Technology & Engineering
Support Reactions
Support reactions refer to the forces that arise at the points of contact between a structure and its supports. These reactions are essential for ensuring that the structure remains stable and does not collapse under load. Engineers must calculate and design for support reactions in order to ensure the safety and stability of structures.
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3 Key excerpts on "Support Reactions"
eBook - PDFStructural Analysis
Principles, Methods and Modelling
- Gianluca Ranzi, Raymond Ian Gilbert(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
The supports prevent movement of the structure in differ-ent directions, and when the structure is loaded, restraining forces (called reaction forces or simply reactions ) develop at the supports. The specific reactions depend on the type of support. Typical supports and their role in the behaviour of two-dimensional structures are dis-cussed below and in subsequent sections. The two-dimensional representation (usually in the x – y plane) is very useful for practical applications as many structural systems, such as beams, columns, and frames, can often be conveniently idealised and analysed as two-dimensional structures. In these cases, it is usually assumed that the depth (or thickness) of each member is small when compared to its length and, because of this, the member can be modelled as a line element and its depth can be ignored. The case of reactions in three-dimensional structures is dealt with separately in Section 2.11. A support provides a restraining action at a particular point in a structure and this can be represented by one or more reactions depending on the number of movements that are prevented. A number of common support conditions are illustrated in Table 2.1. A roller support , for example, prevents movement in the direction perpendicular to the supporting plane. Such restraint is provided by means of a single reaction force applied at the support point in that direction . A pinned support (or hinged support ) allows rotation but does not permit translation in any direction. The reaction consists of a single force ( R ) whose line of action passes through the pinned support and whose magnitude and direction depend on the magnitude and direc-tion of the applied loads. The reaction R can therefore act in any direction and, for conve-nience, is often replaced by its vertical and horizontal components ( V and H , respectively). The reaction at a pinned support cannot include a couple (as rotation is permitted to occur freely).
eBook - PDFBasic Theory of Structures
The Commonwealth and International Library: Mechanical Engineering Division
- J. S. C. Browne, N. Hiller, G. E. Walker(Authors)
- 2014(Publication Date)
- Pergamon(Publisher)
1 2 BASIC THEORY OF STRUCTURES Loads may be subdivided into (a) dead loads, due to the self weight of the structure, and which are always present, (b) imposed loads, which embrace all loads not included in (a), for example; weight of machinery, stores, vehicles, cranes, and people, as well as loading due to wind or water pressure. Unlike dead loads, imposed loads may, or may not, be present. Loads, including own weight JAAL (// % y// Structure '// Y // v -· %■ ,////j ^//////////////W Reactions at supports FIG. 1.1 Generalized diagram of the forces acting on a structure Reactions at supports are just as much forces acting on the structure helping to maintain it in equilibrium as are the loads. They differ from the latter only by virtue of the fact that loads are those forces applied to the structure by active agents, such as gravity, wind or mechanical means ; whereas reactions are those forces applied to the structure due to the resistance of a supporting medium, such as the ground. Think, for ex-ample, of a piece of cast iron resting on the floor. The lump of cast iron is in equilibrium under the action of two equal and opposite forces. One is the load, which is the weight of the piece of cast iron; that is the mutual gravitational attraction existing between it and the earth; and the other force is the reaction provided by the floor. The load is under direct con-External forces (a system in equilibrium) EQUILIBRIUM OF STRUCTURES 3 trol in the sense that a larger or a smaller piece of iron can be substituted, whereas the reaction of the floor adjusts itself to variations in the load, so that equilibrium is maintained. This automatic adjustment of the reaction to the applied load is similar to the way in which a spring stretches more when subjected to a larger pull and so exerts a reaction which is bigger by just the same amount.
eBook - PDF- Alfred Meistermann(Author)
- 2017(Publication Date)
- Birkhäuser(Publisher)
8: The three types of support, the different ways of representing them in statical systems and examples Support reaction — Double supports can absorb forces from several directions. They are fixed and articulated. — Restraints are triple supports and can absorb forces from different directions, as well as moments. The correct choice of support is very important in construction, and must therefore be represented in statical systems. > Chapter Loads and forces, Statical system Support forces Let us assume that a beam is supported on a spiral spring rather than masonry. The spring is compressed by the load from the beam, thus creating a counter-force to the load that the beam exerts. This force is called support reaction. > Fig. 9 If the beam does not move, the reaction force of the spring is exactly the same as the force exerted by the beam. Put simply: action equals reaction. > Fig. 10 It is not possible to see this in the masonry that usually provides support, but it is compressed just like the spring, so that it can generate the support reaction force. When calculating a construction it is necessary to know the magni- tude of the forces that the supports have to apply to support the structural element above them. The support forces are therefore always calculated immediately after investigating the loads. Applying the above-mentioned reaction action load beam support force spiral spring 17 Fig. 10: Action = reaction Fig. 9: Support force Conditions for equilibrium law that action = reaction, it is possible to set up three theses for each structural element that make it possible to calculate the support forces. These three principles are the fundamental tools for statical calculations. They are also known as the three conditions for equilibrium: > Fig. 11 ∑V = 0 All vertical loads together are the same as all the vertical Support Reactions. This means: the sum of all vertical forces equals zero.
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