The bridges: condition, replacement and the engineering backlog

Bridges are the least discussed part of a road network and the part that fails most consequentially, because a closed bridge does not slow traffic down; it removes a route entirely.

The inherited problem

A large number of structures built in the same period, to the same standards, with a similar design life. Structures built together reach the end of their lives together, which converts a maintenance question into a replacement programme.

They were also designed for the vehicle weights of their time. Modern freight lorries are substantially heavier, and a bridge that is structurally sound for its design load may not be sound for what actually crosses it.

What an inventory requires

Every structure surveyed, rated by condition and load capacity, and entered in a register that determines inspection frequency and restriction.

That register is the whole of bridge management. Without it, decisions are made after a failure rather than before one.

The wartime layer

A number of crossings were destroyed, and temporary structures — modular military bridging, pontoons, single-lane bypasses — were installed to restore movement. Those are exactly that: temporary, with lower capacity and higher maintenance, and each is a permanent replacement waiting to be funded.

The engineering conclusion

Replacement is expensive and predictable, which makes it the kind of problem a state can actually plan for if it holds the data. The failure mode is not lack of money in a given year; it is not knowing which structure is closest to the end.

Bridges are the item on which deferred maintenance carries the highest interest: every postponed year multiplies the cost of the eventual repair. Reading a bridge's tonnage limit is part of my job when moving heavy equipment, and when that limit drops both the route and the cost change. Compiling the inventory is dull work; not compiling it eventually becomes an accident.