A new energy dissipating device and its application to pinned connections in precast structural systems

RC structures consisting of single story precast frames with pinned beam-to-column connections are widely used in Turkey and globally, mostly as industrial type of structures. The seismic design of these type of structures is performed as the plastic deformations concentrate on lower end of column where the horizontal rebars are intensified while the beam-to-column connections remain in the elastic range. Some of these type of precast structures had suffered extensive damages during recent major earthquakes (Ceyhan (1998), Kocaeli (1999), Düzce (1999)) in Turkey. The damages had been generally concentrated on the pinned connections as failure of the dowels or at the column base as large plastic deformations. The damages on the structural members are suffered because of plastic deformations occurred on mechanical properties of materials by cyclic plastic effects during earthquakes. Existing seismic codes aim to limit the plastic deformations and to concentrate them into special regions located in structural elements, thus a part of the energy that is released during earthquakes can be absorbed by plastic deformations. However, because of the need to large displacement for the formation of limited structural damage and the need to hysteresis in the regions where the energy is exhausted, the non-structural elements can be extremely damaged. Even though the damage on the structural and non-structural elements can be repairable, repair technique will affect the usability and repairing time of structure. The form and amount of damage, especially in the industrial buildings, might cause by-damage on high cost investments that are located inside the structure. As well as economic losses might be larger if the manufacturing process in these structures cannot be activated for a long time. In recent years, energy dissipating devices began to be installed into the structures to provide additional damping and/or stiffness, so that the behavior of the structures under seismic loads can be improved and the demand of the structures can be reduced. Energy dissipating devices can be installed into the structures which are going to be built as well as the existing ones. It is possible to concentrate the damage on the energy dissipating devices, which are easier to replace than the structural elements. The studies in this Ph.D. thesis can be grouped in two main parts. In the first part a special lead extrusion damper (LED) is developed. The LED has some differences from its similars with some features that significantly simplifies the manufacturing process. In the second part of the study, experimental and numerical studies are conducted on the application of LED to the pinned beam-to-column connections used in precast construction to improve their seismic behavior. Dynamic tests are performed with variable frequency and displacement thresholds to obtain some characteristics of LED through an experimental setup in Structural and Earthquake Engineering Labotatory (STEEL) of Istanbul Technical University. The force-displacement relationships, energy dissipating characteristics and equivalent damping properties of LED are amongst the obtained results. Earthquake acceleration records, which are downloaded from PEER Groundmotion Database with a target design acceleration spectrum that is defined in Turkish Earthquake Code 2007 (TDY 2007), are used in the experimental and numerical studies. The east-west component of Erzincan earthquake (1992) is adopted in shake table tests with different scales, while ten acceleration records are used in numerical studies. The nearly 1/2 scaled specimens which are representing one half of the single story precast RC frames with pinned beam-to-column connections are tested on the shake table facility. Pinned beam-to-column connections of precast members are made of two dowels with a diameter of 14 mm that protrude from the top of the column end enter into waiting sleeves with a dimension of 60×40 mm inserted in the beams. The sleeves are filled with nonshrinking mortar and the dowels are bolted at their top. In the first stage the bare connection and in the second stage the connection equipped with the LED is examined through shake table tests. The LED is installed in the beam-to-column connection region diagonally with 45o from beam and column elements. Top displacement, column base rotation, beam-to-column connection rotation, rebar deformation at the base section of the column, the accelerations on the top of the foundation and beam of the specimen, the displacement and force of LED are recorded during the skake table tests and the differences between two specimens are examined. The numerical models of the specimens are built with using Seismostruct software. The beam and column are idealised with frame elements while the connection of the beam to the column is idealised by a bending hinge. The infinitely rigid frame type elements are used for the accurate modeling of geometry and mass location of specimens. The additional masses are concentrated on the elements that are defined on the top of the beam. The LED is represented by a bilinear link element with its initial stiffness, yield force and post yield hardening ratio characteristics. The 0.10, 0.25, 0.50, 0.75, 1.00, 1.25 and 1.40 scales of the east-west component of Erzincan earthquake acceleration record are combined and an acceleration-time function is built for the nonlinear time history analysis. The results that are obtained from the numerical models are compared with the experimental results. It is observed that i) the behavior of the connection equipped with LED at different inelastic behavior levels can be represented sufficiently, ii) the general behavior characteristics of the connection equipped with LED is improved in comparison with the bare connection. The deformations of beam-to-column connections, the deformations at the base section of the column and the damage propagation on the columns are diminished in the connection equipped with LED. Finally, the improvement of seismic performance of an industrial type precast RC building with LEDs is exemplified. In the frame direction, the LEDs are installed into the beam-to-column connections diagonally. The LEDs are installed horizontally via chevron braces and girders in the orthogonal plane. A three dimensional analytical model of the industrial type building is generated by using Seismostruct software. The force-displacement relationship of LED, which is obtained from the accomplished experimental works, is assigned to the numerical model. Nonlinear time history analysis are performed for the bare and the LED installed structure for ten distinct acceleration records. Seismic performance of the bare structure and the structure with LED are compared by means of average values of maximum structural demands. The LEDs reduce the top displacement demand by % 40 in the frame plane and % 90 in the orthogonal plane. Similarly, the curvature at the base section of the column decreases % 54 and % 93 respectively. The deformations on unconfined (cover) concrete, confined (core) concrete and rebars at the same section lessen by % 60. As a general conclusion, the behavior of the pinned beam-to-column connections used in precast RC structures under seismic loads can be improved with the installation of the developed LED. Hence, the LEDs can be used for seismic retrofitting of precast frame type structures with pinned beam-to-column connections.