Weighing Your Options
Posted: Jul 06 2018
Did you ever try to cross a creek by walking on a fallen log? As it creaked and moaned, you couldn’t help but wonder if you would make it across without falling in.
Or maybe you over-packed your grocery bags only to wonder if the cut-out handles of the thin plastic bags will last long enough to get your eggs inside the house.
We all weigh our options…
Engineers who design bridges are no different; they too have to weigh all the options.
Engineers have to project the traffic patterns ~ not just how many vehicles but what kind, how big, and how much weight each is allowed to carry. They need to know what the bridge is going to be used for. Is it just for people and how many people? Will it carry cars and trucks, a train, or all of it?
Engineers need to collect all the data and weigh all their options in order to know if a bridge is able to perform its job properly. One of a bridge’s main jobs is to carry weight. Weight is an important factor when it comes to traffic on a bridge.
Some bridges are used more than others. The Brooklyn Bridge, for example, is a high traffic bridge with people constantly walking, bicycling, or driving over it. Back in the early 1800s each of its four cables were designed to carry 11,200 tons in order for the bridge to have a load capacity of 18,700 tons.
But something doesn’t add up…
Four cables, each holding 11,700 tons would be 46,800 tons. So why is the bridge’s load capacity only 18,700 tons? What happened to the other 28,100 tons of capacity? The engineers had to consider the weight of the bridge itself, the dead load. Bridges, like buildings or a log across a creek, don’t just have to hold what we put on it (called a live load), has to hold its own weight too.
135 years later, the Brooklyn Bridge is still working great!
When considering weight, some loads bridges carry change while others don’t. These are “dead loads” and “live loads.”
Dead loads on a bridge are materials that don’t change over time like the bridge’s structure or frame ~ cables, piers, and roadways.
Live loads are things like people or vehicles traveling on and off the bridge. Another type of live load factor is environmental stress. This includes snow, rain, and wind. To make things even more challenging, extreme temperatures cause some bridge components to grow and shrink, changing the load capacity.
Can you think of anything else that might affect the weight of a bridge?