Trusses
A truss is special beam element that can resist axial deformation only.
Consider the structure below:
The joints in this class of structures are designed such that no moments
develop in them. The only significant force that develops in each member
is the axial force. The axial force is constant along the length of each
member and generates an axial stress that is uniform throughout the cross-section.
Such members are modeled as truss elements. Trusses are commonly used
in architectural and structural applications such as bridges, roofs, power
towers, and others.
A truss element is defined by two nodes. Each node has 3 degrees of
freedom which are the displacements in 3 orthogonal directions. The truss
element shown below is pinned at the left node and an axial force P is
applied at the right node. Axial direction is along the length of the
beam or truss and not in either direction of the cross-section.
The axial stress (Sx)
= P/A,
and the axial displacement of the right node (Ux) =PL/AE
where:
P = axial force along the length of the truss
element
A = the cross-sectional area of the truss
L = length of the truss
E
= modulus of elasticity.
The above equation can be written as Ux=P/(AE/L)=
P/K where K=AE/L
suggesting that a truss element is analogous to an axial spring of stiffness
k=AE/L.
Joints
The joints coincide with the pierce point of the weldment profile. It
is recommended that you locate the pierce
point at the center of gravity of the weldment profile to avoid unintended
results.
When the pierce point is at the center of gravity, axial loads generate
axial stresses only.
Material Properties
The modulus of elasticity is always required.
Density is required only if gravitational loads are considered.
Restraints
You can only apply translational restraints to truss joints. There are
3 translational degrees of freedom at each node (joint). Fixed
and Immovable (No translations)
restraints are similar for a truss joint as no rotations are considered.
You can apply zero or non-zero prescribed translations. If trusses and
beams meet at a joint, you can apply rotations but they apply to the beams
only.
Loads
You can apply concentrated forces at joints and reference points. You
can also apply gravity. The program calculates gravitational forces based
on the specified accelerations and densities. Note that only axial forces
generated in each element are considered. Note that a truss ignores any
forces applied normal to it.
Meshing
There are no options in meshing beams and trusses. Beam and truss members
are displayed as solid cylinders regardless of their actual cross-section
shape.
A straight structural member identified as a truss is represented by
one truss element. The variation of the axial deformation is linear and
the axial stress is constant throughout the cross section and along the
truss.
Results
You can view axial stresses and forces, displacements, and deformed
shape plots. Forces and stresses of a truss member are constant throughout
the cross section and along the truss. The displacements vary linearly
between the ends. Forces, strains, and stresses in directions other than
the axial direction are set to zero. In a stress plot, each truss element
appears in one color. The force in a truss member equals the axial stress
multiplied by its cross-sectional area.