NUMERICAL AND EXPERIMENTAL INVESTIGATION OF A CHILEAN BRIDGE-SOIL SYSTEM

An actual traditional Chilean highway bridge is analyzed in order to carry out a detailed nonlinear analysis of the structure subjected to strong ground motions. The selected non-skewed bridge was built in 2008; it has five spans of 30 m long and is supported by four multi-column bents and two seat-type abutments. The superstructure is supported over traditional Chilean elastomeric bearing pads and is located in a particular seismic area, where several bridges experienced significant damage during the mega Chilean earthquake in 2010 (Mw=8.8). In this context, a campaign of field tests was performed, allowing to estimate the predominant vibration frequency, the dispersion curves, and the shear wave velocity profiles of the soil using surface waves and the H/V spectral ratio methods. Overall, the soil displays a predominant vibration frequency of 0.5 Hz and has an average shear wave velocity of 280 m/s throughout the first 30 meters depth. Moreover, gravimetric studies indicate that the bedrock is located at 220-280 m depth. These local soil conditions make that the structural seismic demands can be significantly amplified. Based on the geophysical campaign, the soil profile beneath the bridge was also determined. In a parallel effort, the dynamic properties of the bridge were identified from acceleration records that were measured under regular traffic excitations. Frequency analysis and the stochastic subspace identification (SSI) method were employed to estimate the modal frequencies and mode shapes of the structure associated with low amplitude excitations. Overall, the longitudinal deck frequency was 1.98 Hz, the transversal deck frequency was 2.4 Hz, the vertical deck frequencies (for each span) were 3.85-3.99 Hz, and the warping modes of the deck were 4.50-4.64 Hz. The four multi-column bents were also measured, identifying natural frequencies of 3.8-4.2 Hz along the longitudinal direction. A 3D bridge-foundation-ground model was made in the software OpenSees. This simulation shows that the local amplification related to a reduced soil domain of 32 m beneath the bridge can be secondary in comparison to the amplification that occurs through the single soil column when the bedrock is deep. Additionally, it was observed that the radiation damping is 1-3% for the studied traditional Chilean bridge. The soil experienced plastic deformation due to the Soil-Structure-Interaction, generating hysteretic damping and also residual ground displacements of the bridge support (implying residual structural stresses). Based on these analyses, it can be observed that the Chilean Bridge Design Code (BDM) can be ineffective to estimate the seismic demand in base of traditional geotechnical prospections when the bedrock is deep, observing that geophysical techniques should be also used to classify the site amplification. In case that the basin is deep, seismic hazard study are required to evaluate the seismic demand for design of Chilean bridges.