DAMAGE INDEX OF ANGLE BRACE BASED ON RESIDUAL DEFORMATION

Low-rise steel building structures are expected to accommodate disaster prevention activities after an earthquake. Hence these structures are required to maintain their seismic performance not only for a single seismic event, but also in the event of aftershocks. Therefore, it is very important to evaluate the residual seismic performance of the damaged steel structures by quick inspections. However, the damage degree of steel structures obtained only by visual inspection is not enough, although for reinforced concrete structures it is common practice to use the crack width and pattern of reinforced concrete members to evaluate the damage. Many encountered damage patterns of angle braces, which are widely used for seismic members in low rise steel structures, are described in reports about the past earthquakes. This paper focuses on the geometrical changes of the angle cross section and out-of-plane deformations due to flexural buckling, and describes how to estimate a damage index based on these residual deformations of the angle brace. In order to find the relation between the damage and the residual deformation, cyclic loading tests of a single angle brace were carried out. The main parameter in these tests is width-to-thickness ratio of angle section: L65x6 (length of leg x thickness), L75x6, L75x9, L75x12, L90x7 and L100x7. In addition, constant amplitude loading pattern at 0.5, 0.75, 1.0, 1.25, 1.5 and 1.75%, is selected to be a parameter of the test. The test results can be summarized as follows: (1) local buckling occurring at the middle under the cyclic loadings. After that, crack appears at one of the free edges and grows to reach fracture; (2) the residual out-of-plane deformation increases proportionally to the maximum amplitude of loading; (3) the geometrical changes of the angle cross section is increased due to the number of the loading cycles; (4) the maximum story drift and the cumulative damage to angle brace can be estimated from the out-of-plane residual deformation and the geometrical changes of angle cross section, respectively. Consequently, by combining the visual information it is possible to evaluate the residual seismic performance of an angle brace.