Goethals Bridge, New York. Photo by Andy Ryan, courtesy of HNTB.
By Ted Zoli
To explore the idea of resilience in the context of transportation infrastructure, it is useful to consider its opposite—fragility. Both terms—fragility and resiliency—can be applied with the same meaning, whether across expansive networks and systems, or down to smaller component levels.
A network, system, or component can be characterized as fragile if any of these four conditions apply:
- It is prone to failure;
- The consequences of failure are significant;
- The failure mode is difficult to identify and/or repair; or
- A long recovery period is required to return to adequate performance.
Resilience, as a design strategy, seeks to overcome these deficiencies. It has four design parameters and abilities:
- Robustness to withstand unforeseen demands;
- Redundancy to tolerate the loss or damage to a component;
- Resourcefulness to identify a problem and to respond effectively; and
- Rapidity to restore functionality quickly.
Many emergent technologies are changing the resiliency landscape. Here are examples in each area:
Robustness: movable barrier technology and its use in work zones.
In urban street and road projects, movable barriers and reversible lanes help reduce congestion and construction time, while enhancing the safety of workers and the traveling public. Peak congestion is reduced by reallocating unused lanes in the opposing direction. A similar concept is being developed by the Federal Highway Administration (FHWA) for on-demand technologies continue to emerge, the ability to re-allocate under-utilized lanes promises to enhance the overall capacity of existing urban networks.
Redundancy: replacing trusses with the cable stayed bridge and network arch.
In structural engineering, long span bridges often are choke points in transportation networks, especially given their greater cost than conventional short span bridges. In the past, long span bridge forms, parti-cularly the truss, sacrificed redundancy on the altar of efficiency, leaving a legacy of high-risk structures such as the I-35 span in Minneapolis, which collapsed in 2007.
Cable stayed bridges and network arches are two bridge adaptations of the truss that have emerged since the 1980s.
These systems are designed with cable components that are highly redundant, with damage or loss easily tolerated without sacrificing functionality. The recent replacement of major truss bridges in metropolitan New York (Goethals, Kosciuszko, and Tappan Zee) are examples of this trend. As bridge designers continue to focus on redundancy for long span bridges, expect to see new forms developed to enhance performance.
Resourcefulness: mapping algorithms and the ability to redirect traffic.
Mapping algorithms have significantly reduced the impact of shocks to the transportation network, whether planned (road reconstruction) or not (accidents). These mapping tools offer dynamic capabilities to quickly reallocate the transportation network to handle peak demands. What has not yet emerged from mapping algorithms, however, is a planning component. This function could help assess where additional infrastructure resources are needed to enhance network performance when there are shocks to the system.
Rapidity: the advent of accelerated bridge construction (ABC).
Bridge crossings are the most constrained part of the transportation network. Therefore, bridge replacement projects are particularly troublesome, since the already constrained resource is further compromised, often for an extended period.
Over the past 20 years, the advent of accelerated bridge construction techniques has dramatically reduced reconstruction times. More recently, lane-by-lane reconstruction has yielded to full closure rapid reconstruction, leading to more cost-effective and less disruptive projects. As these accelerated construction techniques become more common, the overall resilience of the transportation system is improved.
Designing more resilient transportation networks, systems, and components will undoubtedly enhance lifeline performance, particularly as we migrate from legacy structures that were not planned and designed with resiliency in mind. As we look to the future, technologies that enhance robustness, redundancy, resourcefulness, and rapidity will be at the forefront of the transportation sector.
(Ted Zoli, senior vice president at HNTB, will speak Oct. 2 at ARTBA’s TransOvation™ Workshop, part of the 2018 National Convention. This story appeared in the July-August issue of Transportation Builder.)