In-situ test of a existing rc building frame strengthened with k-type steel brace placed at outside of wall girders: Verification of seismic strengthening of a existing RC building

A seismic diagnosis on the then five-story Building 4 at Kanagawa University, built in 1963 and shown in Photo 1(a), was conducted after the Great Hanshin-Awaji Earthquake of 1995. This revealed that seismic strengthening was difficult, so the building was reduced in scale by about a quarter by removing the eastern wing and two uppermost stories of the existing five-story structure. The remaining three-story section as shown in Photo 1(b) was seismically strengthened with a new steel brace. The seismic capacity evaluation after strengthening indicated sufficient earthquake resistance. As it has been decided to demolish this building, we planned a full-scale in-situ test to evaluate the actual strength of this seismically strengthened building. Figure 2 presents a brace mounting diagram. A K-type steel-framed brace was installed within the frame just outside the wall girder with an additional beam attached to the outer face as indicated in Figure 3. The upright frame member of the brace frame extended to the rigid section of the existing RC columns and wall girders. A cross-section of the main member is represented in Table 3. The brace and frame members used had dimensions of H-250×250×9×14 (SS400). The test portion as shown in Figure 4 is the second story with columns cut between the ceiling and the third floor, and wall girders cut on the third floor between the X17-19 and X20-21 lines. The horizontal strength of the brace expected from the strengthening design manual1) was 2,420 kN. A horizontal force was applied as one-way repetitive loading using two hydraulic jacks. Figure 7 (a) shows the relationship between the horizontal force and second story drift obtained in the experiment. There was no loss of stiffness until the loading cycle reached 3,750 kN of horizontal force, and the story drift was less than 1/400 at that cycle. Stiffness did decline above 4,500 kN of horizontal force. Ultimately, the brace yielded and the RC columns suffered shear failure as shown in photos 2(a)-(c).The maximum applied horizontal force was 5,920 kN. The strength calculated using the strengthening design manual was 3,799 kN. The calculated horizontal strength was 5,006 kN based on the actual strength of the materials. The calculated value is the sum of the horizontal component of the yield strength of the brace calculated from Equation (1), and the horizontal strength of the RC columns with the vertical steel frame shown in Fig. 18. Flexural strength Mu of the RC columns was calculated using the ACI stress block method. Shear strength Qu was calculated using Equation (4). The experimental value proved to be 1.2 times the computed value. The push-over analysis was carried out using the section assuming the upright frame members of the brace frame used with the existing RC columns as shown in Figure 20. The analytical model was a one-story plane frame model as shown in Fig. 18. The multi-spring model shown in Fig. 20 was used as the member model for the columns. The structural centroid position of the section was considered as the center of gravity for the steel, reinforcing bars and concrete in consideration of the ratio of Young's modulus for the X18 column. For the X19 column, which is a tensile column, it is the center of gravity of the steel and reinforcing bars ignoring the concrete. These structural centroid positions are shown in Fig. 5 as CL. Elastic stiffness was assumed to be an equivalent cross section. The results demonstrate good correspondence with the experimental results, except for the stiffness degradation at 4,000 kN as shown by the solid heavy line for Case 2 in Figure 19.