The role of structural engineers in sustainable design is widely perceived as being limited to material specifications and structural efficiency. However, innovation in performance-based design (PBD) and building life-cycle assessment (LCA) is presenting greater opportunities for structural engineers to take a leadership role on the sustainable design team. It is essential that engineers use PBD and LCA to protect the high-performance architectural and mechanical systems being placed in green buildings across the country—especially those that are likely to experience a high-consequence natural hazard such as an earthquake, hurricane, or flood during a structure’s lifetime.
As structural engineers, we know that the primary stated intent of the current building code is to prevent major structural failure and loss of life, and not necessarily to limit damage or maintain function. Therefore, designing a sustainable building with a code-based lateral system may have significant consequences in areas at risk from natural hazards. How can the life-cycle analyses of high-performance sustainable systems be evaluated if their use is cut short by a natural hazard? Unfortunately, many owners and clients still have the misconception that a code-based building is "disaster-proof." It is our responsibility as design professionals to educate our clients and offer them an alternative level of performance for their buildings.
To further appreciate the importance of building performance, it is necessary to understand the concept of embodied energy. The embodied energy associated with a building constitutes all of the energy that was used to create the building including extraction, fabrication, processing, transportation, and construction of building materials. The embodied energy of structural components generally comprises one quarter of the embodied energy of the building at the time of construction. However, the structure has the critical role of protecting the non-structural systems and building contents from damage.
In a typical building, the structural system accounts for approximately 10 to 15 percent of the construction cost (Figure 1). Generally, the most common reason for not improving structural performance is the increased up-front costs. However, even if there was a 10-percent increase in the cost of the structural system, this would only equate to an approximate 1-percent increase in the overall cost of the building. Through greater protection of these systems, we are protecting the return on the capital and resource investments made by the owner at a relatively small initial investment. It may also be possible to provide this improved performance to the owner at no premium or even at a cost savings.
PBD and probabilistic hazard analysis offer structural engineers the tools to evaluate and quantify building performance and to establish design criteria outside the baseline requirements of the building code. Common metrics of these design criteria often include demand parameters such as spectral accelerations, story drifts, and story accelerations. One of the challenges for engineers is to convey the results of the calculated probable building loss to the owner. By presenting the risk in terms that building owners are familiar with (dollars, downtime, et cetera), the engineer can provide the owner with enough information to make an informed decision on the best course of action. New standards in PBD are facilitating the evaluation of building performance within this framework. For example, the Applied Technology Council’s Next-Generation Performance-Based Seismic Design Guidelines for New and Existing Buildings (ATC-58) identifies methods for evaluating losses in terms of these metrics. If the client understands the risks associated with code-level building performance, and is interested in the long-term returns of sustainable design, reducing the risk of building damage will likely be perceived as a worthwhile investment.
Once the decision has been made to pursue a higher level of building performance, one of the first questions your client may ask is, "How can we pursue a LEED credit?"
The U.S. Green Building Council’s (USGBC) LEED rating system has become the de facto national standard for green building. In the current standard, LEED Version 2.2, structural issues are generally only covered in the Materials and Resources (MR) and Innovation and Design (ID) categories. There has been precedent for ID credits measuring a reduction in embodied energy over the life-cycle of the building, but none for improved structural and non-structural performance during a hazard event. Despite the lack of precedent, the ID category offers an excellent opportunity to achieve a building life-cycle assessment and hazard mitigation credit, particularly for projects where life-cycle performance is of significant concern (such as data centers, essential facilities, et cetera).
The forthcoming LEED 2009 is expected to chart a significant change to the standard and that may present future opportunities for structural engineers to contribute greatly to the LEED process. Most significantly, the USGBC has introduced "regionalization" into LEED 2009 as a series of bonus points, similar to the existing ID credits. Initially, six credits existing in each rating system will be identified as regional credits similar to current exemplary performance points. Designers will then be able to choose up to four of the six pre-qualified regional credits.
Engineering practitioners want to encourage the USGBC to continue its development of regionalization in future LEED standards by introducing new topics that affect local green building projects such as disaster resilience. By considering the material costs associated with replacing building contents damaged from probable hazards, structural engineers can utilize PBD to demonstrate a quantifiable material waste reduction in the event of a natural hazard through improved building performance.
There is a natural synergy between the long-term perspectives of sustainability and PBD. Owners who are interested in the benefits of sustainable design over the life of their buildings must also consider performance when subjected to natural hazards. As structural engineers, we have the opportunity to become an instrument of change in the industry while extending the services we offer our clients. The tools available today allow us to extend the building life-cycle and mitigate negative impacts on the environment caused by excess material or damage. By encouraging the responsible use of our natural resources and considering total building performance over its life-cycle, we can proactively collaborate and participate in the best practices of structural engineering and sustainable design.