Fine upstanding member

Have you ever pressed pause on your daily routine to consider how wonderfully rich the network is in terms of civil engineering? The scale is mindboggling. Yes, we’re all familiar with those attention-grabbing bridges across the Tamar, Tay and Forth, but they only occupy a few pixels in a vast, varied and vibrant picture. Take a step back to enjoy a broader view.

Our railways are carried by more than 42,400 individual spans, including a handful of timber-built survivors. Rarely given a second glance are 24,000 culverts. Embankments stretch collectively for almost 4,900 miles whilst another 3,900 miles of line are in cuttings; 17,000 retaining walls help to keep both supported. And we also look after enough tunnels to form an underground railway from King’s Cross to Thirsk, 20 miles north of York.

What’s even more remarkable is that, typically, those structures are visually examined annually and subjected to a physical examination once every six years. As we learned in February, that latter activity can involve roped access adventurers defying gravity and humanity’s natural instincts to bring insight from the most inhospitable of places, exposed on a high beam or within a confined space.

And beyond that baseline recording of condition, there is then a need to determine any impact on loading capacity, predict how that might change and the need for intervention. Helping to inform that process is a small army of structural engineers who combine on-site observation and measurement with professional judgement and calculation. But how do they go about that? Seeking an overview, I sat down with Network Rail’s Mark Norman, alongside John Longthorne, Steve Browne and Ana Walpole from AECOM, a company holding Civils Assessment Framework Agreements across four routes. It was not a short meeting.

History lesson

Notwithstanding the considerable sum invested over recent years, Network Rail’s civils asset base remains quite aged. Railway building reached its peak – albeit an unsustainable one – in the late 1840s, meaning many structures are now approaching 170 years old; the Skerne bridge in Darlington is 190. So it is clearly vital to have knowledge of their history, right back to the moment when shovels were first put in the ground, in order to establish a comprehensive management strategy.

The materials used in construction, the available finances, ground conditions, the experience of the workforce and how closely they were supervised all have a bearing on a structure’s ability to safely fulfil its role today. Those built during periods of war bring issues with them for all the above reasons. At a more granular level, if a crack in an arch barrel is known to be stable and longstanding, the associated risk is clearly less than with one that has just appeared. The past can therefore offer valuable context for what the eye now sees.

Where are we?

Asset management is influenced by a host of different factors, but the two fundamentals are condition and capacity. Understanding the former relies on a regime of examinations.

Detailed exams are undertaken at a frequency arrived at through risk assessment, typically every six years. Each one resets Network Rail’s understanding of a structure’s condition. Involved is a full tactile survey, recording every defect and assigning them a rating by type and severity. From this, two measures are obtained:

• an overall SCMI (Structure Condition Marking Index) score, in a range from 0 to 100, indicating the structure’s sustainability (effectively, how close it is to end of life)
• a risk score, based on a 5×5 matrix, quantifying the structure’s resilience in terms of the likelihood and impact of a failure.Both measures are used to inform decisions around the need for, nature and urgency of an intervention. Simplistically, a risk score exceeding 12 or an SCMI below 40 could be regarded as tipping points, although these are far from hard and fast.

Annual visual examinations – generally carried out from ground level – serve the critical purpose of confirming that a structure remains in a safe condition. This is a relative test, comparing what’s currently seen against the absolute position recorded during the detailed exam. It enables the rate and extent of degradation to be tracked.

Number crunching

Capacity reflects the ability of a structure to support both its own load and that of the traffic passing over it. This is established through an assessment, usually carried out every 18 years in accordance with the requirements of more than two dozen railway and highway standards.
There are three types:

• Level 0 requires the engineer to input parameters into a pre-designed spreadsheet which outputs the structure’s Route Availability (RA) rating. The parameters – describing dimensions, materials, condition etc – are used by macros, automated tasks within the spreadsheet, to perform a simple analysis of the structure. This level is used for common construction types and proves sufficient for most structures across the network.
• Level 1 requires the engineer to carry out a bespoke set of calculations by hand, perhaps because the structure is unusual in form or a more detailed analysis would be beneficial to achieve greater accuracy.
• Level 2 generally entails complex analysis to model the structure’s behaviour and requires a high degree of engineering competence. This can prove time consuming – sometimes taking many months – but has the potential to unlock latent capacity.

It’s worth making the point that technology’s encroachment into this area only goes so far. Today’s engineer might be able to do on a laptop what their predecessor did at great cost 30 years ago by logging into a university computer with punch cards, but that in no way lessens the skills needed to provide the right initial properties and accurately interpret the emerging results. A software tool is only as good as the person using it; there is no substitute for an intuitive understanding of load paths.

Negative influence

Getting on for half of our underbridges are brick in construction. This is good news as the Victorians excelled at building arches. Generally they bring few problems, being immensely strong; where issues do arise, inappropriate repairs can be the cause. It might come as a surprise but we are still learning about arches. Until relatively recently, our instinct was to intervene with modern techniques and materials, however this often resulted in the formation of hard spots which create tension around their edges when a load is applied.

We know now that consistency is key, allowing an arch to work as a homogenous structure in compression; its ability to move is fundamental in order to redistribute the thrust of a moving vehicle and deal with temperature fluctuation.

At almost 9,400 in number, our inventory of metallic underbridges (cast/wrought iron and steel) imposes far stiffer challenges from an assessment perspective, due to their complexity and material deficiencies. Every structural member has to be assessed, with the intention of confirming that each is capable of resisting the load applied to it. That’s the basic equation. Where there are multiple members fulfilling the same function – cross girders, for example – the convention is only to assess the worst case as this will serve to limit the structure’s RA rating.

What shouldn’t be underestimated is the volume of data that has to be mined for assessments. The engineer will inspect the structure with different eyes to the examiner, looking for such things as torsional buckling, distortion, twist and distress. They will also record and measure areas of corrosion, recognising that section loss has a critical impact on load distribution.

Quality control was far from rigorous 150 years ago. Whilst there was a host of practical reasons for this, it has left the railway with an unwelcome legacy: yield strengths (the point at which a material begins to deform plastically) are quite variable.

To address this, a prescribed yield strength is adopted for each material type, arrived at through years of analysis. For railway purposes, the figure is lower-bound but, if necessary, can be revised upwards through sampling. With wrought iron, such were the shortcomings of the manufacturing process that a conservative approach is always taken. There can never be any confidence about its make-up.

But whatever the material, one commonality with all structures is their susceptibility to the overarching power of water: brickwork to freeze-thaw action, steel to corrosion, piers to scour, tunnel linings to mortar loss, foundations to settlement, slopes to instability. Asset management is water management in a great many respects.

Looking harder

It is possible to find latent strength within a structure by gaining a deeper understanding of how it works. The method of achieving this, as part of a Level 2 assessment, is through a computational tool known as Finite Element Analysis. A mesh is generated based on the physical characteristics of the structure, potentially comprising many thousands of linked cells; each one of these is then assigned a set of properties and an algorithm applied which calculates how they interact on loading. Results can be output as a coloured model showing the movement of that load through the structure, with the associated stresses, strains and displacements.

The process is highly sophisticated and, despite modern computing power, can often take many hours to run for larger models. But the benefits can be considerable, perhaps demonstrating that a structural member has more capacity than previously determined because its behaviour was not fully understood, or revealing the source of stress concentrations that were causing a beam to buckle. Such insight – including the ability to model imperfections, deformations and damaged members – leads to more informed judgements, allowing a targeted repair to be designed or precluding the need for one altogether.

Now what?

Based on the findings of the assessment and examinations, the engineer will recommend a preferred form of intervention for each structure, if needed. Providing a framework for this are the criteria set out within Network Rail’s Structures Policy, encompassing:

• the criticality of the route and likely future usage
• the ease of access, both in terms of traffic volumes and physical constraints
• structure type, capacity, overall condition and risk score
• the nature, location and severity of the defects
• capital and whole-life costs.All these factors influence the respective benefits of renewal against recurrent strengthening or repair, and the associated timescales.

The word “intervention” has the potential to mislead, as it does not necessarily involve physical works. The recommendation might be to invoke a higher level of assessment, increase examination frequency or carry out on-site testing, installing strain gauges and deflection poles to measure a structure’s real behaviour under traffic loading.