Railroad bridges are different than other kinds of bridges because they are subjected to unique dynamic forces. An interesting historical fact is that even today railroad bridges are designed using the Coopers loading system, a system developed in 1894 and based on the loading of two consolidation-type steam locomotives.
The Northwest Railway Museum has a collection of bridges representative of those that transformed the west. Timber trestles, open and ballasted decks, a pin connected truss and even a voided-slab concrete span are all critical structures on the Museum's railroad. All these structures are inspected annually by an independent railroad bridge inspector, and periodically by Museum staff.
During bridge inspections components are inspected and rated. Elements that remain effective but show any signs of deterioration are placed on a watch list. On each subsequent inspection, the "watch" parts can be closely examined and an informed decision can be made about when to replace a part.
To change a cap, the stringers and
bridge deck are jacked and supported
from adjacent pile caps. This removes
the weight from the affected cap and
allows it to be changed.
With the weight off the cap, the old one is removed and a new one is inserted.
Shims are inserted to make up any difference in height between the old and
new cap. Then the weight is placed back on the cap and everything is bolted