Durable details (additional sub-section)

In order to meet durability requirements, some suggested guidelines are given below: 1. Specifying a steel grade that does not require painting. As the majority of steel bridge

durability problems involve corrosion of the steel after failure of the protective paint system, ‘weathering steel’ can often be an effective alternative to ordinary painted steels. ‘Weathering steel’ is a low-alloy steel that corrodes at a much slower rate than standard steel grades. The corrosion induces a stable patina of fine-grained rust which remains adhered to the base metal and slows the rate of corrosion to a level which enables the steel to be left in standard atmospheric conditions unpainted. A small corrosion allowance on thickness, whose magnitude depends on environment, still has to be made.

Weathering steel has advantages for health and safety (by eliminating the risks of maintenance painting at height or inside box girders), for the environment (by eliminating

3-2/clause 4(1)

3-1-1/clause 4(3) 3-2/clause 4(4) 3-2/clause 4(5)

emissions of solvents into the atmosphere when the paint cures) and for reducing costs (by eliminating whole-life maintenance costs associated with repainting the structure). It should not however be used in coastal or aggressive chemical environments or other areas where a high concentration of chloride ions is present, as the functioning of the patina is inhibited.

Guidance on the use of weathering steel is available directly from producers and also in Reference 5.

An even more effective, but very expensive, alternative to weathering steel is stainless steel.

2. Avoidance of corrosion traps in detailing. Durability problems tend to start at corrosion traps on the steel structure. Durability can therefore be much improved if the detailing avoids corrosion traps as far as possible. This issue is particularly important for non- painted weathering steels. In addition, it is recommended that water contaminated with de-icing salts is kept well away from steel components by effective fail-safe drainage systems.

3. Avoidance of details that cannot be easily painted. For structures that contain painted steelwork, many durability problems can be avoided by ensuring that there are no areas where access is difficult for applying paint.

4. Sacrificial thickness and fatigue checks for inaccessible components. If areas are totally inaccessible during the design life then they can be increased in thickness so that they are not overstressed if part of the section is lost due to corrosion. In the absence of guidance in EC3 (a National Annex may give guidance), it is recommended that designers use the provisions in BS 5400: Part 3.4 For a design life of 120 years, this gave recommended values of sacrificial thickness to apply to each inaccessible surface as follows:

(i) 6 mm at industrial or marine sites (ii) 4 mm at other inland sites

(iii) 1 mm in addition to the excess under (i) and (ii) where free drainage cannot be specified.

In addition, EN 1993-1-9 requires that inaccessible components are checked for fatigue using the ‘safe life’ concept. Potentially, this would require more onerous partial factors in the fatigue check of the inaccessible component, although it is likely that the ‘safe life’ approach will be used in the UK for all details, whether accessible for inspection or not, as discussed in Chapter 9 of this guide.

5. Careful specification of the painting system. The designer is recommended to ensure that the protective paint system is carefully and accurately specified. Of particular importance is the specification of the initial surface preparation works as these works form the foundation for the rest of the paint system.

6. Careful specification of the fabrication and erection works. Some durability problems can be caused by poor fabrication and erection procedures. Steel bridge structures designed to EN 1993-2 should be fabricated to EN 1090-2 in which the fabrication procedures are designed to ensure durable steel components.

7. Elimination of slip in joints. To prevent slip and consequential possible wear and ingress of moisture between plates in connections, 3-2/clause 2.1.3.3 requires permanent connections to be made using one of the following:

. _{Category B preloaded bolts (no slip at serviceability limit state (SLS)} . _{Category C preloaded bolts (no slip at ultimate limit state (ULS)} . _{fit bolts}

. _rivets . _welding.

4.2. Replaceability (additional sub-section)

3-2/clause 4(6)requires that components which cannot be designed with sufficient reliability to achieve the design working life should be replaceable. Typical components which should be replaceable, along with suggestions for complying with 3-2/clause 4(6), are as follows: 1. The corrosion protection system. Ensure that the corrosion protection system can be

replaced safely at the end of its design life.

2. Stays, cables, hangers. Carry out design checks to ensure that the structure is still adequate if a cable is removed. Ensure that the cable connection detail allows the cables to be replaced in the future. This is discussed in more detail in section 5.1.4 of this guide.

3. Bearings. Ensure that bearings are detailed so that they can be simply removed from the structure without excessive effort. Provide jacking stiffeners so that the structure can be safely jacked up to enable replacement of the bearing.

4. Expansion joints. Ensure that the expansion joints can be replaced without damage to the bridge deck.

5. Asphalt layer and waterproofing. Ensure that the structure can withstand replacement of the surfacing and waterproofing.

6. Guardrails, parapets, wind shields and noise barriers. Ensure that these components can be easily removed from the structure without damage occurring to the main bridge. Components, such as parapets, which may be susceptible to errant vehicle impact should be designed so that their foundation (e.g. deck cantilevers) and anchorage is stronger than the parapet post. This will ensure that repairs, if required, are only required for the parapet and not the bridge deck – 3-2/clause 2.1.3.3(2) refers.

7. Drainage devices. Ensure that drainage systems are able to be cleared at regular intervals by providing sufficient rodding eyes at accessible locations. Ensure that the drainage system can be easily replaced if needed.