Materials In Action

Wood, concrete, and steel have an environmental impact on building construction, operation and end of life
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Building materials can be durable but good design and consideration for future use are equally important. A study by FPInnovations1 examined service lives of buildings in Minneapolis/St. Paul. The author investigated building demolition in 227 residential and non-residential buildings. Some 66 percent of non-residential wood structures were over 50 years old, while a similar percentage of concrete buildings were under 50 years old, and nearly 90 percent of steel buildings were under 50. However, the most common reasons for demolition were not related to material degradation, but to changing land values, lack of suitability for current use, and lack of maintenance for non-structural components. The relative ease of expanding and modifying wood-frame structures may have contributed to their longer life.

Because of the unpredictability of future building needs, a design that lends itself to renovation or adaptation can extend a building’s life span and reduce waste.

Wood is particularly versatile and flexible, which makes it an easy construction material for renovations. For example, Ardencraig House in Vancouver, British Columbia, comprises four townhomes designed within the framework of an existing heritage home and garage. Over 90 percent of the wood in the original structure was retained in adapting the house. Salvaged materials from deconstruction of the garage were used to construct a coach house behind the main structure. Salvaged framing members were used to strengthen roof trusses and increase the space available for insulation.

Strength

The strength of a building material refers to its ability to withstand an applied load without failure. Several types of load can be applied—tension, compression, torsion, bending, and shearing.

Steel is one of the strongest materials for tensile strength, the amount of stretching a material can take before breaking or failing. It is also one of the few materials that is equally strong in tension and compression. There are many different steel alloys, but they all have similar stress versus strain ratios. All steel alloys have the same modulus of elasticity, which refers to the material’s stiffness, or the ratio of the material’s allowable stress versus strain. Steel’s modulus of elasticity is 29 million pounds per square inch (psi), compared to concrete’s 5 million and wood’s 2 million. However, every steel alloy represents a different yield strength, which is the highest force a material can take before it deforms. The most common alloy, carbon steel, or ASTM A36, has a yield strength of 36,000 psi; ASTM A441 has a yield strength of 40,000 to 50,000 psi; ASTM A572 has a yield strength of 42,000 to 65,000 psi. Building codes provide an allowable stress between 33 percent and 75 percent of steel alloy’s yield strength. The steel industry is creating new and stronger alloys. Common carbon steel, ASTM A36, for example, is slowly being replaced by ASTM A572 Grade 50, which is 77 percent stronger.

Flexure test machine for testing full size panels for bending movement and stiffness

Photo courtesy of CertiWood

 

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Originally published in Architectural Record
Originally published in November 2013

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