In this study, the unified requirements of IACS on longitudinal strength of ships are verified through the nonlinear time domain analyses in irregular waves. The formula for the horizontal shear force, bending moment and torsional moment for ships of large deck openings are proposed based on the calculation results for existing ships. Also, external hydrodynamic sea pressure, accelerations and motions are calculated using linear strip theory and the corresponding design formulae are proposed. The calculated results are compared with the existing classification rules and will be incorporated into the rules in the near future after more detailed verification.