# Concept of Demand & Capacity in Structures

In the context of structures, the concepts of demand and capacity are crucial for ensuring the safety and functionality of a building or other engineering structures.

**Demand** refers to the forces, loads, or actions that are applied to a structure. These forces can arise from various sources, such as gravity, wind, earthquakes, live loads (e.g., people, furniture), or temperature changes. The demand represents the maximum level of stress or load that a structure must withstand without experiencing excessive deformation or failure.

**Capacity**, on the other hand, refers to the ability of a structure to resist or carry the applied loads without failure. It represents the maximum load or stress that a structure can safely withstand. The capacity of a structure is determined by its design, material properties, and structural elements such as beams, columns, slabs, or foundations.

To ensure structural safety, it is essential to compare the demand on a structure with its capacity. This process is known as structural analysis and is carried out by engineers to assess the adequacy of a structure's design. The analysis involves calculating the internal forces and stresses within the structure under various load conditions, and then comparing these values with the material strength and design limits.

If the demand on a structure exceeds its capacity, it can lead to structural failure, which may manifest as excessive deflection, cracking, or collapse. To prevent such failures, engineers use various design approaches, including safety factors and codes of practice, to ensure that the capacity of the structure is greater than or equal to the expected demand. This ensures a sufficient margin of safety for the structure.

In summary, the concepts of demand and capacity in structures are fundamental to ensuring structural safety. By analyzing the demand on a structure and comparing it with its capacity, engineers can design structures that can safely withstand the applied loads and provide the required functionality throughout their intended lifespan.

## Definition of Demand (Loads on structures)

Demand refers to all external actions (self weight) gravity, wind, earthquake, snow all are external actions. If differential settlement occurs stresses are induced in structure. Internal stresses are also called load effects.

## Definition of Capacity (Load capacity of structures)

Overall ability of a structure to carry an imposed demand is called capacity.

## Analysis of Capacity and Demand

- To find out whether capacity is enough to carry demands or not.
- To enhance capacity, either change the material or its geometry.
- Failure occurs when capacity is less than demand. To avoid failure, capacity to demand ratio should be kept greater than one or at least equal to one.
- This can be done by increasing the area or reducing the loads or increase the strength of the material by replacing the material
- Steel Reinforcement area should be opposite to the direction of demand e.g. in slabs the demand direction is in the direction of compression and tension i.e. along the length of bending. So steel is placed along the length and along the width.