A live load is a strain on a structure that fluctuates as a result of changing circumstances. The people inside an office building are an example of such a load; they come and go from the building and move around within it throughout the course of the day. Furniture is another example, as are accumulations of snow on roofs and other environmental pressures. Buildings must be designed with potential live loads in mind, as failure to accommodate shifting conditions can lead to a structural failure.
In many regions, the building code includes specific live load definitions and stipulations. Designers must think about the rated occupancy of a building, including special use circumstances that might change the load requirements. For example, factories are more likely to have heavy equipment that could add to the live load. Likewise, a rehabilitation center with a capacity of 70 people might contain a number of people in power wheelchairs, with a combined weight higher 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 fixtures that are functionally fixed in place. Built in cabinets and counters, for example, could theoretically be removed, but will likely stay in place for an extended period of time. These loads behave more statically and are easier to design for, as the engineer can calculate the fixed strain they exert and account for this in the design plans.
Live loads move around, which has the potential to pose some problems. They can change stress patterns in a structure, and may at times weigh some areas down more than others. A classic example can be seen in the snow that collects on the roof of buildings. In an area where snow is infrequent and engineers don't consider the risks, ceiling collapses can occur in abnormal winter weather, because the roof isn't prepared for the live load. The movements of loads within a structure via stairs and elevators can create problems by shifting the distribution of weight.
Tolerances for engineering need to be very high, for safety reasons. When engineers consider the potential live load of a building, they need to design for loads in excess of that, to be safe. This ensures that the sudden movement of a piece of equipment won't punch through a floor, or that a heavily loaded elevator doesn't destabilize a building as it plunges down the shaft.