How do you build an earthquake-proof bridge when the geological conditions are too wobbly for conventional supports, and the locals are demanding an architectural wonder to rival the Golden Gate Bridge? That was the unique challenge facing California’s Department of Transportation when it was tasked with building a new span connecting Oakland with Yerba Buena Island. The San Francisco area takes great pride in its skyline, but it’s also notorious for its seismic activity. So this new span had to have brawn as well as beauty. Fortunately, an answer was found, taken straight from the playground.
The simplest way to understand how the San Francisco-Oakland Bay Bridge works is to think of the engineering principles of seesaws and swings. Imagine building a seesaw in a muddy, waterlogged playground. It requires only a small spot of firm ground to build the single central support to carry the weight of the two equal spans. Two children sit at either end of the seesaw, with their feet on the ground. The children can easily keep it balanced by producing relatively small forces with their legs, without having to hold the weight of the seesaw or themselves.
But what if one span is made much longer to avoid a big puddle? The child seated on this long span, now furthest from the pivot point, has to push harder through the mud to keep the seesaw horizontal and balanced. But their slender legs can’t cope with the compression forces. To rebalance the system, the feet of the child on the short span are cast in a concrete block and used as an anchor. This poor child’s legs are now in a state of permanent tension, and likely to snap. Stress is alleviated by pre-stretching a rubber cord by just the right amount and hitching the concrete block to the short span. The seesaw is now balanced, and the legs of both children can go back to dealing with much smaller forces.