1. Field of the Invention
The present invention relates to a method and apparatus for determining the vulnerability of a structure to earthquake.
2. Description of the Related Art
Conventionally, as methods for determining the seismic vulnerability of a structure are used (1) a method of obtaining the natural frequency of a structure by vibrating the structure, (2) a method of evaluating the antiseismic performance of a structure using a checklist, and (3) an antiseismic diagnostic method of calculating an antiseismic index through structural computation.
In method (1), a method of obtaining the natural frequency of a structure by vibrating the structure, a structure is vibrated using a vibration generator or a weight to give a shock, and then the natural frequency of the structure is obtained based on thus generated vibration. This natural frequency is compared with a reference value to determine the antiseismic performance of the structure.
In method (2), a method of evaluating the antiseismic performance of a structure using a checklist, a structure is evaluated using a checklist containing predetermined items such as conditions of the ground underneath the structure, year of construction of the structure, and structural Specifications. The antiseismic performance of the structure is determined based on total points assigned in the case of this evaluation.
In method (3), an antiseismic diagnostic method of calculating the antiseismic index of a structure through structural computation, any of the following three methods are used, depending on the specific case. In the first method, the ultimate strength of each layer is calculated based on the cross-sectional areas of walls and columns. In the second method, the flexural strength of each layer is obtained based on the assumption that beams and floor slabs are rigid as well as on the basis of calculated bends of individual columns and walls and calculated ultimate shear strength of the layer, and subsequently the toughness of each layer is calculated based on the relationship between this flexural strength and the ultimate shear strength, quantity of reinforcing bars, and the like. In the third method, a simple nonlinear frame analysis is conducted while taking into consideration the strength and toughness of beams.
The above-mentioned methods for determining the seismic vulnerability of a structure have included the following problems.
In the method of obtaining the natural frequency of a structure by vibrating the structure, a large-scale apparatus is required for vibrating the structure, and the structure may be damaged when the apparatus is installed or when a shock is given to the structure. Also, this method requires a reference natural frequency as a basis of comparison, such as a theoretical natural frequency, an empirically derived natural frequency, or a natural frequency measured in the past.
In the method of evaluating the antiseismic performance of a structure using a checklist, the evaluation is rough and susceptible to the subjectivity of an examiner; the vibration characteristics of the structure cannot be obtained quantitatively.
In the antiseismic diagnostic method of calculating the antiseismic index of a structure through structural computation, investigation for obtaining data for use in structural computation and entry of the thus obtained data require considerable time and expenses. Further, the diagnosis requires high-level knowledge of structural computation, so participation of a specialist is indispensable.