Base isolation benefits of rocking and uplift



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Texas Tech University


During strong ground motions, the lateral forces acting on a building may cause it to rock and uplift. In order to avoid this rocking and uplifting, unnecessary dead weight, oversized projections, or other special techniques have traditionally been used to anchor the buildings to their foundations. These precautions, however, directly or indirectly lead to higher costs for buildings. However, as shown in this study, the buildings can benefit from rocking and uplift. Therefore, new construction technologies may be developed to use rocking and uplift as a means of base isolation.

Previous works on the rocking and uplift problem were conducted primarily to study the criteria of overturning, uplift ratio, maximum displacement, shear force at the base, and motion response of model structures. In most of these studies, structures were restrained to rock in the excitation plane only. Therefore, results obtained from them may not be valid for true 3-D rocking. A few works based on the complex 3-D finite-element method were tried with success, but they suffer from expensive computation time. Consequently, a new method that uses moderate computing time while considering 3-D rocking and uplift is needed.

In this study, a real building and its interactions with its foundation are modeled by a rigid cylinder standing on distributed linear springs and dashpots. To simulate the low tensile strength of soils, springs are assumed to bear compression only. Whenever springs are about to be in tension, uplift occurs. The external excitations are simulated earthquakes with acceleration components in horizontal and vertical directions, and full 3-D rocking of the model structure is considered. The exact system equations are derived by the Lagrange equation method and solved by numerical integration. Parametric studies are carried out to obtain the influence of different parameters on the motion responses such as vertical displacement, acceleration, and tilt angle time history. The stresses of several selected points inside the model structure are also calculated by using simple beam theory, and their time histories are plotted. These results are first compared with results of the "bonded case" where the base of the building is always attached to the foundation. Then, they are further compared with the results of the "fixed-base case," where the foundation is assumed to be rigid and the model structure is fixed on it.

These plots clearly show that during an earthquake, both the accelerations and stresses of the model structure are reduced by allowing rocking and uplift. Rolling motion is clearly shown in all simulation cases. Furthermore, rolling can smooth the rocking motion and consequently reduce impact effects. Therefore, it can be concluded that this new model is an improvement over the 2-D models for simulating 3-D responses of structures.