Symmetry and Antisymmetry - Civil Engineering (CE) PDF Download

 Symmetry and Antisymmetry

Symmetry or antisymmetry in a structural system can be effectively exploited for the purpose of analyzing structural systems.Symmetry and antisymmetry can be found in many real-life structural systems (or, in the idealized model of a real-life structural system). It is very important to remember that when we say symmetry in a structural system, it implies the existence of symmetry both in the structure itself including the support conditions and also in the loading on that structure. The systems shown in Fig.1.15 are symmetric because, for each individual case, the structure is symmetric and the loading is symmetric as well. However, the systems shown in Fig.1.16 are not symmetric because either the structure or the loading is not symmetric. 

 Figure 1.16 Non-symmetric (asymmetric) structural systems

For an antisymmetric system the structure (including support conditions) remains symmetric, however, the loading is antisymmetric.Fig.1.17shows examples of antisymmetric structural systems.

It is not difficult to see that the deformation for a symmetric structure will be symmetric about the same line of symmetry.This fact is illustrated in Fig.1.18, where we can see that every symmetric structure undergoes symmetric deformation.It can be proved using the rules of structural mechanics (namely,equilibrium conditions,compatibility conditions and constitutive relations), that deformation for a symmetric system is always symmetric. Similarly, we always get antisymmetric deformation for antisymmetric structural systems, as illustrated in Fig. 1.19.

   Figure 1.19 Deformation in antisymmetric systems

Let us look at beam AB in Fig.1.20(a), which is symmetric about point C.The deformed shape of the structure will be symmetric as well (Fig.1.20(b)).So, if we solve for the forces and deformations in part AC of the beam, we do not need to solve for part CB separately. The symmetry (or antisymmetry) in deformation gives us additional information prior to analyzing the structure and these information can be used to reduce the size of the structure that needs to be considered for analysis.

Figure 1.20 Symmetric beam system AB and its deformation under load

To elaborate on this fact, we need to look at the deformation condition at the point/line of symmetry (or antisymmetry) in a system.The following general rules about deformation can be deduced looking at the examples in Fig.1.18 and Fig.1.19:

1.  For a symmetric structure: slope at the point/line of symmetry is zero.
2.  For an antisymmetric structure: deflection at the point/line of symmetry is zero.

These information have to be incorporated when we reduce a symmetric (or antisymmetric) structure to a smaller one.If we want to reduce the symmetric beam in Fig.1.20 to its one symmetric half AC , we have to integrate the fact the slope at point C for the reduced system AC will have to be zero. This will be a necessary boundary condition for the reduced system AC .We can achieve this by providing a support at C, which restricts any rotation, but allows vertical displacement, as shown in Fig.1.21 (Note: this specific type of support is known as a “shear-release” or “shear-hinge”). Everything else (loading, other support conditions) remains unchanged in the reduced system.We can use this system AC for our analysis in stead of the whole beam AB.

Figure 1.21 Reduced system AC is adopted for analysis for beam AB

Similarly, let us consider an antisymmetric system, a simply-supported beam AB which is antisymmetric about the mid-point C (Fig.1.22(a)). We know that the deformed shape will also be antisymmetric (Fig. 1.22(b)), and the displacement at point C will be equal to zero. Therefore, for the reduced system, we consider one antisymmetric half AC , with a support condition at C which allows rotation but does not allow vertical displacements there (Fig.1.22(c)). Everything else remains same as in AB.

Figure 1.22 (a) Antisymmetric simply-supported beam AB ; (b) Antisymmetric deformation pattern for AB ;  (c) Reduced system AC is used for analysis

Having a priory knowledge about symmetry/antisymmetry in the structural system and in its deformed shapehelps us know about symmetry/antisymmetry in internal forces in that system.(Symmetry in the system implies symmetry in equilibrium and constitutive relations, while symmetry in deformed shape implies symmetry in geometric compatibility.)Internal forces in a symmetric system are also symmetric about the same axis and similarly antisymmetric systems have antisymmetric internal forces. Detailed discussion on different types of internal forces in various structural systems and on internal force diagrams are provided in the next module (Module2:Analysis of Statically Determinate Structures).Once we know about these diagrams we can easily see the following:

1. A symmetric beam-column system has a symmetric bending moment diagram.
2. A symmetric beam-column system has an antisymmetric shear force diagram.
3. An antisymmetric beam-column system has an antisymmetric bending moment diagram.
4. An antisymmetric beam-column system has a symmetric shear force diagram.

Symmetry/antisymmetry for internal forces can be appreciated in a better way after we go through Module 2.However, two examples are illustrated in Fig.1.23 for internal forces in symmetric and antisymmetric systems.

Figure. 1.23 Internal force diagrams for a) a symmetric system, and b) an antisymmetric system

One should remember that although the examples shown here are for (primarily) one-dimensional (or linear) systems, the concept and use of symmetry/antisymmetry is not only limited to these systems.It is applicable to two- and three-dimensional systems as well. Therefore, we will also find “line of symmetry” and “plane of symmetry” in addition to “point of symmetry”. However, these concepts are complex and are not explored in detail in this course.