When envisioning a new structure, engineers often have to balance design choices against the environmental impact of materials used. It is estimated that 40 to 50 percent of greenhouse gases are produced by the construction industry, according to the California Integrated Waste Management Board. Lessening the impact of construction on the environment is a work in progress.
Researchers are searching for designs and materials that are less harmful to the environment. The team compared the sustainability of two structural systems commonly used for spans exceeding 250 feet: the steel cable system and the steel truss system.
Thoughtful selection, by the architectural engineer, in the initial stages of the design process, can reduce environmental impact related to the construction process. The findings show that the selection of steel cable structural system for long spans has considerably less environmental impact than a steel truss system to achieve the same structural requirements, through the entire life cycle of the structure.
Like a spider web, the cable system utilizes tension to sustain a structure. In contrast, the truss system uses compression members in the span of the structure. Most structures in nature rely on tension as it is the most efficient way to transfer forces. They typically minimize compression members and maximize tension members. In a tree, for instance, the trunk is the only compression member, and the branches and leaves rely on either a combination of tension and compression, or tension alone, for their support.
For the study, the researchers designed and compared the most efficient configurations for a steel truss and a steel cable structural system. Both systems were intended for the same function and with the same requirements. To analyze the sustainability of each structure, they used the life-cycle-assessment (LCA), which estimates the environmental impact of a structure during material extraction, manufacturing, construction, use, maintenance and end-of-life phases.
The results show that the cable system has 29 % less mass, 65.1 % less embodied energy (the energy required to produce the structural system, including both materials and placement) and generates 67.2 % less CO2 emissions, than the truss system.
The findings imply that understanding how material selection can affect efficiency is one way to mitigate environmental damage from construction. The researchers are continuing to investigate and compare environmental impacts of other structural systems that can serve the same purpose.