reinforcement cover, crack width restrictions, and minimum steel plate thickness This neglect of durability in design has led to premature serviceability failures in numerous road structures.
In unique circumstances, Transport Infrastructure Ireland may allow a Departure from Standard when meeting the standard is impractical Design Organisations encountering such situations should engage with Transport Infrastructure Ireland early in the design process to explore this option Any proposals for Departures from Standard must be submitted by the Design Organisation in line with GE-GEN guidelines.
All departures from established standards and specifications must receive formal approval before being integrated into a design layout The Design Organisation is required to document the use of any departures and the rationale behind them This documentation should be included in the Technical Acceptance Report, as stipulated in DN-STR-03001 regarding the Technical Acceptance of Road Structures on Motorways and Other National Roads.
Improved Durability – Conceptual / Preliminary Design Stage
Choosing the right structural design for a specific location significantly impacts its longevity This section outlines the essential criteria for structural forms and layouts that are proven to enhance durability Notably, the incorporation of gabions in any structure is strictly prohibited.
The standard design life for all structures is set at 120 years, with specific exceptions outlined: replaceable structural components such as bearings, waterproofing systems, expansion joints, parapets, and safety barriers have a design life of 50 years; short-term structures, including CCTV masts, high mast lighting, and bridge gantries, also have a design life of 50 years; temporary structures are designated a shorter lifespan of 10 years; and the design life for environmental noise barriers is specified in CC-SPW.
00300 - Specification for Road Works Series 300 - Fencing and Environmental Barriers
Bridges up to 60 metres in length, measured between the front faces of the end supports and with a skew angle not exceeding 30 degrees, must be designed to be fully integral.
Fully integral construction shall be defined as:
A monolithic connection between the substructure and superstructure with no expansion joints or bearings at the end supports
A monolithic connection between the bridge deck and intermediate supports eliminates the need for bearings, while steel plate girder decks utilize bearings for support In both scenarios, it is essential for the bridge deck to maintain continuity over the intermediate supports.
Integral abutments must be carefully designed, detailed, and constructed to effectively handle all imposed actions, preventing future pavement repairs due to issues arising from poorly constructed abutments.
Bridges longer than 60 meters or with a skew angle greater than 30 degrees should ideally be designed as fully integral structures If fully integral construction is not feasible, a semi-integral design should be adopted for these bridges.
Semi integral construction shall be defined as:
Construction of the end supports involving no expansion joint but with bearings (for example, with an end screen wall)
A monolithic connection between the bridge deck and intermediate supports, without bearings, is essential for structural integrity Alternatively, in the case of steel plate girder decks, the bridge deck is supported on bearings Regardless of the method, it is crucial for the bridge deck to maintain continuity over the intermediate supports.
The design of fully articulated bridges, which rely on bearings at all support points and include expansion joints, is permitted only in exceptional cases and must be approved by TII through the Structures Technical Acceptance process (refer to DN-STR-03001) When such a design is authorized, it is crucial to ensure adequate inspection and maintenance access to the joints and bearings, as outlined in this document.
Run-on slabs are generally discouraged due to potential maintenance issues, unless specific circumstances warrant their use If the Design Organisation determines that a run-on slab is necessary, they must notify TII through the Structures Technical Acceptance process as outlined in DN-STR-03001 at the earliest opportunity.
All bridges must be designed as fully continuous structures over intermediate supports, incorporating both the slab and beams where applicable Partial continuity, typically involving only the slab, is not allowed due to challenges in inspecting hidden surfaces If the Design Organisation determines that a fully continuous structure is not feasible, this must be communicated to TII through the Structures Technical Acceptance process as outlined in DN-STR-03001 at the earliest opportunity.
There are serious inspection, construction and maintenance problems associated with in-span discontinuities, generally referred to as ‘half-joints’ Half-joints shall not be provided in bridge decks
Deck hinges must not be used in bridges, unless there are adequate facilities for inspection and maintenance and specific agreement with TII is sought
During the conceptual and preliminary design stage, it is essential to incorporate effective drainage solutions for bridge decks and adjacent roads The design of bridge decks should prioritize rapid and efficient water shedding For additional information, please refer to Chapter 3.
All bridge decks shall be provided with an adequate surface water drainage system or bridge deck drainage system
Intermittent side inlet gullies are prohibited; instead, a continuous run of gullies must be installed along the entire length of the bridge deck where gullies are necessary.
The design for structures shall ensure that bridge decks shall project beyond the substructure in such a manner as to prevent water running down piers and abutments
Mainline road drainage must not be routed over or through any structures However, it is permissible to carry drainage over buried structures, provided there is adequate fill above the structure to support the drainage run and maintain a minimum clearance of 600mm between the top of the buried structure and the underside of the drainage pipe.
All bridges should utilize buried structures, considering key factors such as cost, cost-benefit analysis (including overall earthworks balance), clearance, spans, and aesthetic considerations.
All structures must meet the specified design life outlined in this document For corrugated steel buried structures, a combination of sacrificial steel thickness and galvanization is necessary to ensure this design life Additionally, a secondary protective coating system must be applied to permanently accessible surfaces, with a maintenance interval of six years; however, this secondary system will not be considered when calculating the structure's overall design life.
Improved Durability – Detailed Design Stage
Enhancing the lifespan of a bridge can be achieved with minimal extra cost through effective detailing of its structural elements This chapter outlines essential design requirements that contribute to improved durability For reliable design details, organizations are encouraged to consult the CIRIA Report C543, titled “Bridge Detailing Guide” (Soubry, 2001).
According to IS EN 1992-1-1, the nominal concrete cover (cnom) specified in drawings is the sum of the minimum cover (cmin) and an allowance for deviation (Δcdev) Despite any conflicting information in IS EN 1992 or IS EN 206-1 and their national annexes, the minimum cover for ensuring durability of reinforcement in concrete structures (cmin,dur) must adhere to the requirements outlined in Table B.1 of Appendix B of this standard.
For national road structures, the suitable concrete exposure class must align with Appendix A of the standard, unless an alternative is approved by TII through the Structures Technical Acceptance process outlined in DN-STR-03001, which covers the technical acceptance of road structures on motorways and other national roads.
According to Figure 2.1, in situations where a detail cannot be waterproofed or accessed for maintenance, a minimum concrete cover of 80mm must be provided for the reinforcement on the concrete faces directly beneath the movement joint, regardless of other standard requirements.
Systems for the drainage of surface water from bridges shall be so detailed that water is not allowed to fall freely from the bridge deck
Openings through bridge decks for surface water drainage and the routing of surface water discharge through steel box sections shall not be permitted
Closed drainage systems must be durable enough to endure cleaning-related damage, a frequent issue on existing bridges Additionally, they should be resistant to damage from common chemical spills on road surfaces Furthermore, the design and detailing of these drainage systems must account for structural movement.
Drainage waters from bridge decks shall not be discharged into the drainage layers behind abutments
Drainage systems that are integrated into structural elements, such as gullies in beams and pipes in columns, should be avoided For short-span bridges, it is essential to collect surface water from the bridge deck effectively.
All structures must incorporate a positive drainage system to effectively manage water infiltration through the surface and prevent accumulation on the waterproof membrane It is essential that the top surface of all bridge decks is designed with sufficient slopes to eliminate ponding, particularly around deck movement joints.
Effective drainage systems must include sufficient inspection, rodding, and cleaning facilities Rodding access should ensure that lengths are either straight or nearly straight, typically not exceeding 45 meters for straight runs Additionally, all gullies must be completely trapped to maintain optimal functionality.
The Design Organisation must document the recommended cleaning and maintenance procedures for the proposed drainage systems to minimize traffic management requirements, ensuring this information is included in the Safety File.
All bridge abutments, buried structures, and earth-retaining structures must incorporate a positive drainage system for their earth faces, as outlined in the TII Publications Specification for Works These drainage systems should also allow for future access and maintenance through rodding.
To prevent corrosion and deterioration in bridge decks, it is essential to include drainage holes in structures like box beams and voided slabs, in accordance with Chapter 2 requirements Box members must feature sealed access hatches or manhole covers to prevent water leakage Additionally, effective ventilation and drainage holes should be incorporated to minimize condensation and eliminate water ponding inside the box These ventilation and drainage designs must also deter access and nesting by birds and animals.
Bearing shelves and expansion joints in structures must include an effective drainage system connected to the road drainage system, ensuring accessibility for maintenance, cleaning, and rodding.
In the design of structures, it is essential to incorporate drip checks at all edge beams, deck ends over abutments, and specific areas like copings on retaining walls and reinforced earth bridge abutments and wingwalls This design feature effectively prevents water from flowing back along horizontal or vertical surfaces, ensuring proper drainage and structural integrity.
The ideal drip check design is an unreinforced concrete downstand If opting for a groove type drip check, it is essential to ensure that the full cover specified by TII Publications (Standards) is maintained, measured from the inside of the groove to the outermost reinforcement, including links.
Bridge deck waterproofing systems shall be spray applied, satisfy the requirements of DN-STR-03009
Waterproofing and surfacing of concrete bridge decks must enable non-destructive testing Additionally, waterproofing membranes exposed to foot traffic should be durable enough to endure this use while ensuring they are not slippery.
Improved Durability – Materials
Selecting suitable materials that withstand deterioration can substantially lower the overall life cost of structures, even with a slight increase in initial construction expenses This chapter outlines essential criteria for choosing materials that are proven to improve the durability of various structures.
In structural designs that incorporate steel-reinforced concrete, the minimum concrete grades specified by IS EN 206 must be adhered to: Grade 32/40 for superstructure concrete, Grade 25/30 for footway or verge infill concrete, Grade 32/40 for substructure concrete above base level and foundation, and Grade 40/50 for pre-cast concrete.
The design of structural concrete with steel reinforcement must adhere to the maximum water/cement ratio specified in Table B.1 of Appendix B of this standard.
When designing concrete mixes for buried components, it is essential to prioritize durability by considering the chemical composition of the soil and groundwater, alongside the specified strength requirements Additionally, the construction of all columns necessitates the inclusion of kickers (starter stubs).
The Splash Zone shall be that part of a bridge or other structure subject to spray from the adjacent road surface
The Splash Zone for a structure is defined as the area extending across the carriageway and 8 meters on either side beyond the edge of the hard shoulder or carriageway, aligning with dimension ‘x’ in Clause 4.2 of IS EN 1992–2 and the Irish National Annex Additionally, it includes the bridge deck sides and soffit when the minimum clearance above the carriageway is less than 7.5 meters, corresponding to dimension ‘y’ in the same standards Importantly, the Splash Zone also encompasses the parapet edge beams of the bridge structure.
In the Splash Zone of a structure, all exposed structural concrete must either be air-entrained or possess a minimum strength class of C40/50 according to IS EN 206 Furthermore, exposed concrete in this area should incorporate one of the following options: either consist of at least 50 percent ground granulated blast furnace slag or utilize surface impregnation in compliance with CC-SPW-01700.
The Specification for Road Works Series 1700 outlines the requirements for structural concrete, specifically excluding precast pre-tensioned pre-stressed beams made of Grade 50/60 concrete or higher as per IS EN 206, which do not require ground granulated blast furnace slag Additionally, the concrete will be reinforced using type 1.4301 or 1.4362 stainless steel in accordance with IS EN standards.
10088 where the steel shall be embedded in concrete, otherwise type 1.4362 or 1.4436 Stainless Steel to IS EN 10088
For all structures associated with national roads, including those in grade-separated interchanges, it is mandatory to use stainless steel reinforcement Type 1.4301 or 1.4362 in accordance with IS EN 10088 This requirement applies specifically to reinforcement within parapet edge beams, beneath movement joints on bearing shelves, and in bearing plinths.
Reinforcement for connecting parapet edge beams to the bridge deck must adhere to specific standards: it should either consist of stainless steel reinforcement Type 1.4301 or 1.4362 in accordance with IS EN 10088, or it must be safeguarded by a bridge deck waterproofing system, ensuring that the cover on the face of the parapet edge beam exceeds 80mm.
Where the design for structures contains run on slabs, stainless steel Type 1.4362 or 1.4436 Grade
500 to IS EN 10088 shall be used in connecting the transition slabs to the abutments
Where weathering steel shall be used in the design for structures, the environmental criteria shall be
"severe" as defined in DN-STR-03002 - Weathering Steel for Highway Structures
Fasteners and anchorages, such as bolts, washers, and sockets, used for attaching roadside structures like gantry bases and retaining walls, must be made of stainless steel type A4-70 or A5-80 The design should incorporate measures to prevent electrolytic corrosion between dissimilar metals Additionally, stainless steel should extend at least 100mm below the finished concrete surface for anchorage systems.
Detailed Requirements – Inspection and Maintenance
Incorporating provisions for general inspection and maintenance into structural design is essential, particularly for elements like bearings that have a shorter lifespan than the overall structure This chapter outlines the minimum requirements necessary to ensure effective future inspection and maintenance of structures.
Safe access must be ensured at all structures for various essential activities, including routine inspections, cleaning, maintenance, and painting Additionally, provisions should accommodate the replacement of prestressing tendons or stay cables, as well as inspections within closed cells or box sections Access should also be available to components requiring maintenance or replacement throughout the structure's lifespan, such as bearings, joints, anchorage locations, drainage systems, pipes, manholes, lubrication for moving parts, and lighting systems Furthermore, the design must facilitate jacking at bearings for their removal and replacement.
To ensure safety and maintain structural integrity, public access to inspection and maintenance facilities must be restricted using appropriate barriers and covers Additionally, measures should be implemented to prevent the colonization of accessible areas by plants, animals, or birds.
To ensure safe internal inspection of voided elements in bridge structures, access must be provided according to specific guidelines For mainline overbridges, access to abutment galleries should be from the minor road above, with permanent steps installed for ease of entry Access to voids is required from the underside of the bridge, with entry points strategically placed to avoid traffic interference These access points must be large enough for casualty evacuation and clearly marked with emergency signs and appropriate lighting Visibility of entry points to passing traffic should be minimized, and access should not appear on the main bridge elevation, except for necessary doors All permanent services must withstand environmental conditions, and access ladders or steps should be equipped with guardrails Walking surfaces must be non-slip, compatible with waterproofing, and designed to prevent tripping hazards Additionally, the interiors of steel box sections requiring access should be painted light for better visibility, and a permanent lighting system with a reliable power supply is recommended for access routes and chambers.
In structural design, it is essential to maintain a minimum lighting level of 30 lux Emergency routes must be equipped with additional lighting, ensuring at least 0.2 lux intensity, powered by a separate battery-operated supply Furthermore, clear warning notices and signs should be placed on all mains power boards and valves to enhance safety for individuals accessing voids within the structures.
All access points to galleries and voids and the like in the Design for Structures shall be secured from unauthorized access by means of lockable steel doors or grills
Inspection platforms shall be provided in front of the abutments to overbridges for both integral and non-integral structures in accordance with the requirements of CIRIA C543 ‘Bridge Detailing Guide’
Where bridge bearings shall be used in the design of structures they shall be replaceable without requiring the removal of any structural concrete or welding of structural steelwork
Provision shall be made in the design for structures to allow for jacking during any subsequent bearing replacement
The design of structures must ensure that during the essential work of replacing bearings, only one traffic lane is closed at any given time.
Service ducts and pipes, including drainage systems, must not be positioned on or near the external surfaces of any structure Additionally, attaching these service ducts and pipes to the exterior of a building is strictly prohibited.