Earthquake Resistant Performance of Reinforced Concrete Building Retrofitted by Multi-Story Steel Braces

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Reversed cyclic loading tests were carried out for RC plane frames strengthened by multi-story steel braces. Two specimens with a quarter scale to actual buildings were tested which had three bays with each 1000 mm span length and two stories with the height of 800 mm, placing multi-story steel braces at the center bay. The failure type of RC unit frame containing multi-story steel brace was chosen as a test parameter. Specimen No. 1 was designed to develop the rotation of base foundation due to the uplift of multi-story steel braces. On the contrary, Specimen No. 2 was designed to result in flexural failure at the base of steel brace in a first story which is caused by both yielding of whole longitudinal bars in a RC tensile edge column of the brace and pull-out of anchorage bars bridging between horizontal chord of steel brace and RC foundation beam (called as the failure of Type 3). Cross section of a steel brace was H-shape with 60 mm width and 60 mm depth, which was built by weldi … More ng flat plates with 6 mm thickness. Concrete was cast in the horizontal position using metal casting form. Concrete compressive strength was 30 Mpa by cylinder tests.Top lateral force was applied alone at the center of the frame specimen by two oil jacks, keeping each column axial force constant. For footings of Specimen No. 2 were connected to RC reaction floor by PC tendons. For Specimen No. 1 designed to cause the uplift of multi-story braces, on the other hand, two footings under the steel brace were not connected to the floor, but lateral reaction force was supported through round steel bar inserted between RC footing subject to axial compression and steel reaction plate settled on reaction floor.For Specimen No. 1, uplift of base function occurred at the top drift angle of 0.2 %. Collapse mechanism was formed at the top drift angle of 0.7 %, developing flexural yielding at the end of boundary beans and the bottom of first story bare columns. Lateral resistance capacity in Specimen No. 1 decayed gradually due to concrete compressive failure at these hinge regions after attaining the maximum strength at the top drift angle of 1 %. Lateral resistance capacity dominated by base rotation of multi-story braces in the test agreed well with that computed by quick analysis taking the confining effect of both boundary beams and foundation beams on uplifting into account. For Specimen No. 2, all longitudinal bars in RC edge column of a brace yielded in tension at the top drift angle of 0.3 %. Lateral force resistance reached the maximum, forming plastic hinges at all beam ends at the top drift angle of 1 %. Hereafter lateral resistance diminished abruptly by the concrete compressive failure and the fracture of column longitudinal bars at the bottom of both edge columns of steel brace. Less