3.1.7.2. NDFiberSection

This command constructs a fiber section in which each fiber uses an nD material in a beam-fiber stress state. Unlike a standard Fiber section (Bernoulli theory with uniaxial materials), an NDFiber section captures axial-shear interaction in 2D and axial-shear-torsion interaction in 3D. Section geometry is defined with patch, layer, and fiber subcommands inside a braced block, the same way as for a Fiber section.

section NDFiber $secTag { $subcommands }

Argument	Type	Description
$secTag	integer	unique section tag
$subcommands	list	braced block of patch layer and fiber commands referencing nD materials

Note

1. Direct beam-fiber materials. J2BeamFiber and ElasticIsotropicBeamFiber are nD materials formulated directly for the beam-fiber stress condition and are the usual choice for steel or other J2 beam fibers.

2. Wrapping general nD materials. A three-dimensional nD material can also be placed in an NDFiber section. OpenSees obtains a BeamFiber2d (2D) or BeamFiber (3D) copy by wrapping the material with BeamFiberMaterial2d or BeamFiberMaterial, which performs static condensation to the beam-fiber stress state. This approach can be expensive for complicated constitutive models.

3. Use section NDFiberWarping instead of section NDFiber when warping behavior is required in 2D models.

4. Further discussion and verification examples are available in Fibers of Higher Dimensions on OpenSees Digital.

Example

The following example defines a 3D NDFiber section for a hollow steel tube using J2BeamFiber and a circular patch.

1. Tcl Code

nDMaterial J2BeamFiber 1 29000.0 0.3 60.0 0.0 145.73
section NDFiber 1 {
    patch circ 1 8 4 0.0 0.0 2.25 2.50 0.0 360.0
}

2. Python Code

nDMaterial('J2BeamFiber', 1, 29000.0, 0.3, 60.0, 0.0, 145.73)
section('NDFiber', 1)
patch('circ', 1, 8, 4, 0.0, 0.0, 2.25, 2.50, 0.0, 360.0)

Code Developed by: Michael H. Scott