The results indicate that the influence of load types,chord boundary conditions,load sequences and axial tension on chord can be ignored,but the boundary condition of branch pipe and the axial compression on chord may affect the bearing capacity of the joints.

In order to resolve the problem that the damage value under two level loads isn t equal to one, a cumulative nonlinear damage model and its math formula for fatigue are put forward by using the usual cumula- tive damage theories, avoiding the equivalent damage and considering the effect on material for load sequence, domanial latent damage and damage s uneven character.

In order to solve the problem of fatigue damage under two-level loads not always equal to one,based on the conventional accumulative damage theories,an accumulative linear damage model and its math formula have been proposed with the effect of load sequence and domanial latent damage.

Taking the steel tube concrete ribbed arch bridge for example,this article establishes the finite element model of space analysis according to the structural finite element theory and studies the influence of the loading sequence on the deformation and stress of the main arch bridge in the span drel construction so as to choose the loading sequence of safer spandrel construction.

For the traditional Miner s rule can not consider about the load sequence,fuzzy mathematics was used to analyze the fuzziness of fatigue damage due to the stress beside the fatigue vague.

Since the traditional Miner rule ignores the influence of the load sequence on the fatigue life, the fuzzy rules are used to analyze the fuzziness of the fatigue damage caused by the stress nearby the fatigue limit under different load sequences.