As the sole purpose of most bridges is to support traffic, it’s a pretty basic question. So it may come as a surprise that when COWI first looked into the matter, there was no consensus among long span bridge designers as to the loading they should design for, and little or no guidance in the design codes.
As no major bridges are known to have collapsed from overloading, one can assume that the loading used for design must be large enough, despite the wide range of loads used by different designers.
But what happens when a bridge was designed for a much lighter load than current codes require, but it’s been in service for over 40 years with no apparent distress? This was the situation faced by COWI engineers when they were checking the load capacity of Lions’ Gate Bridge in Vancouver, BC.
Was the bridge safe, or was it so close to its limit, that a slightly heavier traffic load than normal could bring it down? The straw that broke the camel’s back as they say.
Stats vs. traffic emulating: Both work
With no known method for calculating traffic loading on long span bridges, COWI set to work with experts in statistics and traffic engineering from the University of British Columbia, to find a method. Not one, but two methods were derived, one based on theoretical statistics, and one that generated random traffic emulating real traffic observed on the bridge. Although very different ways of working out the problem, it was encouraging that both methods produced similar answers!
Averages and single point loads:
What was known was that the longer the length of bridge, the less the average intensity of the traffic load per metre of lane length. The maximum load on a short span bridge would be the heaviest truck to go over it. But a long bridge will never be filled with the heaviest possible vehicles. There will always be empty trucks, cars and buses in the traffic stream that reduce the average load per unit length.
Although average traffic can be calculated in this manner, traffic is not even: there will be clumps of heavy vehicles in the traffic. To simulate this, the design loading was represented in two parts: a uniform load, and a single point load superimposed on the uniform load at whichever location gave the maximum effect on the structure.
Later, this was improved to represent the total load as a uniform load with a single heavy vehicle in the traffic. The heavy vehicle was the same as used for the design of short span bridges.
This simplified loading represents the worst case of hundreds of thousands of combinations of traffic expected to occur during the life of the bridge.
Through this study, COWI proved that the three-lane Lions’ Gate Bridge was able to carry traffic safely – provided there was a weight restriction on heavy vehicles.
Expanding the study:
Unlike the Lions Gate Bridge, most bridges have four or more lanes and no posted limit. So COWI, assisted by a small grant from the Canadian Federal Government, extended the study to include all long span bridges.
COWI also conducted research into what safety factors should be applied, and what the design loading should be on second, third and other lanes. This was particularly useful for the upgrading of the ten-lane West Gate Bridge outside Melbourne, Australia.
The Committee on Loads and Forces on Bridges of the American Society of Civil Engineers (ASCE) endorsed the traffic loading derived by COWI, so it became known as the ASCE loading. The ASCE loading then became the basis for the new American bridge design code AASHTO-LRFD.
After further refinement, the loading that COWI derived was adopted as the basic traffic loading in the Canadian Highway Bridge Design Code, CAN/CSA-S6, when the code was extended to apply to bridge spans of any length (it had formerly only applied to bridges with spans up to 150 m.)
The most recent study applies to the Angus L. Macdonald Bridge that, like the Lions’ Gate Bridge, has three lanes and a load restriction. As part of the extensive bridge renovation project, COWI derived a design loading specific to this particular bridge. It consists of a uniform lane loading with the heaviest truck allowed to use the bridge superimposed on it. In this case, the heaviest truck is a fire truck.
The ability to accurately derive design loading for this specific case has allowed the renovation work to proceed with full confidence, even though the design loading is less than that demanded by the bridge design code.
COWI North America pioneered the concept, performed detailed design, derived the traffic loading to be used for design, analysed the bridge at every step of reconstruction, designed key parts of the contractor’s erection equipment, prepared technical specifications, and is providing technical assistance on site.