4.1.3. Examples

• Dead loads (self-weight of the structure)

Dead loads are the permanent static weights that are imposed on a structure. This includes the weight of the materials used in the construction of the building, such as beams, columns, floors, and roofs. These loads are constant and do not change over time, providing a steady force that the structure must support.

Think of dead loads as your own body weight. Just like a chair must support your weight consistently whenever you sit on it, a building must support its own weight plus any additional permanent installations. For instance, if you have a table lamp on your desk, that lamp adds to the weight that the desk must bear. Similarly, every element in a structure contributes to its dead load.

• Live loads (occupants, furniture)

Live loads refer to transient forces that can vary in magnitude and location over time, such as the weight of people, furniture, and movable equipment within a building. Unlike dead loads, live loads can change based on how many occupants are present, the amount of furniture being used, and the activities taking place. This unpredictability requires that structures be designed with a margin of safety to accommodate varying conditions.

Imagine a party at your house with several guests. Depending on how many people you invite and where they choose to sit or stand (e.g., on a couch or a dining chair), the loads on your floor change. If everyone clusters together on one side of the room, that area of the floor will bear more weight than the other. Similarly, live loads fluctuate based on who is inside a building and what they are doing.

• Wind loads (when considered steady)

Wind loads are forces exerted by wind acting on a structure. These loads can be considered static under steady conditions if the wind speed is constant and uniform. However, they can also become dynamic in nature during gusty or turbulent conditions. Engineers must account for these loads in the design process to ensure buildings can withstand potential wind pressures without failing.

Consider a tall tree swaying in the wind. When the wind blows steadily, you can observe a steady bending force on the trunk. If the wind picks up and gusts occur, the tree sways erratically. In building design, engineers treat steady wind as a persistent force, similar to how a steady breeze constantly pushes against the tree, which must be strong enough to resist this force.

• Gravity

Gravity is the force that pulls objects toward the Earth. In the context of static forces, gravity acts uniformly downwards on all parts of a structure. The gravitational forces acting on a structure must be accounted for in design, as they are present at all times and affect the overall stability and load-bearing capacity of buildings and other infrastructures.

Think of gravity like a giant hand pressing down on an object at all times. For example, when you hold a book in your hand, gravity applies a constant force pulling the book down. If the hand were to press too hard (or if the structure could not support it), the book would break or fall. In this way, buildings must be crafted to handle the constant 'weight' of gravity acting on them from above.

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