# Basic Concept of Supports & Restraints in Structural Engineering

Structural supports are structural components that take dead loads, and non-variable and variable forces to the below structure safely and evenly. A restraint is a connection detail between the support structure and the above structure that prevents the above structure from lateral displacement or rotation displacement.Â

Hence, based on the movement they restrain, there are several types of supports.Â Most structures are either partly or completely restrained so that they cannot move freely in space. Such restrictions on the free motion of a body are called restraints and are supplied by supports that connect the structure to some external stationary body.

This article explores the basic concept of supports and restrains in structural engineering and construction.

The concept of support and restraints can be understood by the following example.Â Consider a planar structure such as bar AB shown in Figure (a). This bar would move freely in space with some combined translatory and rotational motion if this bar were a free body and were acted upon by a force P.Â

 Fig.(a)

If a restraint were introduced in the form of a hinge that connected the bar to some stationary body at point A, then the motion of the body will be only of rotational movement about the hinge (Figure (b)). However, point B would move along an arc with point A as the center. Therefore, another restraint is required at B to prevent completely the free motion of the bar.

 Fig.(b)

The supports at A and B, in restricting the free movement of the bar, are called upon to resist the action that the force P imposes upon them through the bar. The resistance method that is developed to counteract the action of the bar upon them is called support.Â

The total effect of these supports may, therefore, be replaced by the reactions that they supply to the structure (Figure (c)). Any support would offer restraint and some degree of freedom; restraints may be replaced by reactions (force/moment) and degrees of freedom may be represented by displacements (deflections/rotation). A degree of freedom (or DOF) represents a single direction in that a node is permitted to move or rotate.

 Fig.(c)

If a support rigidly constrains a given degree of freedom (DOF) for a rigid body then it gives rise to a reaction corresponding to that DOF. Similarly, if support freely allows the motion of a particular DOF then there is no reaction from the support in that direction.

1. Â Any joint in space will haveÂ 6 degrees of freedomÂ which are 6 independent components of displacements i.e. three rotations and three translations.
2. Any joint in plane frame will haveÂ 3 degrees of freedomÂ i.e. two translations and one rotation.

The reactions corresponding to a support and degrees of freedom associated with a joint is used to determine the degree of redundancy in the structural analysis of indeterminate structures.