INFLUENCE OF VERTICAL MASSES ON THE RESPONSE OF GRAVITY-CONTROLLED ROCKING BRACED FRAMES

The implementation of rocking systems is considered as an efficient way to design resilient buildings or to retrofit seismically deficient ones, as it reduces the amount of lateral seismic forces applied to the building. In a controlled rocking braced frame system (CRBF), the columns of the braced frames are designed to uplift from their foundations. Re centring capacity of the building may be provided by adding vertical post-tensioning elements, the gravity loads supported by the braced frames or by a combination of these elements. When the self-centring capacity is solely conferred by the tributary gravity loads of the frames, CRBFs are referred to as gravity-controlled rocking braced frames (G-CRBF). Energy dissipative devices (ED) can also be implemented on the frame (at the column base, for instance) to control drifts. The rocking frame hence exhibits a typical flag-shaped overturning moment – base rotation hysteretic behaviour during a major earthquake event. Two different numerical analysis approaches have been used in past research to study the overall response of rocking braced frames subjected to ground motions. The first approach is based on a 2D-model of rocking braced frames. Frame members are modelled using elastic beam column elements and vertical masses are added to joints to replicate the masses carried by the frame. An ED device is modelled by a nonlinear element and placed between the foundation and the column base plates. A leaning column is linked to the model to capture P-Delta effects. The second approach consists of the use of a single-degree-of-freedom (SDOF) model using a self-centring material calibrated to produce the same hysteresis as the system under quasi-static loading. The study was then extended to rocking frames for buildings of up to 5 storeys. In this case, two identical complete frame models were used except that only one model included the vertical masses. A comparative study between the seismic responses obtained with the use of these two numerical analysis approaches is presented in this paper in order to examine the effects of the vertical masses on the behaviour of the structure. Single storey chevron bracing is used as the rocking frame design and the ED device is selected to be friction based. Parameters used for the self-centring material are chosen to be equivalent to those selected for the rocking braced frame. The key difference between the two models mentioned is the ability to capture inertia effects due to rocking and induced by the vertical masses added to the joints of the 2D-model. Nonlinear analyses are then performed using the OpenSees software. Ground motions representative of the seismic hazard in Vancouver, BC, are scaled and used to compare the responses of the two numerical models. Focus is put on the drift demands, as well as on the vertical base reactions and the horizontal base shear. For single-storey frame, the results showed overall similar drift histories, but the G-CRBF model with vertical masses experienced smaller peak drift values than the SDOF model without vertical masses. Larger force demands were however observed in the G-CRBF model due to the response associated to the structure vertical vibration mode when one column is uplifted. The results also showed that these differences still exist in multi-storey rocking frames but the effects of the vertical masses on the overall response of G-CRBFs tend to be relatively less significant as the number of storeys is increased.