Strengthened Structural Members and Structures: Analytical Assessment

The catastrophic earthquakes that struck several countries over the last 20 years brought to light the susceptibility of the existing building stock (Fig. 1). Old-type reinforced concrete (R/C) structures are characterized by insufficient reinforcement detailing (lack of stirrups for ensuring a certain ductility level, indirect supports, insufficient anchorages of bars), nonuniform distribution of stiffness and/or mass along the height of the building, insufficient foundation system, poor quality of materials, and various other weaknesses such as increased loading due to change of use and corrosion of reinforcement. The majority of multistory R/C buildings in southern Europe were built in the first half of the twentieth century. Structures were designed for gravity loads only by implementing the allowable stress design philosophy which did not allow any control of the mode of failure and the corresponding deformation capacity of the individual members. Taking Greece as an example, the first seismic code was introduced in 1959, and R/C walls were introduced in construction in the 1960s (often without extending up to the foundation of the building). The modern seismic codes were introduced more than 20 years later, in the mid-1980s. The multistory R/C buildings of the 1950s represent the cutting edge of the construction technology for gravity load-designed frame buildings. Information regarding the material, detailing, and geometrical characteristics of representative lowto medium-rise (up to 8 stories) R/C buildings real structures found in the urban areas built between the 1920s and 1960s is presented in Table 1 (Thermou and Palaioxorinou 2013). The lack of a continuous vertical load path along the height of the buildings is a common feature of the buildings of that era. There is no typical floor since the dimensions of the columns and beams change from story to story. Often in-plan column layout does not follow a grid pattern, hence leading to indirect supports. Representative typical floor plan layouts are presented in Fig. 2. The result of these systematic deficiencies of existing buildings is a decreased level of seismic protection, increased seismic vulnerability, and hence extensive damage expected in future seismic excitations. This is a rather alarming issue considering the socioeconomic impact of severely damaged buildings or collapses in future strong ground motions. The recommended solution for this category of buildings, which comprises the vast majority of the existing stock, is retrofitting (strengthening) with a view to upgrading their seismic capacity andmeeting the current standards for