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Support in a structure is a member which helps others member to resist loads. Different types of supports, their reactions and applications for structures and their details is discussed. Supports in a structure transfers the load to the ground and provides stability to the structure supported on it.Contents:
Types of Supports and Reactions in Structures and Their Applications
The types of supports can be mainly classified into two types- External supports
- Internal supports
External Supports and Reactions and Applications in a Structure
The supports which are usually provided externally without disturbing the structural members are external supports. Different types of external supports are as follows:- Fixed support
- Pinned support or hinged support
- Roller support
- Rocker support
- Link support
- Simple support
Fixed Support and Reactions and Applications in a Structure
Fixed supports are also called as rigid supports. Fixed supports are restrained against both rotation and translation so they can resist any type of force or moment. In structural analysis, there are three unknowns to find for fixed support which can satisfy all the three equations of equilibrium. To provide good stability to the structure, at least one rigid support should be provided. Beam fixed in wall is a good example for fixed support.
Fig: Fixed Support - Beam Fixed in Wall
Pinned Support and Reactions in a Structure
Pinned support or hinged support can resists both vertical and horizontal forces but they cannot resist moment. It means hinged support is restrained against translation. Using equations of equilibrium, one can find out the components of horizontal and vertical forces. Best example for hinged support is door leaf which only rotates about its vertical axis without any horizontal or vertical movement. The rotation of pinned support or hinged support is allowed in only one direction and is resisted in other direction. Hinged supports are also used in three hinged arched bridges with two supports at ends and third hinge is provided at the center of the arch which is called as internal hinge. Figure below shows hinge support of Sydney Harbor Bridge.
Fig: Hinge Support of Sydney Harbor Bridge
Roller Support and Reactions and Applications in a Structure
Roller supports only resists perpendicular forces and they cannot resist parallel or horizontal forces and moment. It means, the roller support will move freely along the surface without resisting horizontal force. This type of support is provided at one end of bridge spans. The reason for providing roller support at one end is to allow contraction or expansion of bridge deck with respect to temperature differences in atmosphere. If roller support is not provided then it will cause severe damage to the banks of bridge. But this horizontal force should be resisted by at least one support to provide stability so, roller support should be provided at one end only not at both ends.
Fig: Roller Support on One End of a Bridge
Rocker Support and Reactions and Applications in a Structure
Rocker support is similar to roller support. It also resists vertical force and allows horizontal translation and rotation. But in this case horizontal movement is due to curved surface provided at the bottom as shown in below figure. So, the amount of horizontal movement is limited in this case.
Fig: Rocker Support in a Structure
Link Support and Reactions in a Structure
Link is support allows rotation and translation perpendicular to the direction of link only. It does not allow translation in the direction of link. It has single linear resultant force component in the direction of link which can be resolved into vertical and horizontal components.Simple Supports in a Structure and Their Reactions
Simple support is just a support on which structural member rests. They cannot resists lateral movement and moment like roller supports. They only resist vertical movement of support with the help of gravity. The horizontal or lateral movement allowed is up to a limited extent and after that the structure loses its support. It’s just like a brick resting longitudinally on two bricks. This type of support is not commonly used in structural purposes. However, in the zones of frequent seismic activity simple supported structures can be seen.
Fig: Simple Supports in a Structure
Internal Supports and Reactions and Applications in a Structure
Internal supports are provided internally in the structural member which means an internal support divides the full member into parts. So, the external reactions can be found for each part which will be quite easier for the analysis. Following are the types of internal supports provided in a structure:- Internal hinge
- Internal roller
Internal Hinge Support in a Structure
Similar to hinge support internal hinge also resists translation in both directions and allows only rotation. In structures, for axial members internal hinges are provided and for beam members middle hinges are provided. These can be widely seen in arch type bridges at the center of arch.
Fig: Internal Hinge Support in a Structure
Fig: Three Hinged Arch with Internal Hinge Support
Internal Roller Support in a Structure
Internal roller supports are same as roller supports but they are provided in the middle of structural member.
Fig: Internal Roller Support
This type of internal roller supports are used in tower cranes or harbor cranes so, using the horizontal movement of support heavy materials or members can be shifted from one place to another place.