Fragility Analysis On Seismic Collapse Resistance Capacity Of Reinforced Concrete Frames With Dampers

During an earthquake, the dampers added to the damping design structures can dissipate a majority of energy and help to reduce the structures’ seismic response. The frames no matter with or without dampers should have an enough safety margin to avoid collapse subjected to a maximum consider or mega earthquake. When the dampers are used for reinforcing an existing structure, the collapse-resistance capacity of the reinforced structure is inevitable higher than that of the original structure. When the dampers are used in new structures, the members’ section area or reinforcing bars of the damping structure are smaller than that of a structure without dampers. In this case, how far the dampers can promote the collapse-resistance capacity of the damping structure is worth of being investigated. In this paper, the collapse-resistance capacities of reinforced concrete (RC) frames designed in different seismic fortification levels with and without dampers are calculated and compared, using a collapse fragility analysis based on the incremental dynamic analysis. The main achievements of this dissertation are as follows:(1) According to the Chinese code for seismic design of buildings, six-story and nine-story RC frames in the seismic fortifications levels of 7.5,8 and 8.5 are designed adopting conventional seismic design method (Denoted as ’seismic design frame’). Then the corresponding structures with fluid viscous dampers (Denoted as ’damping design frame’) are designed, the main frames of which are designed with the fortification level of 7,7.5 and 8. The finite element models of each frame are established in the software MARC.(2) The collapse-resistance capacities of frames with nonlinear viscous dampers and with linear viscous dampers are compared. The results show that frames with nonlinear dampers have higher collapse-resistance capacities. Comparing with nonlinear dampers, the linear dampers can produce larger additional damping force, but at the same time apparently increase the axial load of columns connected with the linear dampers, which causes the bottom columns of the frame with linear dampers easier to break and the frame with linear dampers easier to collapse.(3) The collapse-resistance capacities of seismic design frames and damping design frames are calculated and studied. The results show that the seismic design frames and damping design frames have adequate energy-dissipating capacity to achieve the objective of ’no collapse in maximum consider earthquake’ and an enough safety margin to avoid collapse subjected to a maximum consider or mega earthquake. The seismic design frames and damping design frames in the seismic fortification level of 8 and 8.5 can also achieve the objective of ’no collapse in mega earthquakes’, while The seismic design frames and damping design frames in the seismic fortification level of 7.5 don’t have enough collapse-resistant capacities.(4) The collapse-resistance capacities of seismic design frames and damping design frames are compared. The results indicate that the damping design frames in the seismic fortification level of 7.5 have higher collapse-resistance capacities than seismic design frames, but the damping design frames may have lower collapse-resistance capacities in a higher seismic fortification level.(5) Through analysis of frames’collapse modes and the differences of plastic energy demand patterns of seismic design frames and damping design frames, it shows that the plastic energy demand of bottom columns in the damping design frames are more concentrated than that in the seismic design frames, which makes the bottom columns of the damping design frames more vulnerable and reduces the collapse-resistance of the damping design frames. So the amount of the bottom columns’ reinforcing rebar of the damping design frames in high seismic fortification levels are suggested to be equaled to the amount of correspond seismic design frames. Then, the verification shows that the suggestion of strengthen bottom columns can effectively increase the collapse-resistance capacities of the damping design frames.