The U.S. Green Building Council's (USGBC) LEED program rates buildings in a number of environmental categories, awarding points for energy and water conservation; use of sustainable resources; and building occupant comfort. But the incorporation of structural efficiency, which can help owners and developers use fewer permanent materials and lower building costs is not recognized directly.
LEED and other sustainable building programs tend to overlook the importance of structural efficiency, focusing instead on things such as jobsite recycling and the use of low-emitting building materials to reduce the finished structure's overall carbon footprint. Although these are important elements of green design, they do not address the savings potential that can be realized much earlier in the process: at the design level.
A committee of the Post-Tensioning Institute (PTI), Farmington Hills, Mich., has been formed to help bring awareness to the importance of structural design by specifically highlighting the sustainable attributes of post-tensioning systems for buildings. The use of post-tensioning in floor slabs allows for reductions in building volumes and material quantities; can help create a shorter construction schedule; typically offers a smaller carbon footprint; and can potentially extend the overall service life of a building.
Named the PTI DC-100 Sustainability Committee, the group hopes to prove to the USGBC that post-tensioning produces savings over its rebar counterpart in both the concrete and reinforcing categories, and encourage the council to award LEED points for its use.
A focus on how a building's structure is designed long before flooring or dual-flush toilets are considered sets the tone for how the building ultimately is constructed. Consideration of an efficient concrete formwork system during the conceptual and schematic phases, for example, can mean the difference between speedy core construction and a concrete pour that requires considerable redesign around inconsistently placed columns. Similarly, the use of post-tensioned concrete can create a starting point for efficient construction, resulting in significant monetary and time savings throughout the entire design and construction process.
The PTI's sustainability committee was formed by consensus at the May 2009 PTI Technical Advisory Board meeting. Its 12 members include structural engineers, educators, suppliers, and an architect. The committee is chaired by Martin Maingot, PE, SE, an associate with structural engineering firm Cary Kopczynski & Co. in Bellevue, Wash.
Currently, structural efficiency as such is not directly recognized by LEED, although a point can be earned in the "innovation and design" category if it can be proven that the use of a particular material or design approach can reduce permanent building materials. Since its inception, LEED has placed very little emphasis on design efficiencies. It grew out of energy program initiatives with an early composition of architects, realtors, a building owner, environmentalists, industry representatives, and a lawyer. Structural engineers were not included, so LEED has focused mainly on contributions from the architecture and mechanical design communities.
PT's contribution to efficiency
The importance of structural efficiency boils down to cost and schedule, which, for most projects these days, is critical to their survival.
"Efficiency is two-fold," says Maingot. "On one side, we have building materials permanent materials such as concrete, steel, wood, and masonry while on the other, we have methods used to assemble them into a structure, such as formwork, construction sequencing, and detailing. In other words, efficiency involves not only what to build, but how to build it."
The post-tensioning method addresses both sides of the efficiency coin. Its use can significantly reduce the amount of rebar and concrete in a project, and it is highly likely that the construction schedule will be shortened as well. Seasoned post-tensioning crews can pour a post-tensioned floor in one or two days, compared to three or four days required for their rebar-only counterparts. In addition, PT allows for greater freedom in building layout by incorporating larger spans; controlling long-term deflection; allowing for the use of shallow framing members and lighter foundations; reducing floor-to-floor height; and decreasing seismic demand due to lower seismic mass; most of which positively affects architectural and mechanical systems by enhancing architectural expression opportunities. Exterior skin quantities are reduced, as there is less overall building volume to heat and cool.
The sustainability committee's first step will be to generate an in-depth report that details how sound structural design, specifically through the use of the post-tensioning method, contributes to materials efficiency and time savings. A side-by-side study of PT's savings over conventional rebar is planned, comparing the use of each on different building types. The report will be the first step in addressing the contributions of PT to green building design. It will attempt to lay the groundwork for quantifying such considerations in the major green building rating systems used worldwide, such as LEED, Green Globes, and BREEAM. The intent is to provide a convincing argument, develop a strategy for measurement, and extract a point or credit that is specific to the use of the post-tensioning method.
Once the report is released, the committee members plan to present it at conferences, seminars, trade shows, symposiums, conventions, and other industry events. With that, the team will begin the long process of reaching out and working with the different green building rating system organizations to develop specific processes that best fit their requirements.
"The committee's first target is the LEED rating system," says Maingot, "as it is the de facto authority in green building design in the U.S."
Exactly how post-tensioning's use and design efficiencies will be incorporated into LEED has not yet been determined, says Maingot. It is expected that proposed methodologies to quantify efficiencies will develop as the report makes its way through the revision process.
Sheila Bacon Cain is a Seattle-based freelance writer covering the architecture, engineering, and construction industries.