While a low cycle fatigue criteria is well established for mild steel bars under axial loading, the effect of combined axial and bending loading is not fully developed experimentally nor analytically. This paper intends to address this condition that is applicable to the behavior of mild steel bars in a hybrid frame design. The behavior of mild steel plays a significant role in the behavior of prestressed precast hybrid frames. They were used in the PRESSS program [8] to prevent potential collapse of buildings erected using hybrid frames. The mild steel is expected to undergo yielding and large deformation to absorb snd dissipate energy in an earthquake event. A non-linear finite element analysis of a 3-D model of a hybrid frame [Hawileh, 2003], shows that mild steel bars, designed to dissipate energy, exhibit significant inelastic axial and bending strains under cyclic loading. This prompted a need for considering the low cycle fatigue life for the mild steel bars under bending and axial loading. The FE model shows the potential vulnerability of mild steel bars to low cycle fatigue failure. The bar could potentially be subjected to failure due to low cycle fatigue limiting its usefulness to dissipate energy in earthquake loading. A mild steel fracture criterion is therefore needed in the design procedure for hybrid frame by controlling the total plastic strains in the mild steel bar below a maximum limit. This paper will present analytical derivation to account for development of two equations one which predict the axial strain εaxial and one for the bending strain εb both in
展开▼
