Abstract: Beams of single-column pier caps with small aspect ratios are commonly used in urban heavy-load bridges with limited space in the substructure. Compared to conventional single-column pier-cap beams, the thickness of single-column pier-cap beams with small aspect ratios is larger than the diameter of the pier column, and the cap-beam thickness in the longitudinal direction is close to the cap-beam width in the transverse direction. In response to the inadequacy of the two-dimensional plane truss modal method in the code, which can only analyze the transverse tensile force of the cap beam and lacks consideration of the longitudinal tensile force, a calculation method for the transverse and longitudinal tensile forces is proposed upon the three-dimensional strut-and-tie model of single-column pier-cap beams with small aspect ratios. This method first establishes the basic configuration of the three-dimensional strut-and-tie model based on the characteristics of spatial principal stress traces of single-column pier-cap beams with small aspect ratios. Then, it uses the minimum strain energy criterion to derive the optimal configuration as well as the transverse and longitudinal tensile forces. The performance of the method is verified using the finite element integration method and compared with the two-dimensional plane truss modal method in the code. The study results show that: under the action of support loads, single-column pier-cap beams with small aspect ratios exhibit spatial effects in their structural forces. In addition to tensile stress at the top of the transverse cap beam, there is also tensile stress in the middle of the cap beam in the longitudinal thickness direction. The three-dimensional strut-and-tie model proposed effectively describes the spatial characteristics of force transmission in single-column pier-cap beams with small aspect ratios . The calculation method for transverse and longitudinal tensile forces based on the three-dimensional strut-and-tie model has clear physical significance and high computational accuracy. Compared to the two-dimensional plane truss modal method in the code, the three-dimensional strut-and-tie modal calculation method is safer and more economically reasonable for the design calculation of single-column pier-cap beams with small aspect ratios.