To help visualise the loading within bolted connections, joint diagrams have been developed.
A joint diagram is a means of displaying the load deflection characteristics of the bolt and
the material that it clamps. Joint diagrams can be used to assist in visualising how a bolted
joint sustains an external force and why the bolt does not sustain the whole of this force.
The diagram shown above presents the way that the basic joint diagram is constructed.
As a nut is rotated on a bolt's screw thread against a joint,
the bolt is extended. Because internal forces within the bolt
resists this extension, a tension force or bolt preload is
generated. The reaction to this force is a clamp force that
is the cause of the joint being compressed. The force-extension
diagram presented above shows the bolt extension and the joint
compression. The slope of the lines represents the stiffness
of each part. The clamped joint usually being stiffer than
The basic joint diagram is formed by moving the compression line of the joint to the right.
A triangle is formed because the clamped force tending to compress the joint is equal to the bolt
preload. Positive extension is to the right such as that sustained by the bolt, negative extension (compression)
is to the left and is sustained by the joint material.
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