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In architecture and engineering, design load is the maximum amount of stress that can be put on a structure. Weight is one of the key factors that has to be considered to calculate, but it is not the only one. In addition to weight, design load calculations also have to consider other sorts of forces that will face the structure. For example, bridges and tall buildings need to be able to stand up to strong winds while also supporting their own weight and the weight of people and objects that are on them or within them.
Proper calculation of design load are crucial to making sure that a structure is safe. Without calculating design load, structures, or even sections of structures, can become unstable. Instability can lead to collapse, which can be fatal. There have been many tragic instances in which design load was not properly calculated, which led to severe injury and even death of people within the structure.
Design load is also used to define the maximum occupancy of spaces. This is especially important in large spaces where people gather for events. For example, a dance hall or a concert venue will have a maximum occupancy. This limit has not only to do with safety issues, such as evacuation in the event of a fire or other disaster, but also the amount of weight and stress that the space can accommodate, especially if the space is elevated above another space such as a basement or lower floors.
One of the most common maximum occupancy limits that people experience on a daily basis are the ones posted in elevators. If too much weight is loaded into an elevator, exceeding the maximum design load, the elevator might not be able to function properly. Even if the elevator were to successfully carry more than its intended maximum design load, the stress put on the mechanical elements could damage the machine in a way that could lead to problems, potentially serious problems, later on.
The kinds of stress that are calculated for design load are both external forces, such as wind as described above, and forces occurring within the structure. For example, a balcony at a sports arena will have to be able to support not only a full house of fans, but also the stress applied if those fans begin to jump up and down during exciting moments in the game.
If you ever get a chance to watch a documentary on the design of the Burj Khalifa, which is currently the tallest building in the world, you should take it.
The design is incredible. Especially the wind load design. Because that is the main difficulty with very tall buildings.
It's one of the reasons the Burj gets thinner as it gets taller.
At its tallest point, the building actually sways 1.5 meters from side to side. When you consider what that would feel like from the inside, its pretty astonishing.
There have been some real tragedies where the design load wasn't taken into account.
One that springs to my mind was the collapse of a viewing platform in a tourist area a few years ago, called the Cave Creek disaster. A bunch of college students had crowded onto it in order to look at the view and the platform hadn't been built to proper specifications.
In fact it hadn't really been designed by professionals at all, let alone with a live load in mind. It wasn't capable of holding so many people.
It collapsed and fourteen people were killed.
The only good thing that came out of it was that there was a huge inquiry and the government was forced to reevaluate its policies on those kinds of structures.
I remember reading a novel when I was younger, where an inventor from some ancient time was trying to make a catapult.
He knew all the mathematics and designed it carefully. It was going to be the most powerful catapult ever built and would make his king invincible.
Finally the day came for the machine to be tested. He gave the order for the ropes to be pulled tight... and the whole thing broke into pieces.
Why? Because it was too strong.
It was only made from wood, and the building material wasn't strong enough to take the tension needed for it to work.
I thought that was a really clever way of showing a load design fault.