Seismic behavior of existing reinforced concrete frames and its seismic upgrading using energy dissipation devices
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AuthorOliver Saiz, Elena
Universidad de Granada
DirectorBenavent Climent, Amadeo
DepartamentoUniversidad de Granada. Departamento de Mecánica de Estructuras e Ingeniería Hidráulica
Hormigón armadoTerremotosEfectos de los terremotosIngeniería sísmicaEnergíaDisipación
Oliver Saiz, E. Seismic behavior of existing reinforced concrete frames and its seismic upgrading using energy dissipation devices. Granada: Universidad de Granada, 2016. [http://hdl.handle.net/10481/40942]
SponsorshipTesis Univ. Granada. Departamento de Mecánica de Estructuras e Ingeniería Hidráulica; This Thesis received financial support from the Spanish Government under a FPU fellowship of the Ministerio de Educacion, Spain (AP2009-0533).
Earthquake engineering has progressed significantly in the last few decades. However, a high proportion of the building stock located in earthquake-prone regions still exhibits serious seismic deficiencies. In fact, many of these buildings were designed before the appearance of seismic codes, in view of rudimentary anti-seismic design criteria and/or by using obsolete seismic hazard maps. A significant number of the structures that still remain under-designed have reinforced concrete (RC) frames as the main system of lateral resistance. Recent seismic events revealed the poor performance of under-designed RC frame structures (L’Aquila 2009, Lorca 2011 and Emilia 2012), accentuating the need for seismic assessment and retrofitting. In this sense, it is necessary to develop retrofitting strategies that increase the seismic capacity of existing structures and that control the level of structural and non-structural damage, within the framework of performance-based seismic design. To this end, the addition of energy dissipation devices is an effective technique that provides not only the strength and deformation capacity required to protect human life, but also the supplemental energy dissipation capacity necessary to reduce structural and non-structural damage. Among the different types of energy dissipation devices that are commercially available or under development, the so-called “hysteretic” dampers are particularly popular because of their low cost, in comparison with viscous fluid dampers or viscoelastic solid dampers. For this reason, the use of brace-type hysteretic dampers for the seismic upgrading of existing frames has increased exponentially in the past two decades. Nevertheless, adding hysteretic dampers to an under-designed RC frame structure is not straightforward and some important issues must be addressed. On one hand, the connection of the hysteretic dampers to the existing frame requires special attention. The brace may develop high axial loads and the design of the anchoring system to the existing beam-column joint may be costly and difficult to execute. On the other hand, the dampers require the frame to have a minimum lateral deformation capacity, in order to develop their inherent energy dissipation capacity. Under-designed RC frames may not posses this minimum deformation capacity required for an efficient combination of frame and dampers. For this reasons, this Thesis is focused on investigating two main issues: (i) a solution for connecting brace-type hysteretic dampers to the existing RC frame; and (ii) a hybrid retrofitting solution with hysteretic dampers that considers the possibility of including local strengthening procedures (e.g. FRP/SRP), in order to increase the lateral deformation capacity of the main frame. Regarding the first issue, a new solution for connecting concentric braces to beam-column joints of existing RC frames is investigated in Chapter 3. This solution is suitable for connecting either a conventional concentric steel brace or a brace-type hysteretic damper. It consists of (i) two shear-key steel plates fixed to the concrete with anchor bolts —which restrain the displacements of the end-plates of the steel brace— and (ii) a device for minimizing friction forces between the shear-key plates and the end-plates. Minimizing friction produces the effect of eliminating tension forces on the anchor bolts and of reducing the bending moments on the shear-key plates. Consequently, this solution allows using thin plates without stiffeners and avoids brittle failure modes on the anchors, thus reducing the number of anchors, as well as the required effective anchorage depth. To clarify the influence of some parameters (such as the initial gaps between the steel plates and the thickness of the Teflon sheets) a 3D finite element model is developed. As a result, execution provisions and design criteria of the brace-frame connection are proposed. Finally, the efficiency and validity of the proposed brace-frame connection is evaluated through shaking-table tests conducted on a 3x3x3 m3 scaled RC frame retrofitted with brace-type hysteretic dampers. Experimental results show that the braces installed with the proposed brace-frame connection successfully controlled the damage on the main frame and that the hysteretic dampers dissipated most of the energy input by the earthquake. Thus, the proposed brace-frame connection was effective in mobilizing the energy dissipation capacity of the hysteretic dampers.