“Rust
jacking” is a problem that occurs when steel corrodes and causes severe
expansion, up to 10 times greater than the volume of the steel, in this example
resulting in cracking in the masonry above a lintel.
Steel is one of the most widely used of all building materials. The list is as long as it is varied; columns and beams, stud framing, concrete reinforcement, doors, door frames, ladders, sheet metal, lockers, bolts, nails, and on and on. It would be impossible to build a building without using some steel. All building materials have good and bad qualities with regard to the environment. Steel gets gold stars for its high percentage of recycled content, high recyclability value, and domestic production. On the downside, steel is highly energy intensive to make, has poor thermal conductivity properties, is heavy and commonly transported great distances, and corrodes easily. Steel’s good and bad environmental impacts depend on a number of project-specific factors, as with every other building material.
Rust
stains due to corrosion of steel reinforcement.
Steel is so high in recycled content that it often earns automatic green building rating system points for many commercial building projects. In the LEED rating system, three points are available for recycled content. Materials with any percentage of recycled content contribute, by cost, toward established minimum percentage thresholds for the available points. Building materials allowed in the calculation include everything except mechanical, electrical, plumbing and equipment. Because there is so much recycled content in just about every steel building material, LEED has established an allowable minimum of 25 percent post-consumer recycled content to be used for any steel material. Most steel products used in buildings have a much higher recycled content than this, and can be ascertained by asking individual steel products manufacturers.
To illustrate just how much steel contributes to the available LEED recycled content materials points, I provide an example here using cost estimate data for an office building I am currently working on. The cost for all materials for the project (less MEP and equipment) comes in at $11.5 million. As the table shows, the recycled content value of steel alone earns the project one point using the 25 percent default value allowed by LEED. Using a large, national steel manufacturer’s published recycled content data for just four steel items-structural steel framing, concrete reinforcement, stud framing, and miscellaneous steel fabrications-the project easily earns all three available points (two points plus the third Innovation in Design point for exemplary performance).
RECYCLABILITY
Another huge environmental benefit of steel is its high recyclability. According to the Steel Recycling Institute, steel has achieved a more than 60 percent recycling rate since 1970. A 2004 paper by the U.S. Dept of the Interior reports that more than 1,500 scrap yards across the United States process steel from construction and demolition sites by shearing, shredding and baling. The Institute of Scrap Recycling Industries claims that 2 out of every 3 pounds of steel made in the country is manufactured using ferrous scrap and that 81.6 million metric tons are recycled annually, far more than any other material listed (followed by paper at 50 million, aluminum at 5 million, stainless steel at 2 million, and copper at 1.8 million). One reason that steel is so highly recycled is because it is magnetic and therefore easy to separate from the solid waste stream. It comes close to being the perfect recyclable material.
“Look
ma, no rust!” Galvanized coatings on steel framing and fasteners provide
excellent corrosion-resistance.
A positive thing about steel in contact with moisture is that it will not contribute to the growth of mold. It will corrode (rust) though, which is arguably one of steel’s biggest Achilles’ heels. The risk of corrosion is ever present when steel is exposed to moisture, which can occur during construction, when buildings leak and when condensation forms. There are numerous examples of buildings that have fallen victim to the ravages of rust. Exterior brick walls bowing and spalling due to “rust jacking” of steel support angles (Picture 1), staining of exterior building materials (Picture 2), and rust bleeding through paint are common problems.
Coating steel with corrosion-resistant materials helps steel remain rust free. Applying molten zinc (also known as hot-dipped galvanizing) provides excellent corrosion protection for steel in most environments. Steel studs are routinely specified with a zinc coating (Picture 3). Steel columns and beams exposed to weather are commonly coated with corrosion-resistant zinc-rich primers. The thickness of the coatings and the type of exposure both play roles in how corrosion-resistant the steel will ultimately be and how long this protection will last.
Perhaps the best way to guard against moisture related problems with steel is to use stainless steel, which is highly resistant to corrosion.
Increasing the corrosion resistance of steel adds considerably to steel’s environmental impact and this should be taken into consideration during evaluation. Using galvanized steel lintels in a masonry veneer assembly may be the better choice between plain steel and stainless steel when weighing cost, durability and environmental impact.
STEEL PRODUCTION
Steel is a very energy intensive material to produce. In a paper about recycling iron and steel, the U.S. Dept of Interior estimates that the steel industry uses 3 percent of all the energy consumed in the United States.
The amount of steel production has fluctuated in the United States over the years and the industry has had to compete with imported steel building materials as of late. Using foreign made steel has a negative impact on a project’s sustainability due to the increased transportation impacts and unknown recycled content of these products.
Steel is an excellent thermal conductor, which is great when used to heat a home in the form of a wood burning stove but not so great when used as an exterior structural material. The same mechanism that sends heat from the wood stove into the home conducts that same heat across studs and supports in an exterior wall. This is known as thermal bridging. Steel is such a good conductor of heat that exterior steel framed stud walls without a thermal break can cause a reduction of the wall’s overall R-value by more than 50 percent. Energy codes have been slow to address this issue but that is changing.
The city of Seattle has incorporated prescriptive requirements for a thermal break when using steel studs in exterior wall assemblies. For commercial buildings in Seattle, exterior steel stud walls are now required to have an uninterrupted thermal break of at least R-7 between the stud wall sheathing and the exterior cladding. Without this thermal break, a 6-inch steel-stud-framed wall packed with fiberglass insulation ends up testing out to a mere R-7. With continuous R-7 thermal break, that same wall achieves R-22 and eliminates the possibility that condensation will occur within the cavity space. Dow Corning, a manufacturer of insulation commonly used to achieve this thermal break, provides an excellent online Steel Stud Wall Condensation Analysis tool that can determine the condensation potential for exterior steel stud wall assemblies. Another resource is the Steel Stud Manufacturer’s Association, which can found online atwww.ssma.com.
CONCLUSION
Responsible use of steel in buildings can be a good sustainable design strategy. Steel’s inherent high recycled content, high recyclability, domestic production, and durability get high marks. Ensuring that appropriate corrosion resistance is added to steel materials and that thermal conductivity issues are addressed are musts for a building’s measure of sustainability and adds the “responsible” part of “responsible use.” W&C
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