Sustainable Bridges for Every Bridge Engineer

Daniel Whittemore, AI Engineers, Inc., Middletown, CT

Introduction

As "green design" has become mainstream, examples of bridges with notable "green" features can now be found all over the world. . The Kurilpa Pedestrian Bridge in Brisbane, Australia, designed by Cox Architects, uses an array of 84 solar panels to power LED lights to illuminate its deck. In Ballybofey, Ireland, engineers have installed a water-powered light system powered by the currents of the river it crosses. And in San Diego, California, engineers have proposed a new structure that has an LED light show powered every night by 100% renewable sources including wind turbines. Do these stately, impressive crossings represent the leading edge of sustainable bridge design?

Before we can begin to answer that question, we need to first define the term "sustainable bridge." I suspect this term evokes images of structures like those listed above in the minds of many of my fellow bridge engineers when they hear that term. Or, perhaps they imagine picturesque glued-laminated wooden structures blending harmo- niously with their surroundings deep in a national park. Notably, the one common thread would be that these images would appear to have very little applicability to the real world, day-to-day problems faced by bridge owners or bridge professionals. Being pragmatic, many of us would then quickly dismiss the concept of a sustainable bridge as something impractical, or at best, of limited usefulness outside of the arena of green design.

The Triple Bottom Line

In reality, such images and the basic assumptions behind them are off the mark. To reframe the discussion, a sustainable design of any stripe is commonly defined as "development that meets the needs of the present without compromising the ability of future generations to meet their own needs." Extrapolating from this basic definition, a sustainable engineering project such as a bridge can therefore be defined as one that is conceived, designed, constructed, operated, maintained, and eventually put out of service in such a fashion that these activities demand as little as possible from the natural, material and energy resources of the surrounding supporting community.

The concept suggested by this definition is often described as the sustainable triple bottom line. In this visualization of sustainability, what we are looking for are engineering projects that have the most positive impact at the intersection of the People it intends to serve, the Planet on both a micro and macro scale, and the long-term Profit of everyone involved throughout the life of the project. By this test, a sustainable bridge can't be one that serves the people it serves, but imposes a high lifetime cost. Nor could it be an ecologically friendly and economical crossing that serves noone. Rather, a completely sustainable bridge is one that strives to serve the people that use it and the environment it is connected to at a palatable long- term cost. A sustainable bridge isn't, then, defined by any one feature on the crossing, but by looking at the structure in its entire context.

Are the bridges in the first paragraph sustainable? The answer is: they might be, but the features mentioned above, by themselves, don't tell the complete story about each bridge's impact to the triple bottom line. With a definition in hand, we can now start to look at any of the structures above or a proposed bridge project and begin to measure, and hopefully plan for, its sustainability.

Is It Sustainable?

So, with a useful definition applied to the concept, the next logical question is: How are a sustainable bridge project's goals measured and quantified?

In the United States where I practice, there is currently no national standard for the measurement or ranking of sustainable bridges like the U.S. Green Building Council's (USGBC) benchmark LEED^® standard for buildings. The USGBC describes its program as follows: "The LEED (Leadership in Energy and Environmental Design) Green Building Rating System is the nationally accepted benchmark for the design, construction, and operation of high performance green buildings." The system has become the leading national baseline standard for quantifying how sustainable a building project, a school, or now, even a neighborhood planning project is.

At its core, the LEED guidelines break down any potential sustainable building project into one of 5 overarching categories. For comparison's sake, each of these building categories will be matched with a comparable bridge design metric: (Table 1)


These four goals can be described as four questions:

• Where is the bridge located, and who does it serve? (Sustainable Sites)
• How much and of what quality is the water coming off or under the bridge? (Water Use and Quality)
• What sort of energy is consumed by the bridge and the traffic it carries? (Energy and Transportation)
• What materials are used in the bridge, and where did they come from? (Materials and Resources)

Out of the goals listed above, only the LEED category of Indoor Environmental Quality, which is concerned with indoor pollutants, seems to have no directly corresponding equal in a sustainable bridge metric.

Each of these potential sustainable bridge metrics will now be described in some detail as we take a look at the questions that should be asked of any potential sustainable bridge project.

Bridge Sustainability Metrics

Sustainable Sites
The hallmark of a sustainable site is being the right location for a proposed structure. The right location will not only minimize the ecological impact of new construction, but also address the immediate needs of the communities it intends to bridge between. Questions posed in a sustainable site investigation could include the following: (Table 2)



As you read these questions, note the relation between them and the previously defined triple bottom line. Each Sustainable Site question above attempts to address a specific portion of the triple bottom concept by placing a structure in the optimal location, addressing the ecological needs of the environment it spans over and connects, and by looking at the economic needs of the communities. Each of the metrics for this point on should, likewise, address the same questions posed by the sustainable triple bottom line from its own standpoint.

Water Use and Quality
Water is a precious resource for every nation that should be utilized only when required and handled with care during and long after construction. Water has a direct link to the economic and environmental health of every human community, as is evident when it is not readily available. It has been calculated that 1 in 8 of the world's population lack access to safe water supplies, and as a result of that, 3.5 million people die every year from otherwise avoidable water-related disease (source: water.org).

A water use and quality investigation can evaluate the quality and quantity of water used in construction, the water that runs under it, and that which runs off the structure after its installation. (Table 3)

Energy and Transportation
It has been calculated that transportation represents 10% of the world's gross domestic product and is responsible for 22% of global energy consumption and 25% of fossil fuel use across the world (source: http://environment.transportation.org/environmental_issues/sustainability). The proposed purpose of an energy and transportation category is to ensure that the structure is designed and constructed to minimize the energy and transportation needs of the surrounding community. (Table 4)


Materials and Resources
It has been said that the most sustainable structure is the structure that you don't have to build. A material and resource category ensures that the choice in bridge materials is appropriate for the site, and that the materials available for construction are utilized to their fullest. It also attempts to address future maintenance and eventual recycling of the structure and its components. (Table 5).

Potential Sustainability Benefits

After sustainable bridges have been suitably defined and quantified, the inevitable question then becomes what are the tangible benefits for investing the extra layer of effort and resources into such a project?

Hard evidence for the benefits of this type of bridge design is an area that requires more real world examples and both academic and field studies, as has been done previously for buildings. However, from the above metrics, a list of proposed benefits for this type of design could include the following: (Table 6).


Sustainability should be a part of the basic underpinning to the sphere of civil and structural engineering. For example, in retrospect, it is now well understood the mistakes that civil engineers made in the 1940s and 50s in the United States in the planning of the Eisenhower Interstate system. For example, in my native New England, many of the cities were built next to major navigable rivers that became their economic and social lifeblood. When the current day highway system was planned, the highways were often placed directly on the available and under built waterfronts, literally cutting off the downtowns from the economic engines that originally gave them life. Ramps then spurred off from these highway into the downtown areas, cutting neighborhoods in two and cutting people off from each other and from the downtowns that used to sustain them. Major infrastructure dollars in cities such as Boston have been spent trying to reverse the impacts these highways have had to our cities by either completely sinking them underground, or by trying to bridge over them in an effort to reconnect these cities back to the waterfronts they used to command.

From the perspective of the engineers of the time, the highways were (and ares till) engineering marvels. They were sized with the (then) appropriate number of travel lanes to support the projected traffic loads, the structures were designed to carry the appropriately loaded truck load, and the crowns and superelevations were put in appropriately banked for the rated speeds. What wasn't understood or considered much were the whys of where the roads went, who they were serving, and how the pure engineering was impacting the environments or economics of the cities they ran through and cut in two. The tools of sustainable metrics, when wielded appropriately by the engineering community, has the potential to put back into our hands the power to decide why something should (or shouldn't) be built, and not just the how I suspect we too often settle for.

The Sustainability Hat

As has been shown, sustainable bridge design is not about strictly environmental concerns, or only about energy conservation. Instead, it is a more holistic, top to bottom review and evaluation of a bridge project's merit and compatibility with the indigenous human and wildlife populations on both the micro and macro scale. As such, it has the potential to be a useful tool to quantify and determine the true scale of even indirect or unintended deterioration done to our environment, society and the community at large.

I like to envision sustainable bridge design like the following illustration. Given a typical local collector crossing over a waterway, a partial list of the responsibilities necessary to complete this task might include the following: (Table 7).


Each of the items in the lists above is referred to as a "hat" because an engineer (often on smaller structures, a single engineer) dons the hat somewhat independently from the others during the course of a design. For example, the engineer performing the highway design typically dons his/her highway design hat to lay out the geometry for a new structure, then dons a different structural engineering hat to ensure that the bridge is adequately reinforced. Each of the hats can be thought of as occurring at finite, discrete moments during the planning, design, and construction phases of this sample project.

But when held against our definition of sustainable design and the triple bottom line, we find that this analogy breaks down. This is because sustainable design is not an individual task to be performed at a discrete point in the design process. In order to produce a project that is truly sustainable throughout the structure's life, each of the tasks above needs to be considered from a sustainability standpoint. For instance, when locating the bridge, is it better to place it within wetlands or in the dry? When performing hydraulic analysis, how will the constriction and reduced hydraulic opening impact the upstream and downstream flows and ecosystems? During the construction phase, where did the materials used in the structure come from, and are they being used to their greatest effect?

In order to ever hope to meet the burden of our definition of sustainable design, what is required is not a new hat, but a new pair of glasses to look anew at every step in a project's progress. Or, in order to deliver a truly sustainable bridge, sustainable goals need to be considered and accounted for throughout almost every phase of the planning, design, construction and maintenance process. Sustainability has truly become an underpinning of the entire process of civil and structural engineering.

Current State of Sustainable Bridge Design Standards

As discussed, there is currently no leading standard for quantifying sustainable bridges. Therefore, the number of bridges conceived and branded with "sustainable" labels as of the time of this writing is minimal.

These two facts are interrelated. With no reliable standard or best practices established, it is hard for an outsider to distinguish between a conventional and sustainable design and no way to elevate one project's features over another's. Of the bridges that have been built, it is hard to distinguish many of these bridges' claims to the label of "sustainable" (or the more nebulous label of "green") as the lack of an agreed upon standard allows even the installation of just one pertinent feature access to the sustainable title for the entire crossing.

Sustainable bridge design is a modern day topic that requires more academic study, modeling, testing and thought to move forward in a meaningful way. This field of research should be applied to supplying hard data to the following currently unanswered but important and pertinent questions: (Table 8).


A national standard could also serve to regulate the current unregulated nature of the current sustainable materials marketplace. Currently, with no oversight or goals to shoot for, a new material vendor or process can apply labels at will with little to no tangible benefits to back it up. If a bridge owner or designer knows that a given product has a direct impact on a given sustainable metric, the material vendor has a much more legitimate claim on these types of labels.

The civil engineering community is beginning to become aware of the potential in sustainable design. As I write this, the American Society of Civil Engineers (ASCE), with support from the American Council of Engineering Companies (ACEC) and American Public Works Association (APWA), is on the verge of releasing Sustainability Ratings for Engineering Projects. It is hoped that systems such as this and others being developed worldwide will begin to put recognized, applicable tools into the hands of engineers in the US and abroad to the benefit of the clients and the public we serve.

Conclusion

The concepts presented in this article are intended to be jumping off points and are not a comprehensive list of the types of questions that should be addressed in looking at a potential sustainable bridge project. I fully understand and support that different jurisdictions would want to weigh different parts of a metric in a manner that best suits their needs, and have purposely made no indication in this article as to which of these questions are of more importance than others for that very reason.

It is time for the more insular world of bridge engineering and maintenance to join the growing numbers of professionals from all trades that are realizing the tremendous potential of sustainable design. Not to be confined just to the building and planning industries or the "green fringe," bridge professionals involved with all aspects of the typical bridge lifecycle can benefit from a national, standardized set of sustainable bridge metrics. Further study, research and practice of both national and international standards will be required to solidly establish the end value and proper weighting of any set of proposed metrics. In the meantime, the metrics proposed in this paper and found elsewhere can be used as a launching point for potential pilot projects to facilitate further study in this emerging transportation field.

Daniel Whittemore is the vice president in charge of design efforts at AI Engineers, a full service engineering firm headquartered in Middletown, Connecticut, USA. A licensed professional engineer in 6 US states, Dan is a passionate proponent of forwarding the maintenance, preservation, and sustainability goals of bridge owners and professionals everywhere.