In the field of structural engineering, supports play a crucial role in ensuring the stability and load-bearing capacity of a structure. A support is a structural component that helps resist loads by transferring forces to the ground. Different types of supports are employed based on the design requirements and functionality of the structure. These supports can be categorized as either external or internal, and each type has distinct reactions and applications. In this article, we will explore the various types of supports used in structures, their reactions, and their specific applications.
I. Classification of Supports
Supports in a structure can broadly be classified into two categories: external supports and internal supports.
- External Supports: These are typically provided externally, without interfering with the internal components of the structure. They are the primary means of transferring the load to the ground and providing stability.
- Internal Supports: These are provided within the structure itself, often dividing larger structural members into smaller parts for easier analysis. Internal supports help distribute the load internally before it reaches the external supports.
II. External Supports and Reactions
External supports are placed on the exterior of a structure, supporting it from the outside. These supports are designed to resist various types of loads, including vertical, horizontal, and rotational forces. Below are the different types of external supports and their functions:
A. Fixed Support
A fixed support, also known as a rigid support, is one of the most robust types of external supports. It resists both translation (movement in any direction) and rotation, providing maximum stability to a structure. A fixed support can withstand all types of forces and moments applied to the structure.
In structural analysis, a fixed support introduces three unknowns, as it resists three types of motion—two translational (vertical and horizontal) and one rotational. To satisfy the equilibrium equations, these forces must be calculated. A typical example of a fixed support is a beam that is fixed into a wall. This kind of support is ideal for ensuring that the structure remains stable under heavy loads and provides sufficient resistance against all forms of motion.
Example: Beam fixed in a wall, where the beam is unable to move or rotate.
B. Pinned Support or Hinged Support
A pinned support (also called a hinged support) is designed to resist vertical and horizontal forces but does not resist rotational forces (moments). It allows the supported structure to rotate freely at the point of support while preventing translation in both vertical and horizontal directions.
Pinned supports are often used in structures where rotation is necessary, such as in bridges or doors. For example, a door leaf can rotate around its vertical axis without any horizontal or vertical displacement. Another notable use of pinned supports is in three-hinged arch bridges, like the Sydney Harbor Bridge, where the two end supports are pinned, and the third hinge at the center allows for rotation, ensuring the arch remains stable under varying loads.
Example: Hinged support at the Sydney Harbor Bridge, which allows rotation at the center of the arch.
C. Roller Support
A roller support resists only vertical forces and does not prevent horizontal movement or rotation. This allows the supported structure to move freely along a surface while still carrying the vertical load. Roller supports are typically used in structures like bridges, where expansion and contraction due to temperature changes need to be accommodated.
Since roller supports cannot resist horizontal forces, they are usually placed at one end of a structure to allow movement, while other supports, such as fixed or pinned supports, resist horizontal forces at other points. This is essential to avoid potential damage caused by the structure’s expansion or contraction.
Example: Roller support at one end of a bridge, designed to allow the bridge to expand and contract with temperature changes.
D. Rocker Support
Similar to roller supports, rocker supports resist vertical forces but allow horizontal translation and rotation. The difference is that rocker supports use a curved surface at the bottom, which limits the horizontal movement. The curvature of the support ensures that only a limited amount of horizontal movement can occur, providing a controlled response to horizontal forces.
Rocker supports are used in situations where horizontal movement is necessary but should be limited, such as in certain types of bridges or large structures subjected to varying loads.
Example: Rocker support in a bridge structure, where controlled horizontal movement is allowed.
E. Link Support
A link support is a specialized support that allows rotation and translation in a direction perpendicular to the direction of the link. However, it does not allow translation along the direction of the link. Link supports resist forces applied in the direction of the link, which can be resolved into vertical and horizontal components.
Link supports are often used in mechanical structures or systems where rotation and movement are needed in specific directions.
Example: A link support in mechanical systems that resists forces in a specific direction while allowing movement in perpendicular directions.
F. Simple Support
A simple support is the most basic form of support. It only resists vertical movement and is typically used to support a structure from below without preventing lateral or rotational movement. While simple supports do not resist horizontal forces or moments, they can provide basic stability in low-stress situations. This type of support is less common in structural design but may be used in areas of low seismic activity where lateral forces are minimal.
Example: A brick resting on two others, which allows for slight horizontal movement but resists vertical movement due to gravity.
III. Internal Supports and Reactions
Internal supports are placed within the structural member itself, dividing it into parts to make the analysis of forces simpler. These supports are useful when dealing with large or complex structures, as they help distribute loads more efficiently.
A. Internal Hinge
An internal hinge works similarly to an external hinge, in that it allows rotation but resists translation in any direction. Internal hinges are typically used in axial members (such as beams or arches) to divide the member into smaller, more manageable parts. Internal hinges are commonly found in arch bridges, particularly in the center of the arch, where they allow rotation and help distribute forces across the structure.
Example: Internal hinge in the center of an arch in a bridge, providing rotation and load distribution.
B. Internal Roller
An internal roller operates like a roller support but is placed inside the structure. It allows horizontal movement while resisting vertical forces. Internal rollers are commonly used in tower cranes or harbor cranes, where the horizontal movement of heavy materials or components is necessary for lifting and shifting purposes.
Example: Internal roller support used in cranes to facilitate horizontal movement of loads.
Conclusion
The types of supports used in structural engineering play a fundamental role in ensuring the stability and safety of buildings, bridges, and other large-scale structures. From rigid fixed supports to more flexible roller and rocker supports, each type of support serves a unique purpose based on the forces and movements it must accommodate. Understanding the various types of supports and their reactions is essential for designing structures that can withstand the loads they are subjected to, while also ensuring efficient material usage and cost-effectiveness.