In This Issue
Summer Bridge: A Vision for the Future of America’s Infrastructure
June 15, 2018 Volume 48 Issue 2
The articles in this issue, by academic and industry experts, focus on what’s needed to prepare US infrastructure systems for the coming decades.

The Future Design of Sustainable Infrastructure

Friday, June 15, 2018

Author: Michael D. Lepech

As a critical set of systems, infrastructure forms much of the foundation for quality of life and enables national development and progress. However, it also consumes vast material resources and energy (Matos 2017). Thus, it is essential that it be designed using long-term design approaches that consider social, environmental, and economic impacts over many years of use.

What Is “Sustainable” Infrastructure?

Over the past decade the designers, builders, operators, and owners of infrastructure systems in the United States have been pushed by many stakeholders to adopt greater measures of sustainability in all facets of infrastructure projects. This push has resulted in the creation of sustainability-focused tools for the infrastructure sector that can be classified as (1) knowledge-based methods (e.g., European Commission 2013; Ross and Coleman 2008), (2) rating schema (e.g., BREEAM 2012; Greenroads International 2017a; USGBC 2018), or (3) performance-based tools (e.g., Kendall et al. 2008; Ramaswami et al. 2008; Reger et al. 2014; Zhang et al. 2010).

Together these tools have brought sustainability issues to the forefront of infrastructure and building design, construction, and operations practice in the United States. For example, the US Green Building Council (USGBC 2018) has certified more than 92,000 Leadership in Energy and Environment Design (LEED) projects, and the more recently introduced Greenroads (2017b) rating scheme has already certified over 100 state and municipal roadway projects.

But sustainability is now often defined by the criteria used to recognize it (e.g., limited construction material transportation distance or the purchase of renewable energy for construction site use; Ehrenfeld 2007). There is no formal definition based on the perpetuation of natural, social, or economic systems (i.e., sustainability). Rather, prima facie definitions are rooted in practicality and are a result of the problematic ex post facto nature of sustainability. As such, today’s criteria-based definitions can be judged as “sustainable” only from far in the future with little evidence of causality.

Moreover, while attempting to strike a balance between the built environment, the natural environment, and societal considerations, current -sustainability-focused guidelines and points-based approaches are limited in their ability to support rational decision making and tradeoffs and fail to consider the large uncertainties associated with long-lasting infrastructure systems. This article presents a more practical approach for the -sustainability-focused design of infrastructure.

Limit State Design for Sustainability

Limit state design is a hallmark of modern civil engineering theory and practice (e.g., ACI 2014; AISC 2001), yet it has not been applied to sustainability assessment and design of infrastructure systems or built environments.

The master builders of Renaissance cathedrals, who had relied on knowledge-based heuristics to inform their craft, transitioned to mechanics-based design theories that enabled more reliable, efficient, and well-understood structures. These theories then yielded to today’s limit state design approaches that look to safely and economically balance uncertain structural loads and capacities according to accepted professional levels of safety.

A Code-Based Framework

Performance-based approaches that achieve these goals without the constraints of prescriptive design codes will likely grow more common, as is already the case in earthquake engineering (e.g., Moehle and -Deierlein 2004). Analogously, today’s sustainability-focused guidelines and points-based rating systems, which are well-informed heuristics, must yield to science-based assessment and design methods that balance loads and capacities in ways that are safer, more economical, more reliable, and better understood.

A code-based framework has been proposed that -pushes sustainability-focused design toward this limit state approach. The 2010 fib Model Code (fib 2013, sections 3.4 and 7.10) proposes a design method that consists of two essential features: (1) a stochastic lifecycle assessment and service life prediction model for measuring the impacts of infrastructure construction, operations, and maintenance activities; and (2) -sustainability-focused limit states that guide design. The latter are to be considered alongside today’s -accepted ultimate limit states (ULS), which protect against collapse and preserve life, and serviceability limit states (SLS), which ensure proper functionality.

Recognizing the unique nature and environment of every project, the 2010 Model Code does not prescribe sustainability design criteria and limit states for -designers. Where can these criteria and limit states be found?

Learning from Natural Ecosystems

Environmental sustainability limit states for infrastructure design and management are emerging from the study of natural ecosystem services. Natural ecosystems are the foundation of life on this planet: They provide grains, biomass, water, and genetic resources. They regulate the climate, pests, floods, and air and water quality. They support photosynthesis, pollination, and biogeochemical cycles. And they are of cultural, spiritual, and even aesthetic value (Bakshi et al. 2015). Natural ecosystem services include uptake of carbon monoxide, sulfur oxides, nitrogen oxides, and volatile organic compounds. Natural ecosystems are a planetary-scale life support system (Balmford et al. 2002; Costanza et al. 1997).

Until recent decades, engineers did not pay much attention to the dependence and quantitative impacts of engineering activities on natural and social ecosystems. But viewing the engineered environment as loads on natural ecosystems, and looking to understand nature’s ability to carry those loads, can shed light on a path toward limit state design for sustainability.

The balance between built infrastructure and natural ecosystem services exists at multiple spatial and temporal scales and for a variety of natural ecosystem services (Bakshi et al. 2015). For instance, the load on a natural ecosystem may be determined by specific emissions and resource use related to an infrastructure project design (e.g., lifecycle CO2-equivalent [CO2e] emissions, lifecycle water consumption). Capacity may be estimated from knowledge of relevant ecosystems at the selected ecological scale, such as the supply of carbon sequestration as a fraction of the mass of CO2-equivalents sequestered globally by plants, trees, and oceans. At a smaller scale, the supply of water by natural ecosystems depends on features in the watershed such as rivers, the rate of groundwater replenishment, rainfall, and the degree of surface imperviousness.

For information about the capacity of natural ecosystems at multiple scales, various models and databases are becoming available (e.g., US Forest Service 2018). Models of natural wetlands, for example, can quantify their removal of water pollutants and other ecosystem services (Flight et al. 2012).

By viewing natural ecosystem services as a crucial, but limited, resource that sustains life, the definition of environmental sustainability limit states becomes a question of balancing load versus capacity, with an acceptable level of safety that accounts for the inherent uncertainty in the system.

An Illustration: Designing to Address Climate Change

The United Nations Intergovernmental Panel on Climate Change (IPCC) has proposed reduction targets for global CO2e emissions (IPCC 2014). These targets are based on a global surface temperature rise of approximately 2.5°C, avoiding the greatest consequences of climate change and preventing irreparable damage to the biosphere.

Figure 1 

The evolution of global surface temperature up to year 2300 for a range of global CO2e emission scenarios is shown in figure 1(a), and figure 1(b) shows global CO2e emission pathways through year 2100 (IPCC 2014). The scenario of greatest interest here is representative concentration pathway (RCP) 4.5, which corresponds to a global surface temperature rise of approximately 2.5°C.

Based on figure 1(b), to limit stabilized global surface temperatures to a rise of approximately 2.5°C (RCP 4.5), a 30–60 percent reduction in annual CO2e emissions is needed by year 2050 (with year 2000 as the baseline). This reduction represents a sustainability limit state based on the natural atmospheric ecosystem’s carry-ing capacity to take in and sequester (e.g., via plants, trees, oceans) global CO2e emissions.

Building from the 2010 fib Model Code’s sustainability-focused design approach, a proposed probabilistic design approach consists of two types of stochastic -models: (1) service life prediction and (2) lifecycle assessment (LCA) of infrastructure construction, operation, maintenance, and end-of-life activities (Lepech et al. 2014). To combine the two models, future maintenance, repair, and rehabilitation activities and their impacts are described by probability functions. The resulting framework generates a distribution of cumulative sustainability impacts throughout the lifecycle of an infrastructure system, from the beginning of construction to the time of functional obsolescence (end of life), shown schematically in figure 2(a). This framework designs for sustainability through the reduction of impacts over time to meet current or future sustainability goals (i.e., 30–60 percent reduction in annual CO2e emissions by year 2050 versus the year 2000 baseline proposed by the IPCC).

Figure 2 

The comparison of two infrastructure design scenarios (status quo versus a sustainable alternative) is shown in figure 2(b). Based on this, the level of impact reduction associated with a sustainability-focused infrastructure design (lower trendline) versus the status quo (upper trendline) can be calculated at any time in the future with a given level of confidence. Figure 2(b) also shows the probability of failing to meet a sustainability-focused goal by implementing the sustainability-focused alternative, Pf(t), over the lifecycle.

Challenges of Limit State Design for Sustainability

Without doubt, a comprehensive, performance-based approach to sustainability-focused design will be difficult to implement. Significant challenges quickly come to mind; for example,

  • Can infrastructure sustainability reasonably be reduced to a set of ecosystem carrying capacities?
  • How should designers account for infrastructure designs that enhance natural ecosystems (i.e., a negative load on natural ecosystems)?
  • How can ecosystems that have not been studied extensively by groups like the IPCC be considered?
  • Should all sustainability-focused limit states be considered equally important?
  • How would this approach be introduced or adopted in code-based design?
  • What are allowable probabilities of failure for missing sustainability-focused limit states 5, 10, or 50 years in the future?

These (and other) challenges are significant, but it is appropriate to bear in mind that the transition from the Renaissance master builders to today’s performance-based design of earthquake-resistant structures took centuries. And that dramatic shift required the collaboration of many academic disciplines, such as architecture, engineering, mechanics, and statistics, with contributions from economics (economic loss modeling and costing) and public policy (building codes). The transition to limit state design for sustainability will require substantially more collaboration among academics, practitioners, and policymakers, and will draw from the diverse fields of biology, chemistry, and sociology for the proper establishment of ecosystem carry capacities and social norms. On a positive note, the rate at which collaborative thinking and research have moved from theory to practice has accelerated greatly since the construction of Europe’s great cathedrals.

To incentivize the transition to limit state–based design for sustainability there are measures for “socially responsible financing” (Kim 2016), including Green Bonds and Social Impact Bonds (e.g., Pigeon et al. 2012; Reed 2014). Through such bonds, investors can fund the construction of major infrastructure or other projects in return for the promise of reduced environmental impacts or positive social impacts (in addition to financial repayment). The bonds are financial instruments that weigh the potential positive impacts associated with an infrastructure project against other options, including the option to simply do nothing. They must also weigh the risk of not delivering on promised impacts. Such financing instruments are well matched to limit state design for sustainability, which supports rational decision making and tradeoffs and explicitly considers the large uncertainties associated with long-lasting infrastructure investments.


An important outcome of a shift to limit state design for sustainability is the potential to drive sustainability-related innovations in infrastructure planning, design, construction, operation, and maintenance. Innovations in construction materials (e.g., Billington et al. 2014), vehicle propulsion technologies (e.g., Hawkins et al. 2013), or entirely new transportation modes (e.g., SpaceX 2013) will accelerate in the coming decades, and many will reduce environmental impacts, increase access and equity, and reduce the cost of infrastructure systems and services. While guidelines and ratings-based design approaches may struggle to incorporate the sustainability benefits of relentless innovation in infrastructure systems, the fundamental load-versus-capacity nature of performance-based approaches is highly adaptable.

Figure 3 

An example of such innovation for surface transportation infrastructure is the Hyperloop (figure 3). Although still in the early stages of technology development and proof of concept, this transformative innovation would have significant environmental, social, and economic costs and benefits that are complex, inter-related—and highly uncertain. These impacts include significant capital expenditures to build this entirely new form of transportation system (estimated at $6–7 billion to build a line from Los Angeles to San -Francisco), significant savings in travel time over traditional surface transportation (the estimated Hyperloop travel time from Los Angeles to San Francisco is 35 minutes versus a minimum of 5½ hours by car and over 10 hours by conventional rail), the potential to power the entire system using renewable energy generation and storage (solar power arrays deployed along the route combined with a bank of lithium-ion batteries), and acquisition and site disruption of new right of way (SpaceX 2013).

Existing sustainability-focused design approaches would struggle to accommodate the highly uncertain and complex nature of these costs and benefits, and might therefore become a barrier to innovative infrastructure projects that do not achieve a minimum sustainability rating. Limit state sustainability design approaches are well suited to consider (1) a variety of environmental and/or social sustainability limit states that would apply over the roughly 345-mile (570 km) route and (2) the unknown environmental, social, and economic performance of such a system decades into the future.


As reported in numerous academic studies, news events, and anecdotal stories about the condition of existing infrastructure systems, the time is now to think long term about ways to design infrastructure to meet social, environmental, and economic goals. Improved consideration of economic, social, and environmental impacts in the design of infrastructure and the built environment will be the legacy of guidelines and ratings-based design approaches that are being developed and applied today.

New limit state design for sustainability can reinvigorate designers by opening up a range of innovative solutions that meet sustainability goals in ways that are safe and economical. They reject prima facie definitions of sustainability and encourage -designers, engineers, owners, man-agers, and financiers to collaborate with yet other partners—for example, in the natural sciences, social sciences, and humanities—to design systems that deliver socially, environmentally, and economically sustainable benefits.


The author acknowledges the support of the Stanford University Blume Fellowship, the Stanford University Shah Family Fellowship, the -Stanford Terman Faculty Fellowship, the Thomas V. Jones Engineering Faculty Scholarship, and the Nordic Innovation Centre Project (Number 08190 SR). This material is also based on work supported by the National Science Foundation under Grant Nos. 1453881 “CAREER: Multiphysics Modeling for Probabilistic Design and Engineering of Sustainable Infrastructure” and 1334083 “Seeking -Synergy Between Technological and Ecological Systems for Sustainable Engineering.” The author also thankfully acknowledges the input of Drs. Sarah Russell-Smith, Steven Comello, John Basbagill, Bhavik Bakshi, Guy Ziv, and all NICe project participants who have added to this work, as well as the editorial comments of Cameron Fletcher. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the National Science Foundation.


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About the Author:Michael Lepech is an associate professor of civil and environmental engineering and senior fellow at the Woods Institute for the Environment at Stanford University.