Structural Elements

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Last updated: Oct 28, 2021 by Lesley Lang

Table of Contents

What are Structural Elements?

Structural elements are indivisible pieces that combine to form a larger structure. They are considered indivisible as they serve as the basic foundation for all structures, and there are no components or features that could be simplified further [1]. Structural elements are used to split a building or construct into multiple smaller pieces to make analysis easier. There are five primary structural elements: ties, struts, beams, columns, and diaphragms [2]. Structural elements can be combined to create much more complex structural systems, these systems can describe buildings, bridges, or any architectural construction.

Basic Structural Elements

Ties

Ties are structural members that are only subject to axial tension, and no other forces [2]. Load is only applied at the ends of the tie, and they are not designed to withstand any bending forces on the member. Ties are most commonly found in the sloped roof of residential housing buildings.


A tie used in conjunction with a beam/strut element [3]

A diagram showing a diagonal tie supporting a horizontal beam.



Struts

Struts are similar but opposite to ties [2]. Struts can also only be loaded at their ends, and cannot withstand bending forces. Unlike ties, however, struts are subjected only to axial compression forces. Similar to ties, struts are also commonly found in the structure of sloped roofs. 




A strut and beam being used to support a load, both supported on a column [4]

A diagram of a diagonal strut supporting a horizontal beam.



Beams

Beams are members which are primarily subject to bending forces along the length of the member [2]. Beams are usually horizontal members which primarily withstand gravity forces. Beams are more commonly seen in industrial or commercial building which have more of a rectangular shape. 





Columns

Columns are members primarily subjected to axial compression forces, but can also withstand bending forces [2]. Columns are almost always vertical elements which interlace with grids of beams. Columns are also mostly found in industrial or commercial buildings. 





Diaphragms

Diaphragms are flat plate elements. Diaphragms are generally used as floors, or when stiffness is required in walls [2]. Diaphragms span or connect beams and columns. Diaphragms are found in almost every type of structure as they take the shape of floors or walls. 

A diaphragm supported on two shear walls [5]

A diagram of a diaphragm sitting on top of two shear walls



Properties and Classifications of Various Structural Elements

Beams

Beams come in various different cross sections based on the requirements in that scenario. They are always primarily classified by the type of beam, such as I-beam, wide flange, etc..

I-Beam

Beams are usually characterized and listed by their depth dimension. Many catalogues list the full properties of each individual beam including the moment of inertia, cross sectional area, and weight per unit length. 

Beams are characterized by the following dimensions, shown on the diagram to the right:

      • h: Depth of Section
      • w: Flange Width
      • s: Web Thickness

Other dimensions of I-Beams include:

      • Flange Thickness
      • Root Radius (Radius of the chamfer)

There are a variety of different types of I-beams, such as wide flange, and tapered flange. 

For a full table of dimensions and parameters for I-Beams, see https://www.engineeringtoolbox.com/i-flange-steel-beams-d_1317.html 

Different beam types can also be located on Engineering Toolbox.

Cross Section of a Normal I-Beam [6]

A diagram of the various dimensions of an I-Beam cross section



C-Channel

C-Channels are also characterized by their depth dimension. 

C-Channels are characterized by the same dimensions as I-Beams, shown on the diagram to the right:

      • h: Depth of Section
      • w: Flange Width
      • s: Web Thickness

For a full table of dimensions and parameters for C-Channels, see https://www.engineeringtoolbox.com/american-standard-steel-channels-d_1321.html

Cross Section of a C-Channel [6]

A diagram showing the various dimensions of a C-channel



Equal Angle Channel

Angle Channels are also characterized by their depth dimension. 

Angle Channels are characterized by the Depth and Thickness Dimensions, shown on the right:

      • h: Depth of Section
      • s: Thickness

Some channels may not have equal leg lengths, these channels are called unequal angle channels and can also be located on Engineering Toolbox.

For a full table of dimensions and parameters for Equal Angle Channels, see https://www.engineeringtoolbox.com/steel-angles-d_1324.html

Cross Section of a Angle Channel [6]

A diagram showing the various dimensions of an Angle Channel



Area Moment of Inertia of Beams 

Area Moment of Inertia is a key property to consider when looking at beams. It is a property of a beam's cross-section that describes bending, deflection, and stress that occurs in that beam. The higher the moment of inertia of the beam's cross-section, the less the beam will bend. Area moment of inertia can be calculated on both the y and x axis, with Ix denoting that the object is bending along the x axis. Because of this, the x-axis value is the important value as that is the direction that resists vertical load. 

Area Moment of Inertia can be calculated using a formula, however the formula varies depending on the shape of the cross section. A few of the common formulas are shown below. It is also common for suppliers to list the value alongside the specific beam dimensions.


      • Rectangular: Ix = b h3 / 12 ; Iy = b3 h / 12 ; where b is the width, and h is the height on the rectangle. 
      • Solid Circular: I = π r4 / 4 = π d4 / 64   ; where r is the radius of the circle
      • Hollow Circular: I = π (do4 - di4) / 64 ; where do is the outer diameter, and di is the inner diameter 
      • I-Beam: Ix = (a h3 / 12) + (b / 12) (H3 - h3) ; Iy = (a3 h / 12) + (b3 / 12) (H - h) 

Dimensions used for the Area Moment

of Inertia Calculation of an I-Beam [6]

A diagram of the dimensions used to calculate the area moment of inertia of an I-beam

Columns

Many columns may also take an I-Beam shape, and have similar properties. These properties can also be found on the Engineering Toolbox Page.

Overall, columns are generally design to have very good compressive strength, however they may be susceptible to buckling or even bending under high loading cases [7]. 

Classification

Columns are first categorized by the type of column. The 2 most common are steel and steel reinforced concrete, which can be seen in almost every modern day structure. Other, lesser used types of columns include stone columns and posts. 

Secondly, columns are classified by shape. Generally steel columns take a similar shape to an I-beam, but reinforced concrete can take a variety of shapes. Common shapes include: square/rectangular, circular, and hexagonal/octagonal. Each shape may be tapered along the length, 

Lastly, columns are sorted by their 'slenderness ratio', which is the length of the column compared to effective thickness of the column. 


Struts 

Struts are very similar to columns in the sense that they are subject to only compressive forces, similar to a column [8]. struts however, are almost solely constructed of metal or composite materials, depending on the application. 

Almost all struts are hinged at the ends, allowing them to be placed non perpendicular to other structural elements as part of a truss section. 

Classification

Struts are primarily classified by their material. Struts are primarily steel, but other materials such as aluminum or composite materials can be used in a lower load application. 

Next, struts are sorted by their 'slenderness ratio', similar to columns, which is the length of the strut compared to effective thickness of the column. 


Ties

Ties are most commonly used in house framing, and provide support to the roof of the building [9]. Other examples of ties include suspension cables or tie rods, which are ties in a straight, rod shape. 

Ties used in house framing are mostly made of galvanized steel, while ties in other applications may be made of a variety of materials. 

Classification

Ties are almost solely classified by the type of tie as there are many different varieties. The most commonly used ties are hurricane ties, girder ties, and strap ties. 


References

[1] "Structural Elements : 1 : Structural Elements," Missouri University of Science and Technology, [Online]. Available: https://web.mst.edu/~norbert/ge5471/Assignments/Assign%201%20-%20FLAC%20I/FLAC%20manual/FLAC%20manual/f2d416.pdf. [Accessed 22 January 2021].

[2]"Structural Elements and Systems," Civil Engineering Forum, [Online]. Available: https://www.civilengineeringforum.me/structural-elements-systems/. [Accessed 22 January 2021].

[3] V. Ryan. "Struts and Ties," Technology Student, 2009, [Online]. Available: https://technologystudent.com/struct1/strut1.htm. [Accessed 19 February 2021].

[4] Namita. "Axial Capacity of Steel Column or Strut," Civil Digital, November 3 2017, [Online]. Available: https://civildigital.com/axial-capacity-of-steel-column-or-strut/. [Accessed 19 February 2021].

[5] B. Reinhard, T. Roberto,  M. Thomas, S. Erik D. Philipp. "Properties, Testing and Design of Cross Laminated Timber," Shaker, 2018, [Online]. Available: 10.2370/9783844061437 [Accessed 19 February 2021].

[6] "Beams and Columns," The Engineering Toolbox, [Online]. Available https://www.engineeringtoolbox.com/beams-t_58.html. [Accessed 22 February 2021].

[7] "Types of Columns", Designing Buildings, [Online]. Available https://www.designingbuildings.co.uk/wiki/Types_of_column. [Accessed 26 February 2021]

[8] "Columns and Struts", Civil Engineering, [Online]. Available https://1civilengineering.blogspot.com/2016/11/columns-struts.html. [Accessed 26 February 2021]

[9] "Tie (engineering)", Wikipedia, [Online]. Available https://en.wikipedia.org/wiki/Tie_(engineering). [Accessed 26 February 202

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