Oct 06 2021, by Fleetwood Urban
Cracking the Code
Each year in Australia, countless pedestrian structures are built unnecessarily – and expensively – to the full structural specifications of the Bridge Code, AS 5100. With a viable alternative, AS 2156, the question is why?
The Bridge Code, AS 5100, has been the default standard for Australian councils and engineers since the late 1990s. When used for the purposes it was originally created, it’s a fantastic structural guide. However, each year it also leads to significant levels of over-engineering on smaller community access bridges and structures – a situation Fleetwood believes is unnecessary, unsustainable and, increasingly, unjustifiable.
Knowing your options
“It’s really important to remember the Bridge Code is a standard, not a law,” explains Mark Jol, Lead Design Technician at Fleetwood. “It only becomes law when it’s specified into a contract. It’s also written overwhelmingly for high load-bearing applications involving cars, trucks, heavy traffic and high speeds. In those scenarios it’s the gold quality standard. We’ve used it to deliver many outstanding bridges over the years and we still do. But the decision to use AS 5100 isn’t as black and white as many people think – there are real alternatives.”
“A lot of the briefs and tenders we see automatically dictate pedestrian structures must be designed to AS 5100,” Mark continues. “Unfortunately, when that happens there’s little, if any, room for consideration of other possibilities that may deliver far better outcomes for the project, the community and the environment, not to mention the budget!”
“The question I always ask myself is, what type of structure is the brief actually asking for?” Mark says. “Is it a bridge? Or is it really a boardwalk? That might sound simple, but it’s a very grey area and the answer can open up a whole range of options for the way a project is designed and delivered.”
While there are certainly important exceptions, AS 5100 specifications are typically far in excess of what’s required to deliver safe community bridges and access structures for pedestrians, joggers, dog-walkers and cyclists, especially ones situated away from roads, railway lines and waterways. “The result is often over-design, over-specification, over-sizing and a massive amount of wasted resources,” Mark Jol laments. “Time, money, materials, transportation, greenhouse gasses, structural footprint – the bridge ends up consuming far more of all of these things than is necessary. It’s terribly wasteful, especially at a time when sustainability is such a focus, and reducing the carbon footprint of our local communities is front and centre.”
Mark offers the curious example of kick rails. “To fully comply with AS 5100, we must specify 100mm kick rails at the bottom of all balustrades. That’s a safety measure as it stops small pebbles from being kicked from the decking onto the road below – even if there isn’t a road below! It might be completely unnecessary, but you still have to do it. That’s just one example, there are plenty of others and they all add up.”
A ready-made alternative: AS 2156
The good news is a more sustainable, and entirely legitimate, alternative already exists: AS 2156, otherwise known as The Walking Tracks Standard. Together with the structural loading requirements of AS 1170, which Fleetwood already adheres to on all projects, we’ve been successfully using AS 2156 as the base for community bridges and boardwalks for many years now. The secret is identifying when and where it’s appropriate.
“It’s actually remarkable how many structures can be delivered brilliantly under The Walking Tracks Standard,” says Mark. “It certainly isn’t a loophole – AS 2156 is a safe, flexible and highly robust quality standard. It has all manner of applications for community access structures but, without question, its biggest benefits are in project efficiencies. Smaller structures, fewer materials, more compact site footprints, shorter timelines, the list goes on and on.”
CASE STUDY: LAKE PAMBULONG, NSW
To explore the real-world impacts of designing to AS 5100 and AS 2156, we asked the Fleetwood Estimating Team to crunch the numbers on a recently-completed community bridge project at Lake Pambulong near Newcastle. You can download the findings here.
Significant real-world savings
Estimator Braden Matthews works alongside Mark Jol here at Fleetwood. He knows both Standards well and provides just one example of how they can translate into real-world projects: “When you specify to AS 5100, the standard states the structure must be able to withstand 50 tonnes of lateral restraint load at each end of the bridge,” Braden says. “That’s 100 tonnes in total – an incredible amount of force, akin to a vehicle crashing at considerable speed, and has massive implications on the size and cost of the footings we use. With all the additional steel, concrete and other resources required to produce and install, it can easily add another $50,000 to an otherwise modest pedestrian structure that will simply never experience an impact remotely close to that.”
“It’s a similar story when it comes to bridge deck loading requirements,” Braden continues. “Under AS 5100 you need to allow a blanket 5kPa loading on the bridge deck, which equates to 500kg/m2. We recently completed a 34-metre pedestrian bridge under AS 5100 that had to allow for 51.17 tonnes. Based on the global average body weight of 62kg, that’s 825 people using it at once! This bridge is located in a quiet rural area where it would be unlikely to get more than a few people ever on the bridge at the same time. Under AS 2156 this could have been reduced to 300kg/m2, and would have had significantly reduced member sizes and overall cost. Sometimes less really is more.”
Mark Jol agrees, adding AS 2156 is also a highly flexible standard that allows for structural augmentations as required. “If a brief calls for a longer design life, or the location means we do need to augment an element of the design or base structure – increasing its flood loading, for example – we can absolutely do so, quickly and easily with AS 2156,” he says. “Yes, it’s about understanding the standards, but it’s also about evaluating the specific requirements of the brief, and collaborating with the engineering team to deliver the very best solution every time, rather than an expensive one-size-fits-all approach.”
Could you be using AS 2156?
While it certainly isn’t appropriate for every bridge project, AS 2156 is often an option worth exploring in the planning stages. Fleetwood is highly experienced at working with the Standard, so if you’d like to discuss its suitability for an upcoming project, please get in touch.
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