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F.1.17 Triangulation
An arrangement of Frame Members where all members and segments of members between bends or nodes with Structural tubes form a structure composed entirely of triangles.
a. This is generally required between an upper member and a lower member, both of which may have multiple segments requiring a diagonal to form multiple triangles.
b. This is also what is meant by “properly triangulated”.
Explanation
Triangulation refers to frame members being assembled in a way that creates only triangles. Triangulation is derived from a principal in statics, that states that if a member is only loaded at it's ends (i.e. a two force member), the member will only be able to translate axial loads, meaning the loads will be parallel to the direction of the member. In other words, members that are only loaded at their ends will be loaded in tension or compression, never bending.
Note: The two forces need to lie on the same line of action to not create a bending moment on the member. For a member to be in static equilibrium, the extra bending moment would have to be counteracted.
This video explains two force members and the line of action conditions
The following images show examples of two force members, and members with additional forces that would put them in a state of bending.
Additional bending force
Forces sum to pure compression
Axial load is acting as a compressive force in this example
Example of bending (gif)
But why does it matter?
We care about transferring our load paths through two force members because we want to minimize the displacement the members experience under load. In the gifs below, observe how they both use pin joins as the connections for the structure. The one on the left is free to move or displace more than the one on the right. This is happening because the extra member triangulating the right side prevents the pins from moving.
The pin joints are free to move because there is no member creating resistance to the movement.
The red member is triangulating the structure, and preventing displacement of the members.
This Design Judges video explains why loading chassis members in tension and compression is much more ideal than loading them in shear
Summary:
The additional bending moment created by shear has a moment arm which amplifies the load, making shear less ideal for supporting the same loads as tension or compression.
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