Full-scale dynamic testing of an 11-story RC building and interpretation of the results from the seismic-resistance viewpoint

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1 Full-scale dynamic testing of an 11-story RC building and interpretation of the results from the seismic-resistance viewpoint I. Iskhakov * & Y. Ribakov * ' Department of Engineering, College ofjudea & Samaria, Israel * Department of Civil Engineering, Technion - Israel Institute of Technology, Israel Abstract A description is given of the structural scheme of an 11-story RC building with a flat-slab and a braced frame system. The building underwent resonance vibration and impulse tests to the point of cracking and other damage in the load-bearing elements and their joints. The fact that the building is designed for seismic regions gave rise to the problem of interpretation of the results (mainly, the dynamic parameters and the nature of the damage) from the seismic-resistance viewpoint. As is known, the special pattern of the earthquake substantially affects the dynamic behavior of a building. Accordingly, a theoretical analysis was carried out for different patterns. Comparison of the experimental and theoretical data indicated the type of earthquake to which the building adjusts best. In order to adapt it to other types, an optimized damping system is proposed, permitting improved dynamic behavior of the building under different seismic impacts. 1. Introduction Many RC structures are designed according to conventional seismic design practice. In this case the structures survive severe earthquakes with inelastic action in critical regions, such as column - foundation and column - slab joints. El-Attar et al. (1991) investigated a three - story light RC building 1/8 scale model. Bracci et al. (1992) tested an one - third scale model structure designed

2 332 Structures Under Shock and Impact VI for gravity loads only. A part of a three - story building with a beamless frame structural system was investigated by the authors (Iskhakov et al, 1999). A full - scale dynamic test of a building structure performed in this study is a logical extension of the previous work. The dynamic behavior of an 11 - story precast beamless building was investigated. The seismic loads were simulated using a vibration machine. The measured response was then compared to its analytically obtained counterpart with a view to recommendations on using this structure in higher seismisity zones. With this aim, a base isolation system was proposed for implementation. Numerical results demonstrated the effectiveness of the proposed approach. 2. The construction scheme and the structural system The beamless precast structure consists of columns with 6 m spacing in plan in both directions and a beamless slabs (Fig. 1). The columns are precast units with small unconcreted parts in the zones of column-slab connection. The columns are precast for two or three story and have a cross section of 40x40 cm. The slab has a thickness of 16 cm and consists of three types of precast units - overcolumn, between columns and middle plates. In this structure the frames are designed to carry the vertical loads while the horizontal loads are resisted by rigid diaphragms placed in the transverse direction and struts in the longitudinal direction. The thickness of the diaphragms is 16 cm. This construction scheme has precast marginal wall panels on columns. The foundation of the structure is a monolithic raft plate over the whole area of the building. The raft is provided with projecting sockets under each column to ensure anchoring of the columns in the foundation R. D k f--r l^isif-~tt i i i i i i I ^ # # j# o^# #: i!i i i- Struts H *~ g -_ - _ J " m# L- ' " J 1401,320! ^ 60 0 ^ 600 ^4 * 150 Figure 1: The investigated building

3 Structures Under Shock and Impact VI The vibration tests The objectives of these tests were determination of the building dynamic parameters under microseismic oscillations and vibration resonance actions; analysis of the building condition and its joints, diaphragms and struts in the test process and determination of the building dynamic reserve in the elastic and nonelastic stages. Vibration was achieved by means of a machine consisting of 5 blocks, actuated by two electric motors working simultaneously. This machine was installed at the center of the upper floor on a horizontal steel frame connected to the slab with steel bars of diameter 12 mm at 40 cm spacing. The frame consisted of I -beams with 140 mm in height and was placed in horizontal direction with concrete of 7 cm thickness between the elements (Fig. 2). Figure 2: General view of the vibration machine and the frame For recording of the building's dynamic parameters seismoreceivers and an oscilloscope were used. Before the tests the seismoreceivers were checked with a special vibration device. Fig. 3 shows the layout of the measuring apparatus on the floors of the building. In addition, the building was inspected and all faults were recorded. After that the natural oscillation period of the building was determined by applying a shock at the upper level using a concrete block with dimensions lxlxl.5 m. Tests were carried out in steps. In each step the unbalancing loads of the vibration machine were increased corresponding to the required equivalent

4 334 Structures under Shock & Impact VI, C.A. Brebbia & N. Jones (Editors) seismic forces. After each step the building and its joints were inspected. At the end of the tests the oscillation periods of the building in the transverse and longitudinal directions were determined. 180 " 212 S5S, 600 ^ 600 Jj 600 J, 600 J, 600 J Figure 3: Distribution of the apparatus in the building 4. The results of the dynamic tests According to the experimental results, the periods of free oscillation of the building were s in the transverse direction and s in the longitudinal direction, respectively. In thefirststep the unbalancing loads were 1440 kg and the obtained oscillation period 0.65 s; the sidesway at the end slab level was 4.5 mm. After the first step the unbalancing loads were increased to 2720 kg, 3240 kg and 3280 kg successively. Table 1 shows the experimental values of the horizontal displacements due to the effect of a seismic force

5 Structures Under Shock and Impact \ equivalent to 0.15g; the seismic forces at the floor levels, and the shear forces at different levels in the diaphragms. Table 1. Experimental results Story DispL, cm Seismic forces, kn Shear forces, kn The maximum oscillation period obtained in these tests was 0.72 s in the transverse direction and the end free oscillation period was 0.70 s. The increase in the oscillation period in the transverse direction shows that some of the structure's rigidity was lost, due to cracking and local failures. The cracks appeared, at the following sites: in the columns due to the effect of bending and torsion moments (Fig. 4a, b); in the diaphragms due to diagonal tensile stresses (Fig. 4c); in the monolithic joints between the precast elements etc. The local failures took place at the joints of the struts with the columns. 5. Numerical simulation In order to study the influence of different seismic motions on the structural behavior, further numerical investigation was carried out. The following three seismic excitations were used as input in the analysis: El-Centro (1940), Eilat (1995), Kobe (1995). These excitations were scaled to peak ground accelerations (PGA) of 0.1 g, 0.15g, and 0.3g. An initial damping ratio of 2% was assumed for the first vibration mode of the structure. The analysis was carried out using MATLAB routines (MATLAB, 1993). The peak responses of the structure to the above specified earthquakes are presented in Table 2.

6 336 Structures Under Shock and Impact VI *J&+~ *+*. (a) (b) (c) Figure 4: Cracks and local failures: (a) and (b) in columns; (c) in a diaphragm

7 Structures Under Shock and Impact VI 337 Table 2. Peak responses of the structure to different earthquakes PGA El-Centro Eilat 0.3g Displ., cm BS, kn g Displ., cm BS, kn O.lg Displ., cm BS, kn Kobe The results of the numerical simulation show that the experimental structural response is close to that of the analytically obtained behavior under the 0.15g - version. For stronger earthquakes it was proposed to use a base isolation system (BIS) (for example, the sliding Isolation System, Al-Hussaini et al. 1994), designed so that the structural vibration period was 1.4 s. According to "Design provisions for earthquake resistance structures" (The Standards Institution of Israel, 1995) the above - mentioned increase in the vibration period leads to the desired reduction in the structural response. The analysis was performed for the structure with the described BIS, subjected to the specified earthquakes scaled to PGA = 0.3g. The peak response of the base - isolated structure is presented in Table 3. Table 3. Peak response of the base - isolated structure Seismic motion El-Centro Eilat Kobe Roof displacement relative to the first-story columns base, cm Base shear force, kn Conclusion On the bases of the tests it can be concluded that the building stands safely seismic loads with PGA up to 0.15g. However several structural elements were damaged and cracks opening was obtained. In order to obtain safe behavior of the structure under stronger seismic loads was proposed to use a BIS. According to the analytical results implementation of such system significantly reduces the structural response. Thus the structural with BIS can be recommended for high seismic zones with PGA up to 0.3g.

8 References Structures Under Shock and Impact VI [1] Bracci, J. M., Reinhorn, A. M., & Mander, J. B. Seismic Resistance of R.C. Frame Structure Designed Only for Gravity Loads: Part 1 - Design and properties of a One - Third Scale Model Structures. Technical Report NCEER , National Center for Earthquake Engineering Research, SUNY Buffalo, [2] El-Attar, A.G., White, R.N. & Gergely, P. Shake Table Test of a 1/8 Scale Three - Story Lightly R.C. Building. Technical Report NCEER , National Center for Earthquake Engineering Research, SUNY/Buffalo, [3] Iskhakov, I. & Ribakov, Y. Experimental Investigation of the Nonlinear Dynamic Parameters of a Structural Part under Impulse Loading. Proceedings, International Conference on Earthquake Hazard and Rick in the Mediterranean Region, Nicosia - North Cyprus, [4] MATLAB, High Performance Numeric Computation and Visualization Software User's Guide, the Math Works Inc., [5] Al-Hussaini, T.M., Zayas, V.A. & Constantinou, M. C. Seismic Isolation of Multi - Story Frame Structures Using Spherical Sliding Isolation Systems. Technical Report NCEER , National Center for Earthquake Engineering Research, SUNY, Buffalo, 1994.