Getting Started with OpenSees -- Nodes

At this point the user needs to decide which units will be used. In this demonstration, inches and kips will be used for length and force. Seconds will be used for time.

The assignment of node and element numbers is defined in the figure below:

In a 2D problem only the x and y coordinates need to be defined, using the OpenSees node command:

node $nodeTag (ndm $coords) <-mass (ndf $MassValues)>

node 1 0 0
node 2 504 0
node 3 0 432
node 4 504 432

The boundary conditions are defined using the OpenSees fix command:

fix $nodeTag (ndf $ConstrValues)

with three degrees of freedom per node are constrained:

fix 1 1 1 1
fix 2 1 1 1
fix 3 0 0 0
fix 4 0 0 0

where a fixed constraint is defined with a 1, a free constraint is defined with a 0.

Nodal masses are typically defined at the same time as the nodal coordinates. The nodal mass is used to calculate the eigenvalues and to perform the dynamic analysis. Only the nodal mass in the horizontal direction will be defined in this demonstration. Nodal masses can either be defined within the OpenSees node command, or the can be "appended" using the OpenSees mass command:

mass $nodeTag (ndf $MassValues)

mass 3 5.18 0. 0.
mass 4 5.18 0. 0.

The mass value was calculated by dividing the nodal weight (1/2 of the total super-structure weight) by the gravitational constant g (32 ft/sec):

<math>Mass = ((4000kip over 2) over (ten)</math>

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