Tri31 Element

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This command is used to construct a constant strain triangular element (Tri31) which uses three nodes and one integration points.

element tri31 $eleTag $iNode $jNode $kNode $thick $type $matTag <$pressure $rho $b1 $b2>

$eleTag unique element object tag
$iNode $jNode $kNode three nodes defining element boundaries, input in counter-clockwise order around the element.
$thick element thickness
$type string representing material behavior. The type parameter can be either "PlaneStrain" or "PlaneStress."
$matTag tag of nDMaterial
$pressure surface pressure (optional, default = 0.0)
$rho element mass density (per unit volume) from which a lumped element mass matrix is computed (optional, default=0.0; if rho=0.0, then materials are queried for mass density)
$b1 $b2 constant body forces defined in the domain (optional, default=0.0)


  1. Consistent nodal loads are computed from the pressure and body forces.
  2. The valid queries to a Tri31 element when creating an ElementRecorder object are 'forces', 'stresses,' and 'material $matNum matArg1 matArg2 ...' Where $matNum refers to the material object at the integration point corresponding to the node numbers in the domain.

Code Developed by: Roozbeh G. Mikola, UC Berkeley and N. Sitar, UC Berkeley