This paper presents a reduced-order structural framework for the preliminary load-path analysis and wingbox sizing of a Transonic Truss-Braced Wing (TTBW). The formulation integrates a 2D geometrically nonlinear Simo--Reissner finite-strain beam model with a thin-walled semi-monocoque cross-sectional model. This structural formulation enables rapid stress recovery and component-level sizing under material yield and local panel buckling constraints under prescribed static maneuver and static gust load distributions. The framework explicitly captures finite strain and load redistribution induced by eccentric truss attachments. Numerical verification against an independent 3D shell finite element model under common prescribed loads confirms that the reduced-order model accurately reproduces primary deformation, internal force distributions, and stress trends. Parametric sweeps across spanwise, chordwise, and vertical truss attachment locations demonstrate that spanwise truss placement is the primary mechanism for wing bending moment relief and potential theoretical wingbox mass reduction. The presented framework offers an efficient screening tool for early-stage structural configuration trades prior to high-fidelity finite element refinement.