Users:General FEM Analysis/Elements Reference/BeamNL

Contents 1 General Description 1.1 Element Type 1.2 Degrees of Freedom 1.3 Orientation of the local coordinate system 2 Input Parameters 2.1 Parameter Description 2.2 Example of a Complete Input Block 2.3 Use of the shear correction factors KY and KZ 2.4 Use of the rotation parameter THETA 2.5 The torsional resistance IT 3 Element Loading 3.1 Pressure 3.2 Dead Load 3.3 Snow Load 4 Tests and Benchmarks 4.1 Static linear analysis 5 Theory 6 References

General Description

Element Type

• This beam element is a 2 node non-linear 3D-beam for large rotations and small deformations (Green-Lagrange-strains) taking into account shear deformation (Timoshenko-beam element).
• This beam element has 6 DOFs per node (three translations and three rotations).
• The stiffness matrix is obtained using a one-point Gaussian quadrature, which results in an under-integration of the shear terms. This should prevent shear-locking.

Degrees of Freedom

For the BeamNL element use the 3 translatoric degrees of freedom DISP_X, DISP_Y, DISP_Z and the 3 rotatoric degrees of freedom ROT_X, ROT_Y, ROT_Z.

Orientation of the local coordinate system

The Beam1 element uses the following definition for the determination of the local coordinate system (needed for the orientation of IYY and IZZ,...):

• the local x-axis is oriented from node 1 to node 2 of the beam
• the local y-axis lies in the global XY-plane, such that the local z-axis points in the same half-space as the global Z-axis (mathematically spoken: the local z-axis and the global Z-axis result in a positive dot-product)
• the local z-axis is perpendicular to the other two local axis, following the right-hand-rule for x-y-z
• exception: If the local x-axis (i.e. the beam axis) points in the direction of global Z, the local y-axis points in the direction of the global Z-axis. The local z-axis once again follows the right-hand-rule for x-y-z.

In case that a rotation of the local coordinate system is needed (rotated elements, inverse definition of IYY and IZZ,...) an angle THETA has to be specified. This angle rotates the whole coordinate system around the local x-axis, following the right-thumb rule (i.e. the thumb of the right hand points in the direction of the local x-axis).

Input Parameters

Parameter Description

Compulsory Parameters
Parameter	Values, Default(*)	Description
MAT	EL-MAT int	Number for the used Material e.g. MAT=EL-MAT 1
AREA		Definition of the cross-sectional area of the beam
SHEAR_SECTION_Y, SHEAR_SECTION_Z		Definition of the shear-cross-sectional area of the beam in y- and z-direction
IYY, IZZ		Definition of the moments of inertia
Optional Parameters
KY, KZ	0	Shear correction factor; if 0 or no value specified, shear is not taken into account (Bernouolli-beam theory)
THETA	0	Angle of rotation of the local coordinate system around the beam axis in degrees

Example of a Complete Input Block

EL-PROP 1 : BEAM1
MAT= EL-MAT 1
AREA=5.555  IYY=1.222  IZZ=1.333
KY=1.4   KZ=1.5
THETA=90

Use of the shear correction factors KY and KZ

The shear correction factors KY and KZ depend on the cross section of the beam. For a rectangle, KY resp. KZ are equal to 1.2, which gives a factor of α=1/Ki=5/6.

Use of the rotation parameter THETA

The rotation parameter THETA and its use is explained in the section concerning the coordinate system above.

The torsional resistance IT

The torsional resistance IT is interpreted as the polar moment Ipp, i.e.: IT = Ipp = IYY + IZZ.

Element Loading

For the moment, only nodal forces in the three global directions can be applied (i.e. Fx, Fy, Fz).

Pressure

• not defined yet

Dead Load

• not defined yet

Snow Load

• not defined yet

Tests and Benchmarks

Static linear analysis

For the moment, the element Beam1 has successfully been tested in 3D in all its linear static features, including

• bending, axial deformation, torsion,
• shear deformation (separately definable for both local axes),
• rotation around the local axis.

As an example, the structure on the right was part of the final tests.

Theory

The element implementation mainly follows the implementation of a linear 3D-beam element in FELyX ^[1], Schwarz ^[2] and Wunderlich ^[3].

As it is based on the Hermite-form functions, a Bernoulli beam can be modelled by one single element.

References

1. ↑ https://www.rdb.ethz.ch/projects/project.php?proj_id=8314
2. ↑ Schwarz, H.: Methode der Finiten Elemente, Teubner, 1991
3. ↑ Wunderlich, W.: Statik der Stabtragwerke, Teubner, 2004

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