Seismic Fragility Analysis Of Corrosion Steel Structure

The steel structure is widely used in large bridges, offshore oil platforms, industrial and civil buildings because of its good mechanical properties, economic performance and good service performance. And it is the first choice for seismic design because of the steel structure's good ductility. However, steel structure engineering is often difficult to avoid the corrosion by protection and construction measures, which is used in large industrial buildings and marine engineering. Uneven corrosion pit on steel structure surface will not only reduce steel plastic deformation and fracture toughness, but also accelerate the crack propagation. It will affect the steel structure's seismic performance during follow-up period. For the assessment of existing steel structure engineering's seismic performance in the earthquake fortification area, the seismic performance's evaluation law of the corrosion steel structure is an unavoidable problem.By using the IDA method, the analysis is carried on the seismic performance of the steel structure under different corrosion rates in the sense of probability. The main contents of this paper are as follows:(1)On the background of marine atmospheric corrosion of steel structure engineering, the test of NSS simulated indoor marine atmosphere of Q235 steel accelerated corrosion and the monotonic tensile properties test of steel under varying degree of corrosion are carried out. The impact of corrosion on the mechanical properties of Q235 steel is studied. The impact of corrosion on the ductility of the member section of steel structures is analyzed. Take the five-layer and ten-layer plane steel frame structure as examples, the influence of corrosion on the steel structure's natural period of vibration, lateral resistant ability, inter-story displacement angle and the weak story is discussed.(2)In this paper, four ultimate states of the steel frame structure is defined, the methods of determining the limits of the level of seismic performance are proposed, and the quantitative index of ultimate state is given. By using the finite element software SAP2000, the plastic hinge model is used, and the influence caused by the steel corrosion are taken into account to build numerical analysis model. By using the pushover analysis method, the different ultimate states' vertex displacement angle and inter-story displacement angle limit of different corrosion rates are obtained.(3)By using the finite element software ABAQUS, the IDA method is used to analyze the seismic fragility of the five-layer and ten-layer plane steel frame structure of different corrosion rates. Take PGA as the independent variable, the structure's probability demand function of the structural response of different corrosion rates is obtained. Based on the failure probability formulas of different ultimate damage states,the seismic fragility curves of different ultimate damage states of the five-layer and ten-layer plane steel frame structure of different corrosion rates are draught, and the variation and regular pattern of the failure probability of the five-layer and ten-layer plane steel frame structure of different corrosion rates are analyzed. Based on the "non corrosion limit" and "modified limit", the influence on the assessment of seismic behavior of steel frame structure under different corrosion rate is obtained.