3-D nonlinear behavior of an obelisk subjected to the Lorca May 11, 2011 strong motion record

• Behavior of simple objects during earthquakes provides evidence on ground motion.
• Field information on the 2011 Lorca earthquake is full of rotated object examples.
• An obelisk is a simple block structure with complex nonlinear behavior.
• A numerical discrete element model allowed good representation of observed behavior.

A series of very shallow earthquakes occurred in the late afternoon of May 11, 2011 near the city of Lorca and 55 km from Murcia in Spain. The main shock (Mw 5.2) occurred at 17:47 (UTC), causing 9 deaths and over 250 injured with widespread damage and panic.The strong motion generated by the event was recorded in the town of Lorca near the epicenter. It was a short duration record with peak ground acceleration higher than 0.37 g. In the vicinity of the recorded motion (360 m), there is an obelisk composed of 11 frustums resting on a 1-m-high foundation with a total height of 5.71 m. The general motion observed after the event was of the order of 1 cm (translation) for most interfaces, while at the upper two, the motion was of rotation of 2° to 4° in the counterclockwise direction seen from above.In order to understand the behavior of such a structure, numerical simulations of the obelisk were performed with the discrete element code 3DEC, employing a rigid block model. The deformation is assumed to be concentrated at the joints between the blocks, governed in the linear range by the joint stiffness parameters, in the normal and shear directions. In the nonlinear range, a purely frictional behavior is adopted, without tensile strength. An elastic joint was introduced at the base of the model, simulating the deformability of the foundation. The global model deformability was calibrated by means of in situ dynamic characterization.Dynamic analyses were performed, taking into account the large displacement effects that govern the block rocking motion, applying the recorded seismic motion in the 3 directions. Parametric studies were undertaken to assess the influence of the various input parameters. The model results are compared with the observed permanent displacements of the structure.