Seismic Collapse-resistant Capacity Of Moment Frame Supported By Stepped Foundation In Mountainous City

Moment frames supported by stepped foundation(MFSSF) is a typical Structural form for buildings in mountainous city. With the irregular problem caused by different hight of column support, the structure has different seismic performance which need to be investigated. Previous research of MFSSF suggest that the first floor on the upper foundation is the weak region. But with the uncertainty of earthquakes, the collapse-resistant capacity of MFSSF also need to be invertigated.A MFSSF specimen is modeled and analysed using FE package Seismo Struct to investigate whether this program can predict the behaviour of MFSSF correctly.Then 2 regular frames, 2MFSSFs and 2 reinforced MFSSFs are designed respectively. The collapse-resistant capacity and safety margin of different frame are quantitatively evaluated and compared, with a seismic collapse fragility analysis based on an incremental dynamic analysis(IDA). Then 2 MFSSF are modeled without amplification of earthquake intensity on upper foundation to investigate the effect of amplification of earthquake intensity on upper foundation. Main collapse mode of regular frame and MFSSF are investigated respectively to investigate the effect of collapse mode on collapse-resistant capacity and safety margin.Through research of this paper, four conclusions were drawn:â‘  The finite element package Seismo Struct can predict large displacement behavior of both regular frame and MFSSF.â‘¡ The 2 regular frames designed under the same code have same level of safety margin though the collapse-resistant capacity decrease with the increment of height. The 2 MFSSFs have similar collapse-resistant capacity with the regular frame which have the same total with the MFSSFs, but the safety margins are lower than the regular frames. The reinforced frames with pull beam have the same collapse-resistant capacity and safety margins with the MFSSFs which means the reinforcement is not available in improving the collapse-resistant capacity and safety margin of MFSSF.â‘¢ The amplification of earthquake intensity on upper foundation have a notable negative effect on the collapse-resistant capacity and safety margin of MFSSF and is also the main reason why pull beam reinforcement can`t improve the collapse-resistant capacity and safety margin of MFSSF. Because the amplification, there is a relative disc between upper foundation and lower foundation which is fatal to structure especially with the existence of pull beam.â‘£ The unique load path and failure mode are also important reason why MFSSFs have lower collapse-resistant capacity and safety margin than regular frames. By comparing the main failure mode of MFSSF and regular frame, the main failure mode of MFSSF is found that plastic deformation concentrate on the first floor on upper foundation and make it weak region. This failure mode leads to lower collapse-resistant capacity and safety marginof MFSSF.