The fire resistance of collar-plate-stiffened uniplanar tubular K-joints was studied and formulated in the present research. After establishing a finite element (FE) model and its verification against the results of experimental tests, a total of 1440 numerical simulations were performed on 360 FE models of collar-plate-stiffened uniplanar tubular K-joints at four different temperatures (20, 400, 550, and 700°C). The studied joints were analysed under the axial loading and the reinforcing effect of the collar plate was investigated. Results showed that the resistance enhancement coefficient (Rs) increases by increasing the γ, θ, and τ; while it decreases by rising the β. Results of the parametric investigation were utilised to establish new design equations to determine the bearing capacity of collar-plate-stiffened tubular K-joints under the brace axial loads at fire condition.