One thing the industry is having to learn about is “mass timber.” For those not familiar, this usually refers to cross-grain glued wood, similar to plywood. However, mass timber buildings use engineered wood in all shapes and sizes; and very large scale. For example, Perkins + Will’s River Beech Tower concept, to be built in Chicago, would be an 80 story high-rise made from wood.
We’re more used to concrete, glass and steel. So what hazards will we discover when wood construction appears on this scale, as it surely will? Just look at any of the lists of projects that employ mass timber at scale, or plan to employ it.
One obvious concern is fire. Concrete is not flammable, and steel bar typically does not ignite in atmosphere unless it is over 2000 degrees Fahrenheit. Wood, on the other hand, ignites at a few hundred degrees; it will catch fire in an oven on your iron or steel tray. Steel and concrete can survive structurally after being exposed to moderately hot fires. Whether wood and adhesive can do so, remains to be tested.
So codes have to be overcome, for one thing. The River Beech project has to show building authorities that the engineered wood they use is “non-combustible.” Will it be non-combustible under all conditions for which concrete, glass and steel are non-combustible?
I’m going to hazard a guess that when a laminate material is drilled or sawed and left in dust form, or impregnated with fungus, resurfaced, or locally crushed under repeated stresses, that its flammability properties will change. It seems reasonable. Large timbers are very hard to ignite, while sawdust can explode in the presence of a spark. The present state of the material matters.
One mishap that I recall was actually in concrete and steel construction. In Pinellas County, Florida, a number of builders made concrete-and-rebar buildings, standard stuff. But the salty Gulf Coast of Florida was not the environment their process was tested for, and that made a difference. The rebar oxidized over time, and expanded, and fractured the concrete. Repairing this damage was a mess, and for insurers it was a liability mess.
But wood does not rust, so why worry? In a way, rust wasn’t the issue for those coastal condos, so much as differing rates of expansion. Whenever you have two materials that expand at different rates under certain conditions, you end up with damage. Often, the damage is to the material that expands less. So, if the moisture barrier of this engineered wood is breached somehow, will the expansion of wood particles lead to damage to the adhesives used to hold them together? Will they de-laminate, or simply disintegrate?
Since I am a Florida native, no discussion of wood could be complete without mentioning mold and invertebrates. Things that actually eat the buildings here. Perhaps the buildings in Illinois are less susceptible, but humor me.
Mold finds the smallest cracks and pores through which to infiltrate, and then it grows. Typically this starts when moisture is poorly controlled in some part of a building. But once it gets going, mold damages the structure and the inhabitants. Wood, even laminated wood, is more porous than steel. And we can presume it is more conducive to mold growth than is concrete. As mass timber takes off, expect mold to become a concern, especially if you do business in a warm, humid area.
Finally, one might question my choice of the word “invertebrates.” Why not just say “termites” or “insects.” Remember, I said I am from Florida? We currently have invasive giant snails eating the stucco off of our walls, and the synthetic rubber off of our tires.
Snails, slugs, and insects are remarkably adaptable. In laboratory conditions, we often believe we have their measure. But anyone who has dug up a creosote post, or a pressure treated plank that has been dampened a few times in the right (or wrong) conditions, knows that nothing is perfect. Once conditions are right, the little holes appear, and the next thing you know, you’re feeding a colony of something with too many legs. Or in some cases, no legs. Concrete and steel are not perfect either. However, digestibility is not among their faults as construction materials.
Don’t let me discourage anyone from these new materials. Just think about where the risks are likely to come from. For those intrepid few who insure the first very-tall mass timber structures, good luck on your adventure.
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