Experimental studies of structure-foundation-soil interaction effect on upliftable structures

Abstract

The current earthquake-resistant design has been strongly shaped by the capacity design concept, i.e. plastic hinge development at pre-defined structural locations is permitted. However, major earthquakes, e.g. the 2010/2011 Canterbury earthquakes, demonstrated that the repair cost of damaged structures and economic losses due to downtime can be immense. Hence, in recent years the philosophy of ‘low-damage’ design has emerged. This can be achieved by allowing the structure to uplift, i.e. part of the footing is permitted to temporarily separate from the support. This work aims to understand the seismic response of upliftable structures. To investigate the structure-footing-soil interaction, rigid and laminar boxes were constructed for shake table experiments. At the early stage of the research a rigid box was used. With the development of the research a small box of 0.8 m × 0.8 m × 0.7 m and then later a large laminar box of 2m×2m×2m were developed. Shake table tests on single degree-of-freedom (SDOF) models on sand demonstrate that allowing structures to uplift can reduce bending moments and thus the development of plastic hinge. These laminar boxes are the first to have been constructed and used in the Southern Hemisphere. The results showed that the spectrum values of accelerations on the model footing are smaller than those of the free-field dry sand surface. When calculating the response of the model a significant improvement of accuracy was only achieved using the footing acceleration. A novel approach for correctly simulating model structures in one-g environment was developed and substantiated in experiments. Two multi-storey upliftable models were constructed based on both the proposed and conventional approaches. For the first time with the proposed approach key characteristics of upliftable structures, e.g. nonlinear rocking period, can be correctly incorporated in the physical experiments. Using the proposed approach, a numerical equation is derived for calculating the amplitude-dependent rocking period of structures during uplift. The period obtained leads to more accurate estimate of the maximum deformation of upliftable structures. If uplift occurs, the horizontal displacement of a structure can become larger. To control the uplift and thus the structural response, while at the same time still avoiding plastic hinge development in the structure, the weight of the footing needs to be properly determined.