Types of Structures

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Introduction

A structure is a system made up of connected elements (usually a connection of beams and columns is the most basic link of structural components) that resist the applied loads placed on the structure.[1]

There are different variations of structures, but the four basic structures are considered to be trusses, cables and arches, frames, and surface structures. [2] There are many available structures because of the ability to manipulate the properties of these connected elements to suit the conditions of the surrounding environment. Choosing the correct supports and material allow for this.

There is a wide assortment of supports that can be used as connections within structural components. Roller supports, fixed end supports, pinned supports, and tension cables are a few(refer to figure 1). [1]Choosing the correct support is crucial to avoid failure of the framework. For example, if the purpose of a structure were to resist moment, a fixed end support would be picked.

A material property is also an important factor that is considered during the design process of structures. Based on the maximum limits of a structure, the correct design can be created. Limits refer to the maximum deflection, maximum applied external/internal force, and maximum moment.[3] This depends on the primary purpose of the structure being put together , For example, you might want a beam in a structure to have a maximum deflection of 5mm. Based on that, you would choose the appropriate beam that suits this condition.

You can refer to Loads and Load Combinations, Determinacy, Indeterminacy and Stability,Cantilever Method, Portal Method, to further your understanding in finding forces. You may also want to refer to the following methods which explain deflections of structures elements, Moment Area Theorem, Slope-Deflection Method for Continuous Beams,Slope-Deflection Method for Frames with Sidesway, and Conjugate Beam Method.

Example of Various structural connections Figure:1
Example of a Simply Supported beam Figure:2
Example of a Column with fixed end Figure:3
Example of a Frame with fixed end Figure:4
Example Truss system Figure:5

Structural Elements

Beams

Beams are one of the basic building components of structures. A beam is a horizontally placed element in a structure and is usually sitting on or connected to a column[4]. Figure 2 is an example of a simply supported beam utilizing a pin and roller as its two supports. This beam can be susceptible to various forces and can only withstand axial and shear forces. It is however incapable of counteracting any moment forces applied[4]. Beams as mentioned previously, can be connected to columns. The idea of this is to be able to transfer the loads applied on the beam such as permanent loads to the columns and throughout the structure. Furthermore, when beams are under any load they can experience deflection and when this does occur, the beam can be in compression or tension.

Columns

Columns are vertical members almost like a beam that can tolerate axial, shear, and bending moment forces[4]. In a building a column is a vertically loaded member which undergoes compression. Figure 3 shows the basic free body diagram of a column in which shows the reaction forces of a column when it is loaded.

Support Components

Types of supports Supports are used to connect different structural elements together. There are many types of supports that have different reactions that act on them. The most common support reactions that are used in structural elements are:

  • Roller supports
  • Pinned Supports
  • Fixed-End Supports

For roller supports, it provides a vertical reaction, whereas for pinned supports, both vertical and horizontal supports are present, and finally for fixed end supports, three types of reactions exist, vertical, horizontal and bending moment reactions are present [1].

Figure 1 shows what each support looks like, and the reactions that act on each support

Determinate and Indeterminate Structures

Structures are classified to two categories, which are determinate and indeterminate structures.

Determinate Structures

Structures are externally determinate, when they the number of equations of equilibrium are equal to the number of reactions provided by each support[1]. For example: the truss shown in figure 5 Has three external reactions created by the pin at “A” and the roller at “B”, which is equal to the three equations of equilibrium, therefore all the reactions can easily be calculated

Indeterminate Structures

Structures are externally Indeterminate, when the number of reactions provided by the supports, exceed the three equilibrium equations[1]. For example, the frame shown in figure 4 has six reactions that are provided by the two fixed supports, and there are only three equations of equilibrium, therefore it is not possible to find all the external reactions.

For further insight regarding this topic, refer to Determinacy, Indeterminacy and Stability of structures.

Types of Structures

The following are a combination, of structural elements that if joined together form these basic structures. Furthermore, the permutation of the types of structures can further develop buildings, bridges and Towers[1].

Frames

A frame consists of two or more columns and beams interconnected together[4]. Figure 3 shows an image of a frame. The two vertical members of the frame are columns and the horizontal connection between them is a beam. This example is a simple form of a frame. Multiple frames can be easily connected together to form a multi-storey building.

Trusses

Trusses are a system composed of several beams/columns which are connected uniquely together with pins. This specific connection allows for the strength in design. Some members in the truss undergo tension, some are in compression, and some are zero force members [4]. The most common truss systems used are planar trusses, which refer to truss systems that are constructed in a single plane, i.e. not three dimensional (refer to figure 5)[4]. Trusses are used in a variety of different structures. The most common truss is used to withstand or support rooftops in buildings, houses, and towers[4].Figure 5 is shows an example of a truss that contains a zero force member, which is placed in the center of the truss. Refer to the Method of Joints to identify what are zero force members.

Tension Structures

Tension cables are elastic steel members that are used to construct bridges such as the Golden Gate Bridge[5]. Tensile members always remain in tension in the lifespan of the building and are in tension as a result of the external loads applied on the whole system[1].

Compressive Arches

Another form of compressive structures other than columns are, Arches[4]. Unlike tensile cable structures, we can think about Arch structures as a formulated series of columns enclosed by Arches undergoing compressive stress. This system achieves its highest strength from remaining in compression as a result of the encasing by the Arch[1]. Furthermore, its best abilities and most common application are compressive bridge structures.

Basic Equations of Equilibrium

Related Topics: These three equations of equilibrium are the basis of the analysis of structures

  • $ \sum F_x = 0 $
  • $ \sum F_y = 0 $
  • $ \sum M_o = 0 $

Examples

Burj Khalifa

Burj Khalifa is one of the most recent development of a high-rise tower. In fact it is the tallest standing structure in the world reaching 160 stories high[6].As we can see from the figure (6) of the building, there are beams and columns visible during construction.

By Imre Solt (Dubai Construction Update) [GFDL (http://www.gnu.org/copyleft/fdl.html) or CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/)], via Wikimedia Commons Figure:6

Golden Gate Bridge

The Golden Gate Bridge is an example of a structure that is built in tension and is suspended over a large body of water[5]. From the figure(7) we can see that the cords are an example of tension cables.

By Daniel Ramirez from Honolulu, USA (View of the Golden Gate Bridge) [CC-BY-2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons Figure:7

References

  1. 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Kassimali, A. (2011). Structural Analysis: SI Edition (4th ed.). Stamford, CT: Cengage Learning.
  2. http://www.sut.ac.th/engineering/civil/courseonline/430331/pdf/01_Loads.pdf
  3. http://arch.umd.edu/Tech/Tech_II/Lectures/Properties%20of%20Structural%20Materials.pdf
  4. 4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 Hibbeler, R.C. (2012). Structural Analysis (8th ed.). Upper Saddle River, NJ: Pearson Prentice Hall.
  5. 5.0 5.1 http://goldengatebridge.org/research/factsGGBDesign.php
  6. http://www.burjkhalifa.ae/en/TheTower/FactsFigures.aspx