The demand of flush endplate connections is expanding because of easy storable, transportation, and temporary placement on site. In flush endplate connections, it is becoming common practice to install horizontal stiffeners to ensure connection stiffness and strength. However, when the horizontal stiffeners are attached, high stress concentration is observed in the web plate between the flange and the horizontal stiffeners. No theoretical method for estimating the stress in the web plate has been established. In this study, we examined stress estimation formulas for the web plate of an I girder with horizontal stiffeners. In addition, FE analyses were conducted to validate proposed estimation formulas. The estimation formulas were a combination of axial stress and shear stress that varies with the length of the horizontal stiffeners. Theoretical calculations indicated that increasing the length of horizontal stiffeners reduces stress in the web plate, and the stresses showed that the values could be lower than the general axial stresses. In addition, stress from theoretical calculations using bolt elongation and endplate difomation as the spring element closes to that at the intersection between the endplate and the web.