In this series of researches on evaluating the brittle fracture strength in structurally stress-concentrated regions, many kinds of crack tip opening displacement in model specimens were gathered as described in the 1 st to 3 rd report. It was also shown that these data could be arranged in a definite relation to applied strain on a crack and crack size, etc., and the validity of the COD design curve was confirmed. Standing on the above approach, this report proposes a simplified estimation method of the strain distribution in structurally stress-concentrated regions and furthermore shows a series of strength evaluation techniques for such regions with consideration on the effect of welding residual stresses.