generate_sectional_properties.hpp

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#pragma once
 
#include <array>
#include <cmath>
 
namespace kynema::beams {
 
static std::array<std::array<double, 6>, 6> GenerateStiffnessMatrix(
    double EA, double EI_x, double EI_y, double GKt, double GA, double kxs, double kys, double x_C,
    double y_C, double theta_p, double x_S, double y_S, double theta_s
) {
    const double cos_theta_p = std::cos(theta_p);
    const double sin_theta_p = std::sin(theta_p);
    const double cos_theta_s = std::cos(theta_s);
    const double sin_theta_s = std::sin(theta_s);
 
    // Calculate bending stiffness in principal frame
    const double bending_xx = EI_x * cos_theta_p * cos_theta_p + EI_y * sin_theta_p * sin_theta_p;
    const double bending_yy = EI_x * sin_theta_p * sin_theta_p + EI_y * cos_theta_p * cos_theta_p;
    const double bending_xy = (EI_y - EI_x) * sin_theta_p * cos_theta_p;
 
    // Calculate shear stiffness in principal frame
    const double shear_xx = GA * (kxs * cos_theta_s * cos_theta_s + kys * sin_theta_s * sin_theta_s);
    const double shear_yy = GA * (kxs * sin_theta_s * sin_theta_s + kys * cos_theta_s * cos_theta_s);
    const double shear_xy = GA * (kys - kxs) * sin_theta_s * cos_theta_s;
 
    //--------------------------------------------------------------------------
    // Assemble stiffness matrix by blocks
    //--------------------------------------------------------------------------
    std::array<std::array<double, 6>, 6> stiffness_matrix = {};  // initialized to zeros
 
    // Shear-shear coupling (rows 0-1, cols 0-1)
    stiffness_matrix[0][0] = shear_xx;
    stiffness_matrix[0][1] = -shear_xy;
    stiffness_matrix[1][0] = -shear_xy;
    stiffness_matrix[1][1] = shear_yy;
 
    // Axial stiffness (row 2, col 2)
    stiffness_matrix[2][2] = EA;
 
    // Axial-bending coupling due to centroid offset (row 2, cols 3-4)
    stiffness_matrix[2][3] = EA * y_C;
    stiffness_matrix[2][4] = -EA * x_C;
 
    // Bending-axial coupling due to centroid offset (rows 3-4, col 2)
    stiffness_matrix[3][2] = EA * y_C;
    stiffness_matrix[4][2] = -EA * x_C;
 
    // Bending stiffness with axial-bending coupling (rows 3-4, cols 3-4)
    stiffness_matrix[3][3] = bending_xx + EA * y_C * y_C;
    stiffness_matrix[3][4] = -bending_xy - EA * x_C * y_C;
    stiffness_matrix[4][3] = -bending_xy - EA * x_C * y_C;
    stiffness_matrix[4][4] = bending_yy + EA * x_C * x_C;
 
    // Shear-torsion coupling due to shear center offset (rows 0-1, col 5)
    stiffness_matrix[0][5] = -shear_xx * y_S - shear_xy * x_S;
    stiffness_matrix[1][5] = shear_xy * y_S + shear_yy * x_S;
 
    // Torsion-shear coupling due to shear center offset (row 5, cols 0-1)
    stiffness_matrix[5][0] = -shear_xx * y_S - shear_xy * x_S;
    stiffness_matrix[5][1] = shear_xy * y_S + shear_yy * x_S;
 
    // Torsional stiffness with shear-torsion coupling (row 5, col 5)
    stiffness_matrix[5][5] =
        GKt + shear_xx * y_S * y_S + 2. * shear_xy * x_S * y_S + shear_yy * x_S * x_S;
 
    return stiffness_matrix;
}
 
static std::array<std::array<double, 6>, 6> GenerateMassMatrix(
    double m, double I_x, double I_y, double I_p, double x_G = 0., double y_G = 0.,
    double theta_i = 0.
) {
    const double cos_theta_i = std::cos(theta_i);
    const double sin_theta_i = std::sin(theta_i);
 
    // Calculate mass moments of inertia in reference frame
    const double inertia_xx = I_x * cos_theta_i * cos_theta_i + I_y * sin_theta_i * sin_theta_i;
    const double inertia_yy = I_x * sin_theta_i * sin_theta_i + I_y * cos_theta_i * cos_theta_i;
    const double inertia_xy = (I_y - I_x) * sin_theta_i * cos_theta_i;
 
    //--------------------------------------------------------------------------
    // Assemble mass matrix by blocks
    //--------------------------------------------------------------------------
    std::array<std::array<double, 6>, 6> mass_matrix = {};  // initialized to zeros
 
    // Translational mass with CG coupling (rows 0-2, cols 0-2)
    mass_matrix[0][0] = m;
    mass_matrix[1][1] = m;
    mass_matrix[2][2] = m;
 
    // Translational-rotational coupling due to CG offset (rows 0-2, cols 3-5)
    mass_matrix[0][5] = -m * y_G;  // F_x due to α_z
    mass_matrix[1][5] = m * x_G;   // F_y due to α_z
    mass_matrix[2][3] = m * y_G;   // F_z due to α_x
    mass_matrix[2][4] = -m * x_G;  // F_z due to α_y
 
    // Rotational-translational coupling due to CG offset (rows 3-5, cols 0-2)
    mass_matrix[3][2] = m * y_G;   // M_x due to a_z
    mass_matrix[4][2] = -m * x_G;  // M_y due to a_z
    mass_matrix[5][0] = -m * y_G;  // M_z due to a_x
    mass_matrix[5][1] = m * x_G;   // M_z due to a_y
 
    // Rotational inertia with translational-rotational coupling (rows 3-4, cols 3-4)
    mass_matrix[3][3] = inertia_xx + m * y_G * y_G;
    mass_matrix[3][4] = -inertia_xy - m * x_G * y_G;
    mass_matrix[4][3] = -inertia_xy - m * x_G * y_G;
    mass_matrix[4][4] = inertia_yy + m * x_G * x_G;
 
    // Polar inertia with CG coupling (row 5, col 5)
    mass_matrix[5][5] = I_p + m * (x_G * x_G + y_G * y_G);
 
    return mass_matrix;
}
 
}  // namespace kynema::beams