Experimental Study on Seismic Shear Amplification on Walls With Discontinuities

Abstract eng:
RC structural slender walls under large seismic excitation are expected to behave nonlinear due to yielding of the longitudinal reinforcement, which produces plastic hinges usually at the base of the structure. This occurs due to the fact that design uses reduced forces when the response is compared with linear models for an earthquake or spectra. It is common that the base moment reaches its capacity mainly affected by the first vibration mode. However, the base shear could be affected by the higher modes once yielding in flexure has occurred, which might lead that the base shear is underestimated. This phenomenon is known as dynamic shear amplification, which is not incorporated in the Chilean design code. To investigate shear demand due to higher modes, an experimental program is accomplished on five test specimens 1:10 scaled. The test program considered cantilever RC shear wall with rectangular cross section and a total mass of 1 ton, lumped at 5 levels. All five specimens considered discontinuities common in construction and were only defined at the base of the specimens (door opening, setback) and 1 specimen did not consider shear reinforcement or boundary detailing. They are mounted on a unidirectional shaking table to impose seismic input. The specimens are 2.15 m high, 15 cm long and 4 cm thick. The tests are performed to archive nonlinear effects and higher mode excitation in order to study dynamic non-linear shear amplification. Indeed, experimental data of the phenomenon is obtained for walls built according to local practice, and results from a base wall is compared with wall with discontinuities that presented problems for the 2010 Chile earthquake, checking the amplification impact in these structures. Specimens are tested with a generated synthetic signal based on the Constitution record from 2010 Chile earthquake scaled in time, as well as the Llolleo record from the 1985 Chile earthquake. All specimens are tested under incremental ground synthetic (Constitution) record scaling the acceleration from 10% to 200% of the base earthquake, and also with the Llolleo record scaled by 100% and 150%, when possible. Damage is concentrated at the wall base for all specimens; primary due to flexure with some participation of shear. For the synthetic record an average amplification of 1.3 is obtained, and a decrease in height of the resultant equivalent lateral force closes to 0.4hw. By increasing the intensity of the input record, amplification grows to an average of 1.8. While it decreases drastically when subjected to input records with low frequency content (Llolleo). No significant difference is observed in shear amplification with specimens with a base central opening, nor with the flag wall, even though the cracking and failure mode was different for such specimens. Ductility demand shows no correlation when 2 different earthquakes are considered, whereas the frequency content and Arias intensity (Ia) of the input record directly affected the shear amplification.

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