A resilient structure is not simply one that is capable of withstanding the loads it was designed for. It must also be designed to remain operational after an extreme event or if load conditions change, says David MacKenzie, managing director at Flint & Neill, a Cowi company.
“Design is often presented as being an exercise that looks at the load and the capacity – and there’s the design. That’s a slightly myopic view if you think that’s all you have to worry about,” he says.
“Providing resilience is about recognising how the structure is likely to perform in service – and knowing how to do that at the design stage.”
Open and operating
When it comes to major bridges, MacKenzie says: “We have to be able to keep them open and keep them operating. We build a bridge for the travelling public, and once it’s there it’s there; take it away and people get very upset.”
Historically, building “resilience” into a structure simply meant designing it with a very high factor of safety. “When the first major suspension bridges were built they were designed to be resilient,” says Cowi chief project manager Matthew Bloomstine, citing the example of the Brooklyn Bridge, which was originally designed with a safety factor of five on its main cables. “Since then there was almost a contest to make these safety factors lower and lower with time. But now we are starting to design with the service life in mind.”
This may seem like an obvious approach to take – after all, roads are designed with the knowledge that different pavement elements will need to be replaced during their lifetime. But this has not always been the case for major bridges.
Flint & Neill has worked for many years on repairs for the Severn Bridge, which opened 50 years ago: “We have become aware of some significant problems with the hangers, and realised that no-one thought about how to replace them,” explains MacKenzie. “The cost of repairs could have been reduced significantly if that had been thought through at the design stage. When we design now, we always think about how we are going to replace these components.”
Bloomstine has another example: a 6km long road bridge in Sweden that was procured with lowest initial cost as the main focus. “In the first 20 years the client paid that price an extra time in maintenance, and now that is up to two and a half times,” he says. Cowi has since been commissioned to develop a 70-year maintenance strategy to try to ensure future maintenance of the bridge is as cost effective as possible.
Whole life design
North America-based Cowi senior project director Don Bergman believes major bridge owners have only recently started seeing the value of whole life design over the last decade.
“In the past, bridges have been designed for the least initial cost, and many of them have reached the end of their service life after 25 or 30 years,” he says. “That’s just not good enough.
“We are now designing [structures] very specifically for the environment they’re in; ensuring any elements that can’t be replaced will survive the whole service life; and the elements that can be replaced are incorporated into the design so they can be replaced without disrupting the function of the structure.”
There are different ways of looking at resilience. One aspect is the ability of structures to perform throughout their lives and to be easy to maintain. Another is the capacity to withstand extreme events, for example a ship impact, a wind event, terrorist attack or an earthquake.
“Understanding the performance of structures within these events is critical. For example, seismic design has moved from simply ensuring the structure won’t fall down to ensuring it can continue to be operable,” says MacKenzie.
Another aspect of resilience is coping with future increases in capacity. It is something Flint & Neill and Cowi are very familiar with, as they are regularly called in to design retrofit solutions to give existing structures more capacity, such as a project currently underway to strengthen and widen the 20-year-old EJ Whitten Bridge in Victoria, Australia.
Increased load resilience
“One of the challenges of new design is how we design in resilience for increased load in the future,” says MacKenzie. “How do you convince a client to spend a bit more now to provide that additional capacity in the future?”
The State of New York has been convinced, and is currently spending around $3.98bn (£3.07bn) replacing the 65-year-old Tappan Zee Bridge over the Hudson River with a structure that has built-in capacity for traffic growth.
The existing bridge carries around 138,000 vehicles a day – far more than it was designed for – and has twice the average accident rate than the surrounding road network.
Maintain or build new
The state authority has spent hundreds of millions of dollars maintaining the structure, and calculated that it would cost the same amount of money to continue maintaining the existing bridge as it would to build a completely new one, with no improvement to the current traffic conditions.
Design-build joint venture Tappan Zee Constructors has employed Cowi to design the main span for the replacement bridge, known as the New NY Bridge.
It has been designed to meet current traffic and pedestrian requirements, but also with the future in mind: as well as being designed for a 100 year service life, the new crossing will have space for possible bus rapid transit, and has been designed so that another deck can be added at a later stage for a light rail system.
“In North America we are seeing a lot of design-build and PPP [public-private partnerships] where owners are incorporating into their requirements the need for additional capacity in the future,” says Bergman. “Tappan Zee is an example: it’s a twin deck bridge, but one of the requirements was to be able to hang an additional deck that can carry transit in future. If they so choose they could add a further deck between the two existing decks to run transit.”
The role of the private sector in providing resilient structures is an interesting one.
MacKenzie says traditional design-build contracts tend to “incentivise the contractor to produce the cheapest structure for the conditions you require them to build”. But PPP often brings long term private sector responsibility for maintenance, which can result in innovative approaches to whole life costs.
He cites the example of Connect Plus’s 30-year contract to maintain the M25, which includes a clause giving the concessionaire a level of responsibility for maintenance costs well beyond the lifetime of the concession period.
“The idea of trying to encourage better behaviour is enshrined in the commercial model,” says MacKenzie. “Connect Plus has really understood this and takes a very proactive view.
“If you intervene early, you can programme the works. Emergency resilience is much more expensive.”