An allowable stress formula for the conventional structural design is presented in this paper on the basis of the ultimate strength anaysis considered the finite deflection, the spread yeilded zones and the cooling residual stresses of arches as follows : σ1stn/σ1sta+kσ1stm/σ1sta≤ (F.S.) σ1sta=σy/ (F.S.) 0.51+4n-10n2-0.1 (σk/2400kg/cm2) -0.5×10-5λ2 (σk/2400kg/cm2) k=0.55+ (2400kg/cm2/σy) /4 For the 2nd order elastic analysis, an allowable stress formula is also proposed in a simple form : σ2ndn+σ2ndm≤0.9σy/(F.S.)