It was fascinating to see a recent proposal for the Sears Tower to include all glass ‘look out pods” – a cantilevered glass structure of nothing but glass on five sides including the floor and all supporting structure. No apparent supporting structure – or at least what we would think to be supporting structure, like steel – at all. An architect who interned under me years ago became fascinated with all glass structures and developed this into a market segment, designing things like all glass stairways and all glass trusses – very showy stuff. The way he explained it was simple: glass is a material whose structural qualities are much like concrete – excellent in compression, limited in tension, not great in shear. A major difference between glass and concrete is that one can’t cast an eight inch thick slab of glass in whatever shape with reinforcing bars, the fabrication method is that different.
One only need to look at the glass staircase in the Macintosh computer store on North Michigan Avenue, an all glass stairway whose stair treads are perhaps an inch thick. Imagine a slab of concrete one inch thick, and one probably could envision patio blocks. Not the sort of material that one could use to span an entire stairway width.
The same comparisons are true for wood products. I was invited to attend a seminar held by the Canadian Wood Council and the Forest Products Association of Canada last Thursday. Wood has good compressive and tensile properties. In Chicago, it tends to get overlooked due to this fire we had back in 1871, before the days of building codes and fire-resistivity research. A professor of mine once described it this way – if you were given a choice a sitting under a flaming steel beam or a flaming wood beam, which would you rather? Steel melts when heated, whereas heavy timber develops a protective char, preventing further damage. When used properly, wood can be safer than steel.
While wood may be a piece of tree shaved down to a dimension, modern wood technology is based on taking smaller, perhaps scrap pieces of wood and gluing them together in a way that aligns the direction of wood grain to perform to specifications. Wood itself is composed of directional fiber and cellulose, the cellulose acting as glue. Today’s methods essentially take wood fiber and glue them back together with engineered resins.
The new 2010 Winter Olympics Speedskating Oval in Richmond, BC is the world’s largest all-wood structure. Its graceful, curving roof is made up of many pieces of wood, glued and bolted together. By understanding the importance of structural shape and direction of wood grain, the graceful curves allow impressive spans. A “V” truss shape incorporates fire sprinklers.
One of the more dramatic displays came with a British project, Murray Grove, a nine story apartment building in London, constructed entirely of cross laminated timber panels. Imagine plywood but six inches thick. This system could be constructed to be even taller; it offers many sustainability advantages over steel or concrete systems.
This system met or exceeded all firecodes, provided ease of constructability and negligible construction waste.
Using proper connections, wood construction of this type offers superior seismic resistive cabilities, as entire panels can absord twisting without breaking.
Some people think that wood is like taking a chain saw and killing a tree. Not so. The amount of energy, water and carbon involved in making wood from seedling to reforestation in a properly managed forest is less than what it would take to make a unit of steel or concrete. Trees are most efficient at producing oxygen up to a certain point in their lifespans. A properly managed forest respects this and will target selective trees for harvest while supporting the ecosystem, much like how nature takes care of itself.
Canadian forests consistently rank among the world’s best managed and best documented, managing a small portion of forest while leaving the majority of forests in their natural state. This approach makes wood a highly sustainable building product that is just beginning to be recognized by the “green” industry.