The Pont Alexandre III Bridge in Paris crosses the Seine River and was built from about 1896-1900. It was designed for the Universal Exposition, which was to take place in 1900 in Paris. Many other structures were built for the same purpose, some of which are still in use today. The bridge's first stone was laid by the Russian Tzar Nicholas II to symbolize French-Russian friendship and cooperation. The bridge was named after his father, Alexander III.One of the design constraints was that the bridge could not obstruct the view of the Invalides or the Champs-Elysees. This meant the bridge had to be built very low. However, it also had to allow barges and ferries to pass down the Seine underneath. The bridge had to be built nearly flat with an arched bottom to allow the boats to pass freely and for the view to be unobstructed. Some other design constraints were probably that the bridge had to be able to support the hundreds of thousands of people that would be visiting the city and using the bridge to cross. The Pont Alexandre III connected two main attractions of the event, so it had to be built to withstand heavy foot traffic. Another design constraint was probably wind and flooding. If the river flooded, the bridge had to be able to withstand strong water flow. In addition, the bridge would have had to stay strong against winds. 
One thing I find fascinating about this bridge is that the four main pillars with gilded statues are an integral part of the construction. Not only do they provide aesthetic appeal, but they are necessary counterweights to balance the low bridge. I did some research on Paris and found that the city gets a lot of rain during the year, and cold temperatures in the winter, causing snow and ice. The bridge would have had to withstand the various forces of weather in their extremes. Though generally the temperature fluctuation is not major, the temperatures in Paris have risen up to 104 degrees F in the summer and as low as -11 degrees F in the winter. 
The bridge is still in use today, so it was clearly well built and well maintained for the past century. 

     In the documentary To Err is Human, the various structural fails that a bridge or building can go through were explored. The speaker talked about how failure is what causes success, and how failure can enable future structures to be built differently so as to prevent another catastrophe. He also explained that failures are what leads to success. Safety was a concept very thoroughly explored, and the various testing stages and design steps engineers take to build structures that are as close to guaranteed as possible. 
     For example, all structural fails are due to a design flaw of some sort, but that design flaw may be brought to light by nature, time, or stress. They may also be brought about by maintenance (or lack thereof). One type of design flaw occurring on the Tacoma Narrow Bridge was that engineers were taken up in the trends of bridge building, and failed to notice that the Tacoma Narrow Bridge was far too narrow and slender for an area that experienced high winds. The bridge eventually collapsed from the winds, leading engineers to test all bridge models in high speed wind tunnels for stability. Bridges especially are vulnerable to design flaws, like progressive cracking that goes unnoticed and failure from torsion (twist). After each fail, engineers add to the criteria for new structures to be deemed safe. 
     Stresses and materials are important in structural failure because of variability. Different materials respond differently to different loads and stresses. These materials must be tested in model size in relation to the stress to be applied, allowing engineers to determine which materials are best for different structures, loads, and stresses. 
     Today, engineers test new structures for fail without putting human lives at risk by using advanced technology and models. A small version of the structure is created and put through a series of rigorous tests, including stress, wind tunnel, load, and more. The materials are also tested for torsion, load, etc. Tests are administered until the model fails, at which time the flaw is fixed and the tests start over. This process continues infinitely until the engineers can think of nothing else to test the bridge on. Using these methods, engineers have created many bridges that have yet to fail.