The bridge that was overbuilt

A footbridge designed for a crowd load it has never carried has stood for sixty years with almost no maintenance. The engineer was accused at the time of wasting money.

The accusation was arithmetically reasonable. The structure uses about forty per cent more steel than the loading calculations required, and the extra steel cost roughly a fifth of the budget. On the day it opened, that was a visible overspend with no visible benefit.

What the calculation left out was the cost of the alternative over time. A structure working near its limit deflects more under load, and deflection drives fatigue: the slow accumulation of damage from many small cycles rather than one large one. A bridge at sixty per cent of capacity flexes less, cracks later, and needs inspection less often. Three comparable bridges built to the minimum in the same decade have each been strengthened twice, and the total of those works exceeds the original overspend.

This is the standard argument for margin, and it is not an argument for building everything heavier. Margin has a price, the price is paid immediately, and the benefit arrives distributed across decades and partly as an absence of events. Any decision of that shape loses to a decision with immediate visible benefit whenever the two are compared in a single budget year.

The institutional fix is dull and effective: whole-life costing, in which a project is approved against its expected total cost over a stated design life rather than its construction cost. It does not make the engineer right automatically. It makes the question answerable, which is the most that any accounting rule can do.