The Progressive Collapse Resistance Mechanism of Conventional Island Shield Buildings in Nuclear Power Plants

In China, conventional island shield buildings (CISBs), which are situated close to the nuclear islands of nuclear power plants, are class III seismic items. This is the most important level in the seismic fortification standard. Calculations to check resistance overturning under extremely safe seismic action should be carried out. Although strict seismic design has been carried out, many beyond-design-basis events, such as the 3/11 earthquake in Japan, have occurred in recent years. Under the action of such accidental loads, there is still the possibility that the important vertical bearing components of a CISB will be damaged or destroyed, which will lead to the progressive vertical collapse of the CISB structure, and ultimately threaten the safe operation of the nuclear power plant. Therefore, it is necessary to study the progressive collapse resistance mechanism of the CISB in order to further optimize and improve its design. In this paper, we take the CISB as a research object and consider the failure mechanism of its vertical progressive collapse synthetically, using SAP2000 finite element software to study the causation and strength of resistance force and internal force redistribution characteristics due to progressive collapse under different column removal conditions (long-side middle, short-side middle, internal, and corner columns) using the instantaneous unloading method. The results show that the residual structure is at risk of collapse under the corner and long-edge middle column removal conditions, and there is no risk of collapse under the short-edge middle and internal column conditions. The failure and collapse resistance of the corner column only has a beam mechanism, and the long-side middle column forms a catenary mechanism in the Y direction. The short-side middle column can resist collapse by the beam mechanism, while the internal column forms a bidirectional catenary mechanism in the X and Y directions. The axial force of the column in the residual structure is mainly redistributed to column members with adjacent spans to the failure column, and column members with separated spans are less affected.