These video clips and stills are from the Bridge Busting "Olympics" which is an annual event at the University of Oregon. Each year the 120 students of the Introduction to Structures class are presented with the problem of building a horizontal load-bearing system. It must weigh less than a candy bar (6 oz.) and carry at least a single 5 pound weight applied at the midspan. The bridge may only be made of cellulose based materials, non-metallic thread and any type of glue. It must be able to freely span 30 inches without ANY part of the structure descending below the plane of the supports.
Each of the six lab sections could choose one bridge to represent their section in the Olympics. Some chose the most efficient and others chose the most interesting design. Trials were then held among the remaining bridges. Each bridge was loaded in the lab until they either failed, or until 64 pounds was placed on the bridge. The best were "preserved" for the final event.
The Olympic Officials weighed in each bridge. This was recorded on the public information display system (in the old days it was known as a blackboard). There were four columns of information: the name of the bridge, its weight, the maximum load that the bridge carried and the ratio of the load/weight. The goal was to design and build the most EFFICIENT bridge. This was not necessarily a 6 ounce bridge that could carry a great load!! Many design teams pre-tested their materials and designs in order to ensure a higher quality of construction.
Each pair of bridge designers placed their bridge on the abutments. The judges attached the point of loading (a meat hook) and the load was applied. The hook was attached to a cord that was in turn connected to a tensometer that allowed the magnitude of the load to be continuously read. The rate of loading was determined by the individual team. . . . . some cranked the ratchet very slowly and others very rapidly. What effect did this have on the ultimate capacity of the bridges?
As the bridges were loaded their behavior could be observed by the gathered crowd. The instructors were able to illustrate basic principles to all.
The stiffness of the systems were very evident as the loads were applied. Some were very stiff and exploded as structural elements snapped when their ultimate strength was achieved; others (as the example on the right) simply flexed and deformed until they flew off of the supports. In many cases the members of the structure were subjected to loads that were unanticipated. The image illustrates the typical behaviour of a VERY flexible system. It seemed to suprise its builders!!!!
This movie illustrates the failure of a stiff system.
Each bridge was loaded to failure. Some were spectacular and others were quite dull. In each and every case, lessons could be learned that apply to a general understanding of structural design and the role of failure in design. The most efficient bridge design for 1995 was produced by Terry Vance and Ben McRae. Their beautifully crafted, shallow truss bridge held a ratio of load/weight of 477! That would be the equivalent of a 150 pound person carrying 71,550 pounds!!!
The role of failure in design is the subject of an excellent book by Henry Petroski, To Engineer is Human: The Role of Failure in Successful Design. (Published by Vintage Books, ISBN 0-679-73416-3). Bridges fail. It should be one way to learn about their behaviour. And learning from failure is a very positive thing! Not learning the lessons from those failures is inexcusable......The Tacoma Narrows Bridge and the Quebec
Bridge are two examples of failures.
Were the lessons
learned from these?
In some ways yes, but each individual must learn....