Feats and Fails of Engineering, Volume I

07/11/2017
 
A bridge over 57 north of Champaign is being rebuilt. The process for this construction is worth noting- the highway below still needs to be operable during the process, so the bridge is constructed piecemeal. The diagram shows an example of different bridge sections. While one side is being added, traffic can still drive through beneath the other side.  
 
 
In the America’s Cup races held in June, each team’s catamaran had hydrofoils that allowed the boats to sail virtually out of the water. The hydrofoils are long, flat fins that curve into the water beneath the hull. With enough speed, the foil acts like a wing and creates lift in the water, pushing the sailboat up so that it balances entirely on the foils.  While this does not change the weight of the boat, the decreased surface area has less drag.
 
In a similar spirit of racing, Indy 500 cars have a compartment designed to protect the driver in any sort of crash (for more, read Start your Engines). In the event of a crash, the car breaks around the compartment so that the driver is protected. This is similar to the crush factor used in modern street cars. The compartment itself is also designed to withstand immense amounts of impact.  
 
The cars have other safety features in place as well, such as bulges in the underbelly of the car that help prevent it from flipping over during a spin. Called dome skids, the bulges change the air pressure as the car spins, resulting in an applied downward force that increases as the car rotates to 90 degrees from center. For the safety of other drivers, parts such as the wheels, winglets and nose cone are tethered to the car so that they cannot fly off easily.
 
Many designs are purposely overdone in order to guarantee, for example, a high factor of safety or consistent performance regardless of conditions. A couple weeks ago I took apart a fence that was overdesigned for strength. Each fence section was roughly four feet tall and eight feet long. Each fence post joined the ends of two sections, but instead of making a seam over the post with the end of each joining section covering half, the two sections were attached on either side. This meant that nails had to be inserted at an angle instead of perpendicular. 
 
The fence had five 3” nails per end of each section, three for the top horizontal board and two for the bottom. This meant ten screws per post. The fence had more than ten sections, and each vertical board was also screwed in place with four nails apiece. This type of reinforcement would be justified in an environment that experiences more extreme conditions, such as gale winds, falling debris, or heavy snow. However, in the northern Illinois woods where it was built, the added factor of safety was not necessary and instead proved to be an inefficient use of resources.