“…x𝑏 (𝜉, 𝑅, 𝜑) = 𝐫 𝑏 (𝜉) + 𝑅 ⋅ cos (𝜑) ⋅ 𝐞 𝑙 1 (𝜉) + 𝑅 ⋅ sin (𝜑) ⋅ 𝐞 𝑙 2 (𝜉) ⏟⎴⎴⎴⎴⎴⎴⎴⎴⎴⎴⎴⎴⏟⎴⎴⎴⎴⎴⎴⎴⎴⎴⎴⎴⎴⏟ 𝐫 𝑐𝑠 (𝜉,𝑅,𝜑) (9) where x𝑏 , x𝑠 ∈ ℝ 3 denote the current positions of coupled solid and beam material points initially sharing the same location at Γ 𝑐 in the reference configuration (Figure 5B), 𝐫 𝑏 (𝜉) ∈ ℝ 3 defines the beam centerline curve connection cross-section centroids (Figure 6A), and 𝐫 𝑐𝑠 (𝜉, 𝑅, 𝜑) ∈ ℝ 3 represents cross-section position vectors that point from the cross-section centroid to Γ 𝑐 , defined by means of the polar coordinates 𝑅 ∈ ℝ + and 𝜑 ∈ [0, 2𝜋); compare Figure 4. The rate form of Equation ( 8) in the reference configuration is defined by the material time derivative:…”