Georgia E. Thermou

Global Interventions for Seismic Upgrading of Substandard RC Buildings

A methodology is developed in this paper for the design and proportioning of interventions for seismic upgrading of substandard reinforced-concrete (RC) buildings. The retrofit approach is presented in the form of a simple design tool that aims toward both demand re- duction and enhancement of force and deformation supply through controlled modification of stiffness along the height of the building. This objective is achieved by engineering the translational mode-shape of the structure, so as to optimize the distribution of interstory drift. Results from the proposed approach are summarized in a spectrum format in which demand, expressed in terms of interstory drift, is related to stiffness. Design charts, which relate the characteristics of commonly used global intervention procedures to influence drift demands, are developed to facilitate the retrofit design. The intervention procedures considered in this paper are reinforced-concrete jacketing, the addition of reinforced- concrete walls, and the addition of masonry infills. The proposed methodology is also amenable to adaptation to other strengthening methods, such as the addition of cross-bracing. DOI: 10.1061/(ASCE)ST.1943-541X.0000474.

Design Methodology for Seismic Upgrading of Substandard Reinforced Concrete Structures

This article presents a design methodology for seismic upgrading of existing reinforced concrete (RC) buildings. The methodology is based on the modification of the deflected shape of the structure so as to achieve a near-uniform distribution of interstorey drift along the building height, thereby eliminating damage localization. Yield Point Spectra are utilized for the definition of demand and a direct displacement-based design approach is implemented. The fundamental steps of the method are described in detail, including a systematic evaluation of assumptions and limitations. A full-scale tested structure is used as a case study for assessment and verification of the proposed methodology. Alternative retrofit scenarios are set according to target response and performance levels. The role of the target deflected response shape and its influence on the outcome of the retrofit strategy is investigated. The viability of the alternative retrofit scenarios is studied for different ground motions including near-fault earthquake records.

Fiber-Reinforced Polymer Retrofitting of Predamaged Substandard RC Prismatic Members

An experimental study was conducted to investigate the efficiency of FRP jackets in upgrading the seismic behavior of lightly reinforced concrete prismatic members previously damaged under a combination of axial compression and a reversed cyclic lateral displacement history simulating earthquake effects. The test program comprises 13 cantilever prismatic specimens, which, owing to substandard reinforcing details representative of older construction practices in southern Europe, were susceptible to various undesirable modes of damage such as web-shear cracking, longitudinal bar buckling, or lap-splice failure. After repair, the specimens were retested using the same load combination. The efficiency of the repair options considered in the study, which refer to alternative strengthening systems (with glass or carbon wraps), was investigated with reference to the design parameters of the intervention, the type of the applied lateral displacement history, and the mode of failure that had occurred previously in the initial phase of the tests. The results provided valuable insight regarding participation of the FRP jackets in the various mechanisms of resistance, their ability to reverse the effect of initial damage, and to impart deformation capacity to the structural member.

Screening criteria to identify brittle R.C. structural failures in earthquakes

Collapse of structures in severe earthquakes is synonymous with loss of vertical load bearing capacity in the columns and walls of the structural system. This paper identifies criteria that could be used in the context of preliminary assessment in order to rapidly identify from the large inventory of existing, substandard construction, those buildings that are more likely candidates for catastrophic collapse. Proposed criteria include (i) a stiffness index in order to determine the severity of seismic displacement demand and, (ii) a base-shear strength index associated with typical column details representative of the state of practice from the era of the building’s period of construction. The criteria may be used to characterize the primary deficiencies of the building and the level of spectral acceleration that may be tolerated prior to failure. Ten buildings representative of older construction practices used in the Mediterranean countries prior to the introduction of capacity design procedures, which suffered excessive damage or collapse in past earthquakes, are used to proof-test the applicability of the procedure and the practical advantages of spectrum compatible stiffness and strength criteria that may be used in determining a proper retrofit strategy.

Preliminary Seismic Assessment Method for Identifying R.C. Structural Failures

In this chapter, an efficient method for rapid preliminary assessment of the seismic vulnerability of reinforced concrete buildings is presented. The method determines the columns’ limiting shear resistance at the critical storey of the structure, by applying a strength assessment procedure associated with typical column details representative of the state of practice from the era of the building’s period of construction and evaluates the severity of seismic displacement demand and the maximum seismic acceleration that the building can sustain by applying a stiffness index assessment. For application of the method, only knowledge of the basic geometric and material properties of the building is required. The proposed method is applied for verification reasons to two reinforced concrete buildings that failed during the 1999 Athens earthquake. It is shown that the proposed method can be used as a diagnostic tool for identification of both the building’s fragility and the prevailing failure mechanism, allowing the engineers to immediately identify the most vulnerable buildings that are likely to collapse in a potentially strong earthquake, as well as to set objectives for their rehabilitation.

Seismic Rehabilitation of Substandard R.C. Buildings with Masonry Infills

ABSTRACT Seismic deformation demands are localized in areas of stiffness discontinuity, such as in the soft storys of frame structures, where disproportionate damage is often reported in post-earthquake reconnaissance. In many parts of the world, this damage pattern is mitigated using strengthening schemes that include addition of stiffness in the structure so as to limit the magnitude of drift demands. A low-cost retrofitting method is the addition of masonry infills to increase the stiffness of soft storys in low- to mid-rise reinforced concrete (R.C.) structures. This is an easily replaceable remedy in the event of damage that may prove advantageous over R.C. structural systems, owing to the lower forces imparted to the foundation in this retrofit option as compared to more thorough interventions, thereby avoiding extensively invasive retrofit operations in the foundation. Behavioral mechanisms mobilized by masonry infills in successful retrofits are shown to emulate confined masonry behavior. It is also shown that despite their brittleness, well-connected infills can successfully mitigate the occurrence of catastrophic damage by diverting damage localization from the vulnerable regions of the building. The main objective of the current paper is to present a rapid retrofit design methodology, where masonry infills are utilized for strengthening existing substandard constructions in order for their R.C. load-bearing elements to behave elastically in the event of the design earthquake. To facilitate the retrofit design, practical design charts have been derived, to link drift demand to the ratios of infills’ area in plan to the total plan area in the critical floor of the structure. Performance criteria, such as target distributions of interstory drift demand, a target estimate of the fundamental period, as required by the designer, and a limit on acceptable displacement ductility in terms of demand for the retrofitted structure, are necessary design decisions that guide the proposed retrofit strategy. Application of the retrofit design through infills is demonstrated through example case studies.

Retrofit Design Methodology for Substandard R.C. Buildings with Torsional Sensitivity

ABSTRACT Recent earthquakes have revealed the susceptibility of non-ductile reinforced concrete (R.C.) buildings with deficiencies related to stiffness and/or mass irregularities in plan and elevation. This paper proposes a design methodology for the seismic upgrading of rotationally sensitive substandard R.C. buildings. The methodology aims to first eliminate the effect of torsional coupling on modal periods and shapes and then modify the lateral response shape of the building in each direction so as to achieve an optimum distribution of interstory drift along the building height. A case study is used to illustrate practical application of the proposed methodology.

CONCRETE-TO-CONCRETE INTERFACES UNDER CYCLIC LOADING: FINITE ELEMENT ANALYSIS TOWARDS EXPERIMENTAL VERIFICATION

The objective of this paper is to describe a novel experimental setup for testing the concrete-to-concrete interface behavior under cyclic loading. Within the suggested configuration, various investigation parameters can be facilitated, such as the variation of concrete properties, interface roughness and number of transverse dowels. The specimen consists of three distinct concrete blocks in contact, as to create symmetric double interfaces. The middle concrete block has different concrete compressive strength, which suggests interfaces cast in different times. For the specimen preparation, elaborate computer-aided manufacturing (CAM) methods such as 3D design, laser-cutting and CNC-milling have been utilized. Moreover, an ad-hoc numerical analysis is performed, in the form of a blind-test procedure against its forthcoming experimental counterpart. The analysis is based on a three-dimensional nonlinear finite element model, accurately describing the physical specimen properties and loading setup. The analytical results are presented in a form of interface shear stress vs. slip, stress variation contours along the interface area and dowel tensile stress. A comparison with Code-based recommendations is performed and various interesting points are highlighted.

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

Analytical modeling of interface behavior in reinforced concrete jacketed members

The paper presents an analytical model for estimating the composite action of prismatic reinforced concrete (r.c.) members repaired/strengthened by r.c. jacketing. The model considers the slip at the interface between the existing member and the jacket and establishes the mechanisms mobilized to resist this action thereby supporting composite behavior. An algorithm has been developed for calculating the response taking into account the shear phenomena that appear due to sliding at the interfaces as well as the spacing and penetration of flexure -shear cracks. Results calculated using the analytical model are corroborated with selected experiments from the international literature .