Use of thin steel infill plates for seismic retrofit of low ductile reinforced concrete frames

Turkey is located in an active earthquake zone. Therefore, earthquake resistant design of buildings and determining structure performance under seismic effects are critical points for structural engineers. In many regions in Turkey, espicially in İstanbul, most of existing buildings are in danger of collapse because of lack of engineering support and being used poor quality material. Even so some of buildings designed and constructed under control of engineers, they do not have required performance level which is stated in contemporary seismic codes. As a result of this, to improve performance level and ductility of the structure, various retrofitting methods are applied and some structural elements are added to system. For reaching demanded earthquake resistance in codes, most prefered two methods are adding new reinforced concrete shear walls and retrofitting with steel braced frames. Even if positive effects of these traditional retrofitting methos on earthquake resistance of structure are proved experimentally, new strengthening methods are researched because of application difficulties of adding reinforced concrete shear walls and additional masses transfered on existing buildings by them. At this point, the steel plate shear wall systems stand out due to ease of application and low weight. In recent years, steel plate shear walls reveal excellent performance and ductile behavior during earthquakes in several countries. These show us that shear walls which consist of thin steel infill plates are usefull for retrofitting existing buildings against seismic effects. The studies about behavior of reinforced concrete frames which are strengthened by using steel plate shear walls are proceeded. In this study that is presented as master thesis, changes in structural properties such as story shears, energy absorption ability and rigidity of a single span low ductile reinforced concrete frame are investigated after the frame is retrofitted by using thin infill plates. Studies are carried out theoretically as well as experimentally. For that purpose, analytical models and test speciemens is prepared for three different type of frame. First of them is produced to determine structural properties of low ductile reinforced concrete frame which is not retrofitted. Other types of frames are designed as shear walls which are composed of a reinforced concrete boundary frame and thin steel infill plates. It is aimed in this study to see if an additional ductility will be provided adding steel infill plates. Difference between last two type is elements that steel infill plates conntect to. In one of them, thin steel plate is connected to all structural elements such as beam, columns and foundation which is bounded it. However, in the other type, steel infill plate is connected only to beam and foundation, not to columns. The aim here is to determine effects of retrofitting type on structural properties of a strengthened reinforced concrete frame. These frames are ½ scaled form of one of the moment frames of a single story building. The frames have one span in length 1,700m and their story height is 1,525m. Column dimensions are 0.250mx0.200m and beam dimensions are 0.325mx0.200m. Besides, a reinforced concrete slab in dimension 0.82mx0.12m is located to top of the beam for considering slab effects on reinforced concrete frame. Foundation system is continuous footing that has 3.00m length, 0.60m width and 0.50m height. Axial compression strenth of concrete is chosen as 10MPa and axial tension strenth of rebar steel is selected as 275MPa. Given material strenths of reinforced concrete members are determined by considering low ductile behavior of existing buildings which were constructed by using poor quality material and are needed to retrofit as soon as possible. There is not any confinement zone on beam and columns. Also, all reinforcement steel elements are unribbed. Thin steel infill plate which is used in retrofitted specimens has 1mm thickness and is in S235 quality. By using all informations given above, analatical models and test speciemens are prepared. Analytical models are developed by using SAP2000. In this process, strip modelling technique is used for modeling thin steel infill plates. According to this tecnique, steel infill plate is divided to 23 tension-only pieces (strips) which have 45o angle with horizontal axis. Each strip is connected as pinned to existing reinforced concrete beam and columns and plastic hinges are defined at strips' mid-points. As a result of using this technique, in model, beam and colum elements are diveded where they have intersection point with strip elements and plastic hinges are defined at start and end points of these column and beam parts. Foundation is not attached to analytical models and all points at which columns connect to foundation are defined as fixed support and it is not allowed any rotation and translation. Concrete material properties are defined by using linear inelastic methods given in "Specification For Structures To Be Built In Disaster Areas Information Annex 7B" (Turkish Seismic Code, a.k.a. DBYBHY 2007 Bilgilendirme Eki 7B ). This model also known as "Mander Material Model". Inelastic material properties of reinforcement and retrofitting materials are taken also from same chapter of mentioned specification. Sections are defined by using Section Designer, which is a module in software SAP2000. All axial force - moment interaction curves and moment – curvature curves are obtained through this module. After modelling step, systems are anlayzed in SAP2000 with displacement controlled nonlinear pushover method and base reaction – displacement curve of system is obtained. As a result of this process, changes in structural properties of reinforced concrete frame after retrofitting is reported. Connection details are designed accordingly "American Institute of Steel Construction 360–05 : Specification for Structural Steel Buildings (ANSI/AISC 360-05)". At this point, it is assumed that yielding strees of steel infill plate will be a limitation for the system stability. On this basis, it is supposed that the forces which act on connections will reach their maximum when yield occurs on steel infill plate. Based on these assumptions and mentioned code, element load bearing capacities are calculated and details are designed. Connection between thin steel infill plate and reinforced concrete elements is consituted via built-up T-Shape steel base profiles which have 20mm flange thickness and 6 mm web thickness. Behavior of this bolted T-Shape base profile under tension force is determined by using ultimate prying models. Infill plates are connect to the T-Shape base profile by using pre-tensioned M16 bolts. These connections are designed as slip-critical connections. The T-Shape steel base profiles are conntect to reinforced concrete elements by using M12 post-installed anchor rods. All bolts and rots be used in connections are in 10.9 quality (Yielding stres is 900 N/mm2, Ultimate tension strength is 1000 N/mm2 ). After connection details are designed, all datas whic are needed for producing test specimens are obtained. Specimens which are used in experimental studies are produced in İstanbul Technical University Structural and Earthquake Engineering Laboratuary (ITU STEEL). With this aim, first of all, concrete mix design is prepared. In accordance with this design, cubic specimens are produced and tested for determining its compressive strength. Also, all steel elements are tested for determining their tensile strengths. During producing process of frame specimens, strain gauges are located on rebars at nearby to beam – column nodal points, system support points and mid-span of beam because these are probable plastic hinge points on the frame. There is not any confinement zone on beam and columns. Spacing between two confinenment bar is 0.25m. All frame loading tests are performed in ITU Structural and Earthquake Engineering Laboratuary. Results of this experimental study are investigated and compared with analytical results which are obtained through SAP2000 models. This master thesis that summarized above is consist of six chapters. In first chapter, general informations about traditional retrofitting methos, steel plate shear walls and frame systems which are used in this study are given. In chapter two, recent researches about topic are given chronologically. Chapter three is about preparing process of speciments and analytical models developed by using SAP2000. Chapter four contains informations about design of speciments and connection details. In chapter five, results of material tests of rebar, structural steel, anchor bolts and concrete are given. In chapter six and seven, observations and results obtained from experimental studies are given.