Correlation of damage index and natural frequencies of SMRF steel structures

Damage Index quantifies the damage level on a scale of 0 to 1, which represents undamaged to severe damage. The decrease of natural frequency is an indication that damage to the structure is ongoing. The study of damage index and its correlation to the change of natural frequency was performed on three low-rise SMRF steel structures, two stories with one and two bays and six stories with two bays structures. The index was determined according to the Park Ang equation. The study was conducted numerically by using OpenSees software. A validation model based on an experimental study conducted by other researchers was carried out to ensure the FE model could represent the actual structure. Semi-cyclic pushover analysis was performed to obtain the parameters needed to calculate the index. It is found that severe damage with the damage index close to 1 causes stiffness degradation to the structure, which is resulting in a decrease of natural frequency. This study helps to understand the structural behavior and makes damage assessment using natural frequency usable in structural health monitoring.