Technical forum Steel Bridge Group’s latest publication supports an efficiency drive spearheaded by Highways England and Network Rail.
Since 1994, the aim of the Steel Bridge Group (SBG) has been to make steel bridges as efficient, economic and sustainable as possible. It does this by sharing the knowledge and experience of its team of structural steelwork experts from client, designer, fabricator and contractor backgrounds. Where technical queries are raised, it trawls through the nitty gritty of design codes and commonly used bridge specifications to find answers. It does the hard work, so that bridge designers do not have to.
Current SBG chair Chris Hendy explains that the group’s first task was to consider the British Standard for specifying steel – BS 5400-6 Steel, concrete and composite bridges. Specification for materials and workmanship, steel. This allowed designers to choose the level of quality and workmanship for a piece of steelwork.
“Designers were not used to making these decisions, which would heavily influence fabrication,” he says. “So they generally opted for the most onerous specifications to be safe, leading to excessive levels of quality and workmanship.”
Variation among designers
There was also much variation among different designers when it came to treating and testing steelwork details. Without a consistent approach, fabrication errors could be more easily made, especially where two seemingly similar details on similar structures designed by different designers were treated differently.
Although these errors could often be rectified or the steel manufactured with greater care, the knock-on effect was for the cost of steel bridges to go up or for fabricators to lose out. So the SBG decided to step in.
It created a set of model clauses for highway bridges in the areas left open to designer’s judgement. The SBG rationalised the main options designers could specify and brought a more consistent approach to specifying in more specialised areas. It also produced, Guidance notes on best practice in steel bridge construction (SCI P185) which covered aspects such as detailing, material properties, fabrication, inspection, testing, erection and protective treatment. The guide explains the thought process that designers should follow to lead to straightforward, safe and economic fabrication and construction.
This guide, now in its sixth edition and 20th year of publication by the Steel Construction Institute sheds light on many issues related to different types of steel structure
When Eurocode EN 1090-2 replaced BS 5400-6 in 2010, the SBG updated its model clauses in line with the new design code. The same issue of choice had expanded even more in the Eurocodes, whose scope had increased to cover all steel structures. Network Rail and Highways England updated its specifications in line with the Eurocodes in 2016 and the SBG is still there guiding the way.
“Steel Bridge Group: Completion of Appendix 18/1 for use with Specifications for Highway and Railway Steelwork (SCI P418)” is the title for this latest document. Although the Highways England specification, also the basis for that of Network Rail, provides guidance for efficient design, fabrication and construction in the most common cases, there are still areas where the designer must weigh up options and make the final call.
These are generally where structures or details are bespoke, or where the setting is unusual. Transport Scotland is expected to adopt the Highways England and Network Rail specifications. Many local authorities commonly reference these specifications in their contract documents, so the guide will have applications over a variety of road or rail structures in many different settings.
The SCI P418 publication references more than 100 clauses from EN 1090-2 and the Highways England and Network Rail specifications, covering aspects such as geometrical tolerance, welding, bolts and cables, grade of steel, surface treatment and whether trial assembly is beneficial.
A new section of roof being built at Waterloo Station in London has been designed with reference to this guide, comments SBG member Ian Palmer.
“The SCI P418 guide is a useful checklist to make sure no aspect of design has been overlooked and nothing is overspecified. It is there to ensure that a consistent, economic and correct approach is taken when considering the questions unanswered by the Highways England and Network Rail specifications,” he says.
Elsewhere, particular to the rail environment, where major bridge work is carried out during possessions, the guide recommends trial assembly before final construction and refers to further SBG guidance. Other clauses examine workmanship and testing to satisfy fatigue requirements (see box); how to treat steel surfaces in contact with concrete; choice of steel grade; and even how a piece of steel should be marked for identification in a way that will not adversely affect fatigue performance. One of the document’s strongest messages is that collaboration with the supply chain is imperative, so that the issues that influence steelwork design are understood.
Quantified Service Category
Structural steelwork specifications published recently by Highways Englan d and Network Rail include a new parameter called the quantified service category (QSC).
This characterises how hard a component or structure is working, and points to the level of testing required to validate its “fatigue stress”.
“The logic is that, if you allow for a higher fatigue stress, you are permitting fewer flaws in the steel and detailing, which requires more attention and the need for it to undergo more onerous testing to verify its quality,” says SBG chair Chris Hendy. “Designers must specify the QSC for every detail on a bridge and must understand how that value will affect the efficiency of the structure and its cost.”
Designing for a high level of fatigue stress will have the effect of ramping up quality and workmanship and, with it, the need for more expensive and time-consuming testing methods. This could impact on overall cost more than, say, increasing the thickness of steel and specifying a lower level of fatigue stress.
“Increasing the steel tonnage and specifying less onerous quality and workmanship and simpler testing methods, which are cheaper and quicker to perform, might turn out to be the more cost-effective solution,” Hendy suggests. The SBG examined numerous UK highway and rail bridges to understand their retrospective QSC values and concluded that most situations were designed for a fatigue stress range of up to 56MPa, or QSC level F56. As a result it has based its SCI P418 best practice guidance in line with this, but also suggests ways of approaching the relatively few cases where the QSC may need to be higher.