Two-dimensional structural shell equations (extension of the one-dimensional shell implemented in Goma)

[wortiz]

Developer: Kristianto Tjiptowidjojo, June 2014
Description: Two-dimensional shell equations require the following:
- Three shell normal equations - only have one ETM
- Two shell curvature equations - only have two ETM
- Three mesh equations - for structural shell, we only consider
boundary, diffusion, and source ETMs
Example:
EQ = shell_normal1 Q1 SH_N1 Q1 1.0
EQ = shell_normal2 Q1 SH_N2 Q1 1.0
EQ = shell_normal3 Q1 SH_N3 Q1 1.0
EQ = mesh1 Q1 D1 Q1 0. 0. 1.0 1.0 1.0
EQ = mesh2 Q1 D2 Q1 0. 0. 1.0 1.0 1.0
EQ = mesh3 Q1 D3 Q1 0. 0. 1.0 1.0 1.0
EQ = shell_curvature Q1 K Q1 1.0
EQ = shell_curvature2 Q1 K2 Q1 1.0