Tiny Tuesday: How to Grow a Bridge

New construction is by definition an attack on natural order. Steel, concrete, and engineered wood products all require complex manufacturing processes that extract raw materials, burn fuel to transport and refine them, and waste whatever’s left over. So it’s stunning to find a building method in Cherrapunji, India, that consumes no natural resources at all.

These living bridges are made from tree roots. The rubber tree grows a secondary root system above ground. As described in this treehugger article, builders use hollowed branches from the betel nut tree to train the rubber roots in a specific direction, in this case to cross streams and gullies along a path that’s comfortable and safe for up to 50 users at once. The concept is neither new nor flimsy – some of the bridges are believed to be over 500 years old.

The living bridges take 10 to 15 years to complete, which is about how long most highway bridges take in the US. (No joke – I used to design bridges for a living, and state DOTs are just now beginning to champion a system called Accelerated Bridge Construction in which projects will last “only” 3 to 5 years.) So far they all seem to be pedestrians-only, but there’s no reason they couldn’t support the weight of a car. As a comparison, Boston’s Longfellow Bridge carries both trucks and trains, but closes to pedestrians during fireworks displays because the load is excessive.

It looks like the living bridges require constant maintenance to keep them growing flat, but that’s not too different from highway bridges either. Vibration might be an issue, as cracks could take years to repair (naturally, of course). I suppose the biggest shortcoming is the scarcity of rubber trees outside the ultra-rainy Indian state of Meghalaya.

Is there a “living” counterpart for constructing buildings? I’ll explore one such product next week, one that you can actually get in America.

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