An earthquake referred to a sudden slip on a fault and the resulting ground shaking and radiated seismic energy caused by volcanic, magmatic activities, or other sudden stress changes in the earth. Earthquakes are likely to occur worldwide and cause great life and financial losses. It is impossible to predict and prevent earthquakes. However, its casualties can be minimized by designing and constructing safe structures and detecting and improving unsafe buildings. Moderate and excessive earthquakes are usually accompanied by aftershocks. Aftershocks in structures damaged by the main earthquake may cause extensive damage. On the other hand, steel structures in humid regions are prone to corrosion. The standards used in this research to check corrosion are ISO 9923 and ISO 9224 standards for calculating the amount of corrosion according to the life of the structure. The ISO 9223 standard examines the corrosion rate in the first year in different atmospheric environments, and ISO 9224 standard is used to calculate the corrosion rate for the following years.
The simultaneous effect of corrosion, main earthquake, and aftershock can aggravate the damage, but in the current regulations, the effect of aftershock and corrosion are not included in the calculations. In this research, damage resulting from the combined effect of corrosion, aftershock, and the main earthquake is investigated using fragility curves that determine the level of vulnerability of the structure. The three-story steel moment structure was initially modeled using the design regulations, and then the corrosion corresponding to 20 years was taken into account for the columns of the first floor, and it was subjected to incremental IDA analysis under the record of the main earthquake and aftershock, and the maximum drift of the floors was determined as the demand, and then in the following curves Fragility is drawn for the desired structure. The seismic fragility curves were calculated for four damage levels by selecting the relative displacement capacity at seismic performance levels of slight, moderate, extensive, and complete damage from the US Hazus code. The analysis was done using structural reliability relationships and incremental dynamic analysis (IDA) with the OpenSees software platform. According to the results, it can be seen that the corroded structure corresponding to 20 years with aftershock has no significant effect on the structure in the slight and moderate damage, but it increases by 32% in extensive damage levels, which shows that It has the destructive effect of corrosion combined with the aftershock effect.
In the end, it can be stated that if the intensity of the earthquake is low, the presence of aftershocks and corrosion corresponding to 20 years will not affect the performance of the structure, but if the intensity of the earthquake is high, the presence of corrosion and aftershocks can have destructive effects on the structure and even cause the complete collapse of the structure. Considering the mentioned cases and the seismicity of Iran, there is a need to retrofit corroded metal structures.