Abstract: An accurate evaluation of boundary axial stiffness is crucial in predicting the actual response of affected frame beams. Consequently, the influence of the planar stiffness of surrounding concrete slabs, a significant aspect affecting the value of boundary axial stiffness of the beam, is an important part of catenary action research. This paper sheds a considerable light on the stiffness evaluation of the boundary axial restraints provided by adjacent slab systems to the beam considered. Firstly, three components that contribute to the axial stiffness of the restraint are identified, namely, slabs, steel beams and studs. Then the nonlinear deformation functions of the components were proposed, from which the axial restraining stiffness formulas were derived. Due to the fact that only a part of slab length is attributed to the resisting external force if a sufficient slab length is allowed, an ‘effective length' has to be considered. The axial stiffness formulas are then modified on the base of the effective length. Subsequently, the reliability of the proposed formulas was validated against a case study of 20 examples, and good correlations are observed.