AD 353: Execution class for bridge steelwork
With the switch to Eurocodes for the design of bridges, project specifications are now being drawn up based on the use of the execution standard BS EN 1090-2 and the Steel Bridge Group’s Model Project Specification (MPS) document (SCI publication P382). The Advisory Desk has become aware that, in adopting the MPS, specifiers have in some cases chosen to select Execution Class 4 (EXC4) as the ‘default’ for the project, rather than EXC3, as recommended in the MPS. The purpose of this AD Note is to explain the consequence of making that more onerous choice.
BS EN 1090-2 sets out four execution classes, EXC1 to EXC4, EXC4 being the highest class. For the reliability level appropriate to most bridges, EXC3 will ensure the necessary quality of workmanship throughout the structure – in practice EXC2 might be sufficient for many parts of a bridge but it is easier and safer to adopt the same level of requirements generally, rather than try to differentiate between one part of the structure and another.
The differences in requirements between EXC3 and EXC4 arise in only a few clauses of BS EN 1090-2 but have a significant effect upon certain acceptance criteria and levels of inspection. The most notable changes are: thickness tolerance (5.3.2); quality of cut surfaces (6.4.3); acceptance criteria (7.6); extent of supplementary NDT (Table 24).
In detail the increased requirements are:
Thickness tolerance (5.3.2): The default thickness tolerance for EXC4 is Class B, which is more onerous than the Class A normally used at present for both highway and railway bridges. Specifying Class B will result in an increase of 0.5 to 0.7 mm in the thickness of most plates and material cost is directly related to the weight of the plates. The increase in self-weight due to positive tolerance should be allowed for, where appropriate (in lightweight structures of thin plated elements).
Quality of cut surfaces (6.4.3): The cutting processes used in normal production are not capable of producing the ‘Range 3’ quality of surface required by EXC4. To achieve this quality the cut surface would need to be ground, which would frustrate the efforts that fabricators are making to avoid all manual grinding because of health and safety concerns over tool vibration and eye injury. The additional operation would add to cost. Range 3 quality is not usually assumed when carrying out fatigue assessment of free edges
Acceptance criteria (7.6): Fabrication shops using mainly manual welding methods will struggle to achieve some of the additional acceptance criteria for level B+, particularly if the design calls for complex joints where access is poor and the welder may be working in an uncomfortable position. The additional requirements of EXC4 are only really achievable if there are simple, relatively straight joints where mechanized processes can be applied or the welder has good access and visibility.
Extent of NDT (12.4.2): A greater extent of inspection clearly invokes greater time and cost.
The requirements for EXC3 in these clauses correspond to current practice in bridge fabrication, which has offered a quality that has generally been acceptable. Class EXC3 is adequate to ensure that the fatigue reference stress level is for the detail category in the Tables in BS EN 1993-1-9 or, where listed in Table NA.1 in the UK National Annex, the minimum strength level in that Table. Specifying EXC4 would meet the requirements for “special inspection and testing requirements” noted in NA.2.1.2 – for example allowing the detail category to increase from 125 to 140 for free edges – but the increase will rarely have any practical effect for fatigue, as lower category details at or near the specific location will usually govern.
In summary, if the more stringent requirements of EXC4 are specified, the requirements can be met but they are more onerous, will result in increased fabrication cost but will achieve very little benefit.
Contact: David Iles
Tel: 01344 636525