Buildings must be designed to withstand live loads, which are fluctuating stresses caused by people, furniture, and environmental factors. Dead loads, on the other hand, are fixed. Engineers must consider potential live loads and design for higher loads to ensure safety.
A live load is a stress on a structure that fluctuates due to changing circumstances. People inside an office building are an example of such a load; they come and go from the building and move inside it during the course of the day. Furniture is another example, as are snow accumulations on roofs and other environmental pressures. Buildings must be designed with potential moving loads in mind, as failure to meet the moving conditions can lead to structural failure.
In many regions, the building code includes specific definitions and clauses for dynamic loads. Designers need to think about the nominal occupancy of a building, including any special use circumstances that might change the loading requirements. For example, factories are more likely to have heavy equipment that could increase active load. Similarly, a rehabilitation center with a capacity of 70 people could hold a number of people in power wheelchairs, with a combined weight greater than that of the average group of individuals.
This is in contrast to a dead load, which is fixed. The weight of the building itself is part of the dead load, as are the window frames functionally fixed in place. Built-in cabinets and counters, for example, could theoretically be removed, but will likely remain in place for an extended period of time. These loads behave more static and are easier to design, since the engineer can calculate the fixed strain they exert and factor this into the design plans.
Active loads drift, which could create some problems. They can change the stress patterns in a facility and can sometimes burden some areas more than others. A classic example is the snow that accumulates on the roofs of buildings. In an area where snow is rare and engineers do not consider the risks, ceiling collapses can occur in abnormal winter weather conditions because the roof is not prepared for the load. Movements of loads within a structure via stairs and elevators can create problems by shifting weight distribution.
The tolerances for engineering must be very tight, for safety reasons. When engineers consider a building’s potential live load, they need to design for loads higher than that, to be safe. This ensures that the sudden movement of a piece of equipment doesn’t hit a floor or that a heavily loaded elevator doesn’t destabilize a building as it plummets down the shaft.
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