In order to access the spatial accessibility of emergency shelters, the relationship between the supply and demand of emergency shelters in the two dimensions of space and non-space must be comprehensively considered. Meanwhile, it is vital to understand the competitive relationship among emergency shelters. However, there are disadvantages when using the two-step floating catchment area (2SFCA) method and the improved 2SFCA method when addressing these issues. This study proposes the adaptive catchment size 2SFCA (A-2SFCA) method to calculate spatial accessibility values, which can work alongside the two relationships mentioned above. The analysis procedure of the A-2SFCA method has two stages. Firstly, this method adjusts the catchment size of the shelters by observing how crowded they are and repeatedly using this statistic in a service subset. At the end of this stage, every catchment area is determined. Secondly, the catchment areas are used to calculate the spatial accessibility values. The method was used to study a region in the Tianjin urban area in China. The proposed A-2SFCA and fixed-coverage-based two-step floating catchment area (FC2SFCA) methods are employed to measure and compare the spatial accessibility values. The result shows that the spatial accessibility in Tianjin urban area is unstable. The spatial accessibility result obtained from the A-2SFCA method is more reasonable than the FC2SFCA method when analyzing the reasonable catchment areas of emergency shelters. The A-2SFCA method provides a method for determining the catchment size of public service providers, which can be used for the accessibility analysis of various other public facilities.

Urban emergency shelters are very important public safety facilities. This paper analyses the spatial accessibility of emergency shelters in urban Tianjin by simulating the evacuation process. This process is divided into stages by using GIS spatial analysis techniques based on shelter data and house data. A big difference or ascendancy is that this method can find out the discrimination in the service supply and service demand. The difference can reveal the competition among them. The experience results show that, except for Hebei District, which needs to increase the construction of evacuation sites, the spatial distribution of emergency evacuation sites in other districts and counties in Tianjin is reasonable, but there is a need to improve the utility rate of large and medium-sized evacuation sites in terms of planning, construction and management.