Whenever you step on a bridge, every bit of your weight is being transferred – part to one shore, part to the other – down to the bedrock below. If the structure is to continue standing when it takes on a new load, every link in the system has to resist the burden, kilo for kilo. Your weight, technically a ‘live load’ (which is to say transient, as opposed to the resident ‘dead load’ of the structure itself), must be met by the strength designed into the bridge, whether it is a simple beam or arch, one of the myriad types of trusses, or a suspension bridge, either the traditional catenary or the more recent cable-stayed. Most often the bridge will succeed in bearing your weight. It’s rare that someone is individually responsible for a collapse.

LRB 21 February 2013 | PDF Download