How do engineers calculate a reto fitted weight limit for old bridges?

Question

When driving around the country side you may drive over an old stone bridge with a weight limit. How did they come to that limit when the bridge could be 150-300 years old? They cant just keep putting more and more weight on it until in nearly collapses! LOL. Thanks

Answer 1

Based on a few factors, the age of the bridge, material of construction, existing loading, non-destructive testing of high stress points, shape/type of bridge...

They can calculate an average stress and thus give a rating for the bridge. That calculated value is divided by a safety factor, which is usually 2. So essentially the bridge you are driving over is at least twice as strong as the sign will state.

Answer 2

Bridges are regularly inspected for safety. Much of our modern civil infrastructure (Interstate) is approaching 50 years old and at or over its design life. The ASCE reports that two out of every three bridges are in need of repair or replacement. This is a serious predicament, but doesn't get the attention it deserves. Perhaps when more and more bridges get downgraded or removed from service the issue will be taken seriously.

For real old bridges, engineers analyze the structure like they would any other. They take into account the age of condition of the materials and adjust the allowable stresses as necessary.

Answer 3

Load bearing structures have to be regularly surveyed, thus they can be maintained at their design load, until serious faults are observed, the bridge can then be fixed or downgraded.
Many of the older smaller bridges are made of stone and brick, which have high compressive strength, and would probably take ten times the limit designed for, over engineered maybe, but that is why they are still there.
Modern bridges are designed very carefully (too carefully) to make the design efficient with minimum cost, they are given a design factor, several times the working load, eg, a 10 tonne bridge might be designed to a load of 40 tonnes (don't quote me on safety factor figures)

Answer 4

Load Limits for bridges are calculated based on what the bridge is actually constructed of. There are very specific equations and bridge theory used to calculate the load capacities of bridges.

Steel bridges are different than concrete bridges and so on. Each type of bridge has its own set of equations and theoretical basis. There are two basic types of limits:

These are Operating Rating and the Inventory Rating. Ideally you want both of these ratings to exceed the design criteria by which the bridge was designed in the first place.

Over time, the ratings for various bridge may decrease based on deterioration of the abutments, foundations, floods, deck delamination and deterioration due to many years of freeze/thaw cycles and the application of salt to allow vehicles safe passage, and general deterioration due to contact with the elements.

Answer 5

the standard engineering practice is to calculate it and then double it just to make sure

tensile strengths of materials , etc.