Strengthening of rc structures with external energy dissipating rc walls

Earthquakes constitute a great risk in Turkey from past to present. Strong tremors caused even death tolls up to thousand lives in a single shaking. Understanding the response of common structural types to strong ground shakings analyze the probable mitigation measures can lead to construct more reliable structures. In old cities, such as Istanbul, the large portion of the building stock consists of pre-code structures that are vulnerable to strong shakings. To prevent big casualties in earthquakes, performance of a large portion of the existing building stock has to be improved. For a structure that is considered under a considerable collapse risk during an earthquake, there are two feasible options to be followed. The first options is that the vulnerable structure may be demolished and re-built. The second option, obviously, is to retrofit the building and extend its economic life. Either way is acceptable depending on parameters of economical, juridical, social and technical nature. There are several methods for strengthening a building, but especially one of these methods is very commonly used for their ease in construction and rather low cost. This common method is the conventional way at which RC shear walls are added to the structure and/or columns are jacketed. In this thesis, energy dissipating steel cushions are designed to be used in strengthening mid-story residential RC structures. The dissipator cushions were developed and tested at ITU within a European Union-funded research project. The shape of the cushion members is elliptical while the thicknesses tested at ITU vary as 3, 5 and 8 mm. The width of the cushions tested is 10cm, however in this thesis 20 cm and 30 cm wide versions are used. Two reference buildings are modelled in order to compare cladding system and bare frame buildings. First building is a 4 story RC structure and has 3 bay in both X and Y directions. Story height is 2.8 m and bay width is 4.00 m. All beam section are 40x20 cm and all columns sections are 45x45 m. Second building is a 6-story and has 3 bay in both X and Y direction. Story height and bay width is same as the first model. There are 3 different column sections in the 6-story building. They are 70x70, 60x60 and 50x50 cm. First 2 story are 70x70 cm, next 3 stories are 60x60 cm and the top story is 50x50 cm. All beam sections are 50x25 cm in 6 story building. Both frame buildings are analyzed in non-adaptive (1st mode dominated) pushover analysis to obtain reference values. Lateral base shear capacity of the buildings are obtained together with their deformation capacities. The frame members are then examined to reach the overall performance of the structures based on the descriptions given in the Turkish Earthquake Code of 2007. It was found that the both structures, designed to represent average mid-story pre-code structures in Turkey, did not satisfy the Life Safety level since they both indicated the Collapse at target displacement. To increase the lateral load capacity of the both buildings, prefabricated RC walls, to be attached outside, connected with cushions to the structure, are used. The dimension of the RC wall members are (thickness x width) 30x400 cm. The dimensions of the cushion members of the structure depend on the location they are mounted. Under the RC walls, cushions need to bare the weight of the RC walls, therefore large axial loads are carried with a neoprene material placed into the cushion. The reference values for cushion-neoprene composite connectors are again taken from the tests conducted at ITU laboratories. In the 4-story building, 3 cushion members with 30cm width and 8mm wall thickness are used. In the first 2 stories, 3 cushion members with 5mm wall thickness are used. The width of these members are 20 cm. finally, in the last 2 stories, 3 cushion members, which have 20 cm width and 8mm wall thickness, are used. In the 6-story building, the first 3 stories, 3 mm wall thickness cushion members are used. The width of these members is 30cm. In the next 3 stories, 5mm wall thickness cushion members are used, and width of these members is 30 cm. In the top story, 8 mm wall thickness cushion members are used. After strengthening, classic 1st mode pushover analysis is conducted. Lateral load capacity of the building increased up to nearly 2 times. The performance point of the structure receded to the Life Safety limit state. According to the Turkish Earthquake Code of 2007, the strengthened buildings satisfy the Life Safety limit state. Nonlinear time history analyses have also been applied to both structures for the bare frame as well as for the strengthened cases. The hysteretic response of the cushions, as well as the energy dissipating capabilities, can be better seen in a dynamic analysis. The expected behavior of the cushions is to exhibit fat hysteresis loops to dissipate energy during the cyclic reversals. Seven earthquake acceleration records are applied to the models. 28 time history analyses have been conducted in total. From the 28 time history analyses results, maximum relative displacement values are noted. Average values, as well as the standard deviations of the maximum relative displacements of each 7 accelerogram suit are found for the 4 types of buildings. Results show that maximum relative displacement of retrofitted buildings decreased considerably also in the case of the nonlinear time history analyses, rendering the structures with a Life Safety performance level. Another result that has been obtained from the time history analysis is the proportion of the base shear (i.e. fracture of the base shear carried by the cushion-wall system). It can be seen that in both buildings, the shear capacity of the columns and shear walls are nearly equal for the examine configuration. Therefore the lateral load of both frames and retrofitted systems are increased twice. The other aspect, which must be considered, is that the beams adjacent to the cushion members suffer large axial loads leading to collapse since they have low confinement. The beams should be confined before application of the cushions, may be by using technics such as FRP wrapping.