Core Values

Once considered supporting players, structural engineers reinforce their role on the green design team.
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From GreenSource
Michael Cockram

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.

Low-Tech Fabric Structures
Going beyond rectilinear constraints

Photo: The Center for Architectural Structures & Technology 

The Center for Architectural Structures and Technology (CAST) has developed this beam as a physical representation of a moment diagram. The volume of a typical beam is indicated in yellow.

Canadian architect and researcher Mark West provides an example of how the science and art of building design can be integrated to produce structurally efficient form. As the director of the Center for Architectural Structures and Technology (CAST) at the University of Manitoba, West is developing a series of structural members formed with geotextile fabric. Fabric forming frees the design of a component from the constraints of rectilinear molds. “The fundamental reason concrete members are rectilinear is that the molding material comes in sticks and sheets,” West says.

CAST has developed a beam that is essentially a 3-D representation of a structural moment diagram (bending moment is usually the predominant stress in a simply supported beam). Wherever the stresses are greater the beam becomes deeper. The result is a beam with a sinuous curvilinear profile. West estimates he can reduce the concrete in a beam by about a third using the fabric-form method with significant savings in the amount of steel used. This method also saves a substantial amount of material and energy that’s typically used to construct formwork. West’s work is intentionally low-tech, in keeping with his three guiding principles: simplicity, accessibility, and sustainability. “We know we’re on the right track if anyone in the world can make these things with a jigsaw and a roll of fabric,” West says.

 

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Originally published in GreenSource
Originally published in January 2012

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