QUEBEC BRIDGE collapse

Spanning the St. Lawrence River near Quebec City, the massive Quebec Bridge has a history of triumph and tragedy. Completed in 1917 at a cost of more than $22 million, it is the longest cantilever bridge in the world, stretching more than 1,800 feet between its main piers. 

For years, the bridge has been viewed as an engineering marvel, but few people know the full story behind its construction and the two disasters that claimed the lives of 89 workers.

Why don’t we just see what actually happened and then build on from there? Yeah? Let’s do it!

The Case

The St. Lawrence River was the main channel of trade for Quebec during the summer. During the winter, it filled with ice, and trade was completely cut off until the river iced over and travel was possible again across a dangerous ice bridge. The desire to bridge the St. Lawrence River was fueled by Quebec’s need to be competitive in trade.

In the late 19th century, the transportation needs of Quebec led to proposals for bridging the St. Lawrence River. The Quebec Bridge was the longest cantilever structure attempted until that time. In its final design, the clear span was 548.6 m (1,800 ft) long. The bridge project was financially troubled from the beginning. This caused many setbacks in the design and construction. Construction finally began in October 1900. 

The Bridge Design

Tenders were called they were then reviewed by Theodore Cooper(Consulting engineer) . The specifications called for a cantilever structure. The basic configuration of a cantilever bridge is shown below. 

However, suspension bridge designs were allowed, providing they came with their own set of specifications. Earlier, noted French engineer Gustave Eiffel had considered the problem and found that a cantilever design would be superior to either a suspension or an arch bridge for the Quebec site.

Basic configuration of a cantilever bridge
Drawing of the original design of Quebec Bridge

Cantilever Bridges and
their mechanism of load transfer

  • cantilever bridge is generally made with three spans, of which the outer spans are both anchored down at the shore and cantilever out over the channel to be crossed.
  • The central span rests on the cantilevered arms extending from the outer spans; it carries vertical loads like a simply supported beam or a truss—that is, by tension forces in the lower chords and compression in the upper chords.
  • The cantilevers carry their loads by tension in the upper chords and compression in the lower ones.
  • Inner towers carry those forces by compression to the foundation, and outer towers carry the forces by tension to the far foundations.

How/Why did it collapse?

First Collapse in 1907:

The collapse of the Quebec Bridge resulted from the failure of the lower chords in the anchor arm near the main pier. The failure of these chords was due to their defective design.

A grave error was made in assuming the dead load for the calculations at too low a value and not afterwards revising this assumption. This error was of sufficient magnitude to have required the condemnation of the bridge, even if the details of the lower chords had been of sufficient strength, because, if the bridge had been completed as designed, the actual stresses would have been considerably greater than those permitted by the specifications. This erroneous assumption was made by Mr. Szlapka and accepted by Mr. Cooper, and tended to hasten the disaster.

But to save time and money, extra support wasn’t added. The designers hoped that the problem could be fixed as construction went on, but at the end of the workday on August 29, 1907, parts of the bridge collapsed under their own weight, killing 75 workers.

Their main conclusion was that the bridge’s components just weren’t strong enough.

 

Second Collapse in 1916:

The bridge was rebuilt. This time much bigger and stronger. But, all that extra metal made it much heavier, too. When a casting in the lifting apparatus broke, causing the center suspended span to fall into the water as it was being hoisted into place from a barge. Thirteen workers lost their lives in this accident. 

The 50 MN (5,000 t) span sank to the bottom of the river to rest beside the wreckage of the first bridge, which still remains there today.

Second Bridge

The second bridge was finally completed in 1917 and weighed two and a half times as much as the first one. The construction of the second bridge was very well documented by a Report of the Government Board of Engineers, Canada Department of Railways and Canals.

A lesson from the past

The disaster showed what unquestionable power an engineer could have at the time in a project that was improperly supervised. As one result, Galbraith and others formed around 1925 what are now recognized as  organizations of Professional Engineers. General guidelines include that an engineer must pass an ethical examination, be able to show good character through the use of character witnesses, and have applicable engineering experience .

The Quebec bridges aftermath was a reminder that it is important to make sure that the thing you’re building can carry the weight you’re putting on it and it probably would not have happened today, over the last century engineers have developed tons of new mathematical techniques and computer programs that probably would have caught the first collapse long before the bridge was built.

Happy learning 🙂