In the beginning, there were no structural engineers. From Vitruvius to da Vinci to Thomas Jefferson, architect and engineer were embodied in the same person; structure and form were facets of a unified thought process. Then came the industrial revolution, mass production, thermal comfort, and life safety-all with technologies so complex that building design became dispersed among specialists.

“We need to get away from the model where the architect designs the building alone, then all the other disciplines are brought in later to realize the design,” says Wolfgang Werner, sustainability director at the engineering firm Thornton Tomasetti. “It's no longer just up to the architect.” Lance Hosey, architect and CEO of the non-profit GreenBlue, asserts that we should be reimagining the relationship between sustainability and building structure. In his upcoming book, The Shape of Green, he contends that the segregation of architect and engineer is a schism between artist and scientist. According to Hosey, this kind of left/right brain split has led to green buildings that often rely on the “science” of applying technologies like photovoltaics to buildings without considering a broader aesthetic potential of sustainability. Hosey avoids labels like organic or biomimicry but he points out that nature produces very efficient and functional forms. “I began by asking if there are better ways to make a beam, column, or whole building structure to conserve resources. If you look at the way nature produces structure, it doesn't make little boxes. The rectangle is an inefficient geometry for carrying loads,” he adds (see sidebar “Low-Tech Fabric Structures”).

Sustainability could be one of the forces that pulls the disciplines back into a more integrated process. “We're in a generational shift in which buildings were once about bricks and mortar, to where buildings are more about energy and performance,” says Craig Schwitter, managing director for North America for the engineering firm Buro Happold. “We have the responsibility to be attentive to the issues of energy and performance on projects, but structural engineers don't typically get training in these areas.” Schwitter sees structural engineers getting involved early in the process and having a more informed influence on mechanical systems, daylighting, and other areas.

Illustration: Thom Sevalrud

More Than Skin Deep

Several engineers commented that high profile architects sometimes design eccentric buildings that often belie awkward and material intensive structures beneath their sleek skins. But the L.A. firm Morphosis manages to mix green practices with cutting-edge design. The San Francisco Federal Building, completed in 2007, provides a good case study for the integration of structure into sustainable strategies of a building. “Our approach is to look at how all systems can be integrated to enhance the performance of the building on a number of different levels,” says Brandon Welling, project architect for the facility. “From the beginning of the schematic design process we brought in all the engineers because we wanted to set up a model of integration,” he adds.

The design team used a narrow floor plate for the 18-story building, which accommodates cross ventilation from the prevailing winds. Instead of hanging girder beams down from the floor, the engineers at Arup helped develop a system of upturned concrete beams that run the long direction of the tower. Above the slab, the beams are enveloped within the raised floor system. The upturned beams free the ceiling of obstructions and allow ventilation air to move uninterrupted across the thermal mass of the exposed concrete. The team wanted to use thermal mass to absorb heat during the day, and night flush ventilation to cool it at night. In lieu of using beams for the short dimension of the structure, the engineers helped to develop a wave-formed slab that functions as a stiffened corrugated plate. The corrugations not only channel the air through the building, they increase the surface area and the thermal storage capacity of the slab. Because of the passive strategies, the top 12 floors have only minimal mechanical systems in the upper lobbies, according to Welling.

The Durable and Adaptable Building

Though we live in the era of the 50-year building, the U.S. General Services Administration requires its facilities to be built for a 100-year lifespan. That was the case for the San Francisco Federal Building. Engineers are often pushed by the marketplace to design structures that are as cost-effective to build and as materially efficient as possible for an intended purpose and life span. Structural efficiency can also be seen as a green strategy: Using fewer materials reduces the amount of energy it takes to construct a building as well as the embodied energy and carbon in a structure. At the cutting edge of structural efficiency are evolutionary optimization programs whose software is a kind of digital version of the natural selection process. It quickly generates multiple iterations of possible structural variations responding to conditions such as load patterns. Skidmore, Owings & Merrill (SOM) used evolutionary optimization software to develop the structure for the tallest building in the world, the Burj Khalifa in Dubai. Interestingly, these structures often evolve into non-rectilinear forms that echo structures found in nature.

In the San Francisco Federal Building, a wave-form slab supports the floor plate and provides thermal mass. Photo © Nic Lehoux

The upturned beams keep the undulating ceiling plane free of structure and allow ventilation of the thermal mass. Photo © Nic Lehoux

However, the counterpoint to extreme structural efficiency is that sustainable buildings should be built for longevity by making them durable and adaptable to future uses. Extending a building's life can save much of its embodied energy and eliminate the enormous amount of energy and resources spent in replacing it.

When considering the longevity of the San Francisco Federal Building, the design team took into account how the structure would adapt to change. Besides optimizing ventilation and daylighting, the narrow floor plate allowed the designers to use perimeter columns only, which left the interior free of structure. The lack of fixed obstructions is a key aspect of a flexible and adaptable space.

Stewart Brand's book How Buildings Learn has been influential in developing concepts of building adaptability. Brand promoted the idea of shearing layers, in which building elements are replaced at different rates of time. Layers such as interior finishes and “stuff” (furniture, equipment, etc.) are replaced relatively frequently, whereas the structure has a longer lifespan. “We start envisioning buildings as being skeleton and skin and try to get the skeleton to be straightforward and robust,” says David Mar, a principal at Tipping Mar. “We see programming and architectural partitions as temporary,” he adds.

Instead of hiding small shear walls within partition walls, Mar advocates the use of a few strong frames such as steel or concrete rocking frames. “The goal is to make buildings more flexible for future incarnations,” he says.

Determining the balance between durability and efficiency requires the design team to weigh a myriad of factors, including the lifespan of the structure and potential future uses. One aspect to consider is the general building type. Lance Hosey divides buildings into two general categories: figural and fabric buildings. “Fabric buildings make up the consistent aesthetic of the city, such as housing and commercial. For fabric buildings we should be creating structures that are as flexible as possible,” Hosey says. “Figural buildings, such as churches, museums, and civic buildings, tend to keep their function. The Louvre, for example, has been a museum for half a millennium. We can push the [efficiency] envelope in how we shape the structure for buildings that are likely to keep a specific function,” he adds.

System and Material Selection

There are many options for structural system types, but for most large, non-residential buildings the choice often comes down to two: concrete or steel (with many hybrids). If you read much about which material is more sustainable, it's like the two major political parties comparing budget figures—the results can vary wildly depending on who's crunching the numbers. “Every material has its functional sweet spot where it performs optimally,” says Mar. “When you're out of that sweet spot, you're spending more money [and energy] getting that material to work in a way it doesn't naturally. For example, if you have a dynamic facade with an undulating edge, concrete essentially does that for free—you just change the shape of the mold.” A steel frame is less practical for curved forms, but for complex grids or column spans, steel would likely perform better.

When Mar's firm took over the 13-story San Francisco Public Utilities Commission project, it was stalled over budget concerns. The original scheme called for a steel frame, but the engineers reworked the structure using a concrete frame. The primary concern at the time was financial, but the switch also had some green aspects. As with Morphosis's Federal Building, the exposed concrete could be used for thermal mass. Other savings were found in reducing the floor-to-floor height by about one foot per floor.

The Brower Center’s “self-healing” structure is made up of a pair of horizontally post-tensioned moment frames and a vertically post-tensioned circulation core. This system was devised to allow the building to move and flex in an earthquake without permanent deformation. Tipping Mar (right); Timothy Griffith (bottom)

Designing for Resilience

According to Mar, building codes have evolved to protect life, but not necessarily the future viability of the building. “Our approach is to look at the big environmental hazards in an area. On the West Coast we have seismicity—the same thinking could be applied elsewhere to temperature extremes or to storm resistance,” he relates. “Often the building is designed as a throw away—like a car crash that you walk away from, but the car is totaled,” he says. There are currently no seismic guidelines in the traditional approaches to sustainability, and in Mar's view, “It would be inconsistent with the larger green goals to not make a resilient structure.”

Tipping Mar has developed concrete building cores that enable buildings to flex and dissipate energy during seismic events. Vertical post-tension cables allow the cores to yield and then realign to their original position. For David Brower Center in Berkeley, California, the cores are used in combination with two large moment frames at the perimeter, which keep the building free of excessive structure that might inhibit future adaptability.

The Rise of Life-Cycle Assessment

Under the current LEED point system, the structural engineer's influence is often limited to specifying local sourcing and the recycled content of materials. The draft of LEED 2012 proposes a new category for Life-Cycle Assessment (LCA). LCA has become a key way of measuring environmental impact of everything from a bottle of beer to entire cities. In a nutshell, LCA estimates an array of variables such as water use, toxicity, and greenhouse-gas emissions. The typical “cradle-to-grave” model includes the cycle from the material extraction, to manufacture, to use, to disposal or recycling. For structural engineers, LCA can be used for specifying individual materials, such as what type of rebar to use, or looking at the impact of whole building systems.

A key factor in LCA is embodied energy, the amount of energy invested in a material over the course of its life. Also important is embodied carbon, the quantity of CO2 generated by the material over its life span. “As buildings become more efficient in operational energy, embodied energy is going to be a larger share of the total carbon footprint of a building,” says Wolfgang Werner. At the 2011 Greenbuild Expo in Toronto, Werner and Rob Otani presented a software design tool that Thornton Tomasetti is developing to tabulate embodied energy and carbon in structures. The program allows the user to quickly manipulate and compare materials, bay sizes, and other building parameters, while getting feedback on the environmental impacts of each.

Arup structures and materials sustainability specialist, Frances Yang, says embodied carbon is a better metric for addressing climate change. “The amount of energy embodied in a material would be the same whether produced with fossil fuels or renewable energy,” Yang says. “However, embodied carbon would reflect the lower amount of carbon emitted from a material made with 'cleaner' renewable energy.”

Of course the design team has to consider the reality of looking at first cost versus life-cycle costs. Mar starts by looking at how green he can get with a market-rate project. “Ultimately what designers need to consider is the carbon impact per dollar invested as a way to be able to see the most promising strategies for a particular building and find overall optimal solution,” he says.

Design for Deconstruction

Structural engineers have an important hand in detailing sustainable elements– from the reduction of thermal bridging to the structural support of elements such as building integrated wind turbines. And the detailing of structural connections has a crucial impact on whether building parts are reused, recycled, or enter the waste stream. Now that LEED and the building sector are considering the life cycle of buildings and materials, it's perhaps time to look at the end-of-life scenario more closely.

Construction waste constitutes almost a third of the entire waste stream in the United States. The EPA estimates that 92 percent of construction waste comes from demolitions and renovations. In Stewart Brand's model of shearing layers, the structure should be independent of the other layers to facilitate deconstruction or adaptability. Unfortunately, common building practices attach components in ways that are arduous, if not impossible, to deconstruct for reuse.

“One of the things that needs to change is how we see building value,” says Mark Webster of Simpson Gumpertz & Heger. “Buildings that are adaptable and deconstructable should have higher value, since maintaining and altering them will be simpler and cheaper. There will be inherent value in the materials that can eventually be tapped,” Webster adds.

One of the most difficult systems to disassemble is also the most common. “It's hard to imagine how you would deconstruct a cast-in-place concrete structure,” Webster says, whereas a steel or wood frame comes apart readily if it has bolted connections. Precast concrete systems can be detailed for deconstruction fairly easily, according to Webster, but unfortunately, most precast concrete buildings are designed for welded connections and other details that make them difficult to take apart. He is currently working on a proposal that would test a system that uses precast planks on top of a steel frame with connections that would facilitate disassembly and reuse. Concrete also has a relatively low end-of-life value for recycling, so utilizing reusable precast components helps it maintain value. For the Chartwell School (2009) in Seaside, California, architects at EHDD and Tipping Mar engineers targeted adaptability and deconstructability as important themes in the overall design scheme. They developed a system of large interlocking precast concrete pavers in lieu of exterior monolithic slabs. The elements can be moved to access utilities and eventually be reused elsewhere. Components such as partition walls between classrooms were designed to be non-structural to allow for future reconfiguration. The timber-frame structure is made so that the pieces bolt together to facilitate disassembly. The exposed interior frame is accessible, making it much easier to modify for future changes in loading conditions, and to eventually dismantle.

It's clear that there is no single structural solution that will fit every project. A system that deconstructs well may have a higher embodied energy. Another system may not have good thermal properties for a particular building. That's why the leaders in sustainable structural engineering advocate for involving well-informed engineers from the beginning of a project in a more integrated approach to green design.

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Learning Objectives

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Have a basic understanding of the evolving role of structural engineering in sustainable design. Discuss the pros and cons of optimizing structural efficiency versus building for adaptability and resilience. Understand the fundamentals of life-cycle assessment, embodied energy and embodied carbon in structural systems. Address the relevance of design for deconstruction and how structural engineering can impact it.

This course was approved by the GBCI for 1 GBCI credit hour(s) for LEED Credential Maintenance.

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