Economical depth of a plate girder refers to the minimum depth required for the girder to achieve minimum weight. In other words, it is the depth that results in the most efficient use of material, minimizing the weight of the girder while still meeting the required strength and stiffness criteria.

There are several factors that influence the economical depth of a plate girder, including the span length, loading conditions, and material properties. By understanding these factors, engineers can optimize the design to achieve the most economical depth.

1. Span length: The span length of the girder plays a crucial role in determining its economical depth. Longer spans require deeper girders to resist bending and deflection, while shorter spans can utilize shallower girders. As the span length increases, the economical depth also increases to accommodate the additional loads and ensure structural integrity.

2. Loading conditions: Different loading conditions, such as dead load, live load, and dynamic load, affect the design of the plate girder. The magnitude and distribution of these loads influence the required depth to resist bending and shear forces. By analyzing the loading conditions, engineers can determine the optimal depth that balances strength and weight.

3. Material properties: The material properties of the plate girder, including the yield strength and modulus of elasticity, impact its design. Higher strength materials allow for the use of thinner plates, reducing the overall weight of the girder. By selecting appropriate materials, engineers can optimize the depth to achieve minimum weight.

4. Stiffness requirements: The required stiffness of the girder is another important consideration. Girders with higher stiffness can resist deflection more effectively, reducing the need for additional depth. By analyzing the deflection criteria, engineers can determine the minimum depth required to meet the stiffness requirements.

By considering these factors and performing structural analysis, engineers can determine the most economical depth of a plate girder. This depth ensures that the girder meets the required strength and stiffness criteria while minimizing the weight of the structure. The optimization of the girder's depth can result in cost savings and improved efficiency in construction projects.