A new framework for designing bridges to reduce operation and maintenance (O&M) liabilities on the owners has been put forward by the Steel Bridge Group (SBG).
The SBG is a technical forum that is part of the Steel Construction Institute (SCI), comprising experts working for clients, designers, fabricators, contractors and its secretariat. Its purpose is to identify and report technical issues with the use of the Eurocodes and to produce complementary guidance reflecting UK best practice.
Representing the SBG at NCE’s Bridges conference on 17 July, Hewson Consulting Engineers director Andrew Hodgkinson introduced its three-tier framework for embedding O&M considerations into bridge design.
Before introducing the framework, he highlighted the pressures of bridge maintenance on owners.
“If you are responsible for a bridge, with increasingly vocal public pressure and a lack in finance, you are pressed on every side,” Hodgkinson said. “There’s a limit to what we can do in terms of our existing bridges – we have to maintain them; we have a liability.
“But what we have to recognise about the bridges we’re building now is that it’s increasingly important that we don’t add to that liability unnecessarily. In other words, [we have to make sure] that the bridges that we are currently designing and constructing are as minimal maintenance as possible.”
To paint a picture of the maintenance liability on bridge owners, Hodgkinson referred back to a survey carried out by the SBG in 2017 that had nearly 34,000 respondents, which represented roughly 29% of the UK’s bridge stock.
Average annual cost of maintenance of steel bridges (£ – 2017 prices)
It was found that the greatest overall spend for bridge maintenance was on maintenance painting, followed by movement joint maintenance, repair or replacement of bridge deck waterproofing, fatigue repairs and bridge bearing maintenance or replacement.
When looking at individual maintenance schemes, it was found that bridge bearing maintenance was the most costly element, followed by maintenance painting, bridge deck waterproofing and then fatigue repairs.
Average steel bridge maintenance scheme cost (x£1,000 – 2017 prices)
Hodkinson further explained that the survey asked specific questions about the types of maintenance activity and the types of issues associated that led to the need to undertake the maintenance. Out of this, the SBG identified the top six O&M liabilities in steel bridges as:
- Bearing maintenance and replacement
- Corrosion protection system repair or renewal
- Bridge expansion joint replacement
- Waterproofing renewal
- Fatigue related strengthening
- Hidden critical elements
Respondents were also asked whether they thought enough attention was given to O&M in bridge design and it was a roughly 50-50 split between yes and no, according to Hodgkinson
“The very fact that people were not convinced means there is more to be done in that area,” he said. “The other key point was that there was a strong feeling that we needed more guidance on how to design for operation and maintenance.”
This feedback was taken away and worked on by the SBG, who drew up a framework.
Hodgkinson did point out that guidance from major clients released since 2017 has tended towards recommending a minimal O&M design approach. For example, National Highways’ CG 300 states: “Designs that have minimal maintenance provide significant benefits in reducing the safety risk to the workforce and reducing disruption to the network.”
Additionally, Network Rail’s CIV/003 states that good practice is “evidence that access and maintenance requirements of asset manager and maintenance organisation have been incorporated within the design”.
“That’s a really good starting point,” Hodgkinson said. “But I think you’ll agree there’s more to be done in this field.”
The SBG has therefore come up with a three-tiered framework for minimising liabilities.
The proposed bridge design framework for minimising O&M liabilities
“The first level is, unsurprisingly, to eliminate, wherever possible, O&M liabilities at source,” Hodgkinson said.
“The second level is to reduce. If we can’t eliminate an O&M liability then we need to reduce the impact of the liability; we need to be looking at things that make it safer and easier for a bridge maintainer to deal with that liability during the lifetime of the bridge.
“The last level is to inform. Now, I think this is often a bit where we go wrong. I’ve had many conversations with bridge owners who’ve said ‘Well, we know what we have to do with this bridge, we know we’ve got to replace these bearings, but nobody’s told us where we can jack it. Nobody’s told us what this bearing stiffener will take in terms of load.’
“So, there’s very much a case that if we can’t eliminate an O&M liability and we have reduced it, that we also need to inform.”
Hodgkinson pointed out that “our ability to affect O&M liabilities reduces the further we get into design”.
“So, it’s really important that, the same way we consider health and safety at an early stage, we consider O&M at an early stage,” he continued. “This is the only way we’re going to stop adding bridges that have significant O&M liabilities to the bridge stock.”
Hodgkinson then moved on to show how the framework can be applied in practice.
He showed an image of a bridge over a railway and said: “This is a bridge that has got a nice access, so I’m assuming this bridge is on bearings, and an abutment gallery has been provided, which means we can access the bearings. And there’s a walkway that’s provided to that abutment gallery.
“But was consideration ever given to the idea of making that bridge integral? I don’t know the answer, but that’s the first step in the framework process, asking that question: in designing this bridge, can we eliminate those barriers?
“I don’t know in the case of this bridge whether or not that would have been possible, but that’s the sort of thinking we would need to go through.”
Hodgkinson then put up an image of a bridge he was familiar with from his work with Hewson: the Pont Briwet Viaduct in North Wales (pictured above and below).
“It’s a combined rail and road bridge, 134m long and fully integral,” he said. “There are no bearings and in fact there are no formal expansion joints on the bridge at all. So, this bridge, in my view, is about as low maintenance as a bridge can get.”
The Pont Briwet Viaduct is a 134m long fully integral road and rail bridge
He also highlighted a snipped from the CD 350 Standard for Highways which states: “Bridges with a skew angle greater than 30 degrees or lengths greater than 60m may be designed as integral structures.”
Hodgkinson said he would like to see an attitude, or even a change in standard, where bridges meeting that description “should be designed as integral structures wherever technically feasible”.
“That poses a challenge because it might mean that as designers we have to do structure integration analysis,” he said. “But in fact the [Pont Briwet Viaduct] is one where we did full source structure interaction analysis to prove the pressures behind the abutments and to demonstrate it was technically feasible.
“So what I’m saying is, if you’re a bridge technical approval authority, ask those questions about whether it’s technically feasible and sometimes don’t take no for an answer; perhaps try and challenge it a bit so we can eliminate the risks.”
Hodgkinson then moved onto the second tier of the framework: reduce.
“The sort of things we need to consider if we’re going to reduce the risk or reduce the liability of bearings are things like providing enough workspace around bearings,” he said. “What we’ve found is that one of the great things about BIM and 3D design is that actually, as well as being a great design tool, it allows us to understand and visualise what space we have, to be able to take a bird’s eye view or take a walk around the bearing and actually look to ensure there’s sufficient space for maintenance. It’s a great tool and I don’t think it’s used enough for considering those sorts of liabilities at the design stage.”
Hodgkinson also suggested the use of secondary bearing plates.
“Do we install secondary bearing plates so that bearings can be removed without needing to pack out grout or even bearing plates?” he asked. “That’s a debate I’m willing to have, but that’s the way it seems to be going at the minute.”
A third consideration he mentioned was having defined temporary jacking points.
“Does the bridge owner know where they can jack the bridge to replace the bearings?” he said. “What we need to try and avoid is situations where significant temporary works have to be put up to be able to replace bearings. If we need bearings, if we can’t eliminate them, we don’t want to have to put up significant temporary works to be able to replace them.
“So those are the sort of things that can only really be addressed at design stage, hence why early consideration is needed.”
Hodgkinson then moved onto the final tier in the framework: inform.
“Hopefully the bearings will only need to be replaced many years after the bridge has been finished, however technology will have moved on and the people in the owner organisation will have moved on,” he said. “The new people in the organisation will need to know what are the assumptions you’ve made? What’s the sequence? How many bearings can be replaced at one time? What concurrent maintenance activities can take place?”
He added that, in his view, this information should all be on the drawings.
“[Having the information] in an operational maintenance manual is great, but they often get lost, so it’s good to have them on the drawings as well,” he said. “What about checking those? Will [the owner] know what load the designer would assume at the checking point? Yes, you can back calculate it, but it would be a lot easier if it was just on the drawing so it’s obvious.”
Hodgkinson said that this three-stage framework can be applied to any of the aforementioned maintenance liabilities, including painting, movement joints, hidden critical elements and so on.
“If you think about it, this eliminate, reduce, inform framework can be applied throughout to these O&M liabilities,” he said. “Hopefully it gives a structure to how you consider that at the design stage.”
Hodgkinson finished by acknowledging the “significant weight” that bridge owners carry with regards to the responsibility of maintaining their structures.
“So, as designers of new bridges, we therefore have a responsibility not to add to that weight unnecessarily,” he continued. “We must ensure that the bridges we design are minimal maintenance and that we embed inspection and maintenance requitement so that our designs are smart from the start.”
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