Structural loads have different properties that we must know when analyzing structures such as spatial application, duration, nature, rate of application, and certainty.
Live loads and environmental loads may be temporary or short-term. These loads last for a short period. For example, typhoons/hurricanes/cyclones only last for several days - it is rare to encounter these in an extended time frame.
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Generally, we can classify loads as either static or dynamic:
Static loads are loads that do not generate acceleration effects on the structure. Examples would include most dead loads (self-weight, weight of objects, and more).
Dynamic loads generate acceleration effects on the structure. An earthquake is a perfect example. When it strikes, the building will be in motion.
In the case of live loads, it may be static or dynamic, depending on how structural codes treat them. We usually describe live loads as a percentage of something. In this way, we treat the dynamic nature of live loads as static.
Rate of Application
Another factor is how nature applies loads when introduced into the structure.
Generally, there are three types: impulsive, quasi-static, and cyclic.
Impulsive loads are forces that are applied suddenly. A direct collision between a car and a structure is an example.
A load that gradually increases and then stabilizes is called a quasi-static load. It's a force that has some resemblance to static loads. An example of which is a steady wind load hitting a structure. At first, the magnitude of the load is relatively low; as the wind gradually blows stronger, so does its strength. Eventually, the steady wind will reach a maximum point. When that happens, the magnitude also reaches a limit.
Cyclic loads, as the name suggests, are loads repeating in nature. When an earthquake strikes, for instance, it creates cyclic loads. Structures sway left and right due to forces that are equal in magnitude but constantly changing in direction – a property of cyclic loads.
Load Certainty
All loads have a probability of occurring. For example,
What is the chance that n number of people will occupy the building?
What is the chance of a building experiencing major catastrophic events such as earthquakes, typhoons, and floods?
Load probability is an essential factor in load analysis. Some loads have a good chance of occurring. Other loads have an unlikely case of occurring, like natural catastrophic events (however, these usually carry high loads).
We need to identify this occurrence to anticipate if the structure will meet these loads during its design life.
Summary
Structural loads have different properties that we must know when analyzing structures.
Spatial application refers to the area in space where the load is applied.
Duration tells us the period of a force acting on a building.
Nature property refers to the classification of loads as either static or dynamic. Static loads are loads that do not generate acceleration effects on the structure while dynamic loads cause acceleration effects on the structure.
The Rate of Application refers to how nature applies loads when introduced into the structure.
Load Certainty deals with the probability of a structural load occurring throughout its design life.
