Cem Demir

Seismic Retrofit of Brittle and Low Strength RC Columns using Fiber Reinforced Polymer and Cementitious Composites

The objective of this study is to investigate the seismic performance of reinforced concrete columns constructed with low quality of concrete and insufficient transverse reinforcement before and after retrofitting. Totally twenty nearly full size specimens with rectangular cross-section were tested under constant axial load and reversed cyclic lateral loads. Both longitudinal and transverse reinforcements were plain bars. Ten of the specimens had insufficient lap splice length of longitudinal reinforcement between stories as well. Both pre-damaged and undamaged columns with these deficiencies were retrofitted with FRP (fiber reinforced polymer) composite jackets or prefabricated HPFRCC (high performance fiber reinforced cementitious composite) panels. The test results showed that all reference specimens, which were not retrofitted, failed with a premature loss of performance either due to buckling of longitudinal reinforcement or loss of bond, while retrofitted ones exhibited a significantly superior performance, particularly in terms of ductility. It should be noted that the enhancement in performance was less remarkable for the specimens with inadequate lap splice lengths. An analytical work is also presented for prediction of the behavior of the specimens both for continuous and lap spliced longitudinal reinforcement cases.

SEISMIC RETROFIT OF INFILLED REINFORCED CONCRETE FRAMES WITH CFRP COMPOSITES

Retrofit of existing poor reinforced concrete frame structures is a major concern for Turkey. Considering the large stock of existing poor buildings and economical situation of the country, research on economical and easily applicable retrofitting techniques is important. Although the infill walls are not considered as structural members during design, the observations after earthquakes have shown that they are important sources of strength, stiffness and damping during earthquakes. Consequently, keeping infill walls in their locations during earthquake by preventing premature damage either due to in-plane or out-of-plane loads can help the structure maintain a significant amount of its unaccounted strength, stiffness and damping characteristics. In this study, 6 reinforced concrete frames, two bare, two infilled and two with CFRP retrofitted infill walls were tested under constant axial load and reversed cyclic lateral loads. At the end of the tests, it was seen that retrofitting of infill walls with CFRP composites in diagonal direction provided significant enhancement in lateral strength and stiffness.

Effects of Loading Rate and Duration on Axial Behavior of Concrete Confined by Fiber-Reinforced Polymer Sheets

In this study, 18 concrete cylinder specimens were tested either under uniaxial compression at different loading rates or exposed to sustained axial stresses after being jacketed externally with carbon-fiber-reinforced polymer (CFRP) sheets. The specimens were cast using medium strength concrete. All the specimens had identical dimensions and level of confinement. Loading rate and applied sustained stress level were the main test parameters. Applied loading rate varied between 0.0002 and 0.04 strain/min. Four stress levels between 0.52 and 0.85 f cc ′ (0.90 and 1.46 f co ′ ) were used in short-term creep tests. Test results showed that the stress-strain behavior of CFRP confined concrete was influenced by the change in loading rate, and CFRP confinement provided considerable increase in the creep performance of concrete. The strength enhancement was more pronounced for specimens loaded at higher strain rates, while specimens loaded at slower strain rates exhibited better deformability. Results obtained fr...

AXIAL BEHAVIOR OF RC COLUMNS RETROFITTED WITH FRP COMPOSITES

Fifteen RC columns and one plain concrete column with inadequate transverse reinforcement were tested under uniaxial compression after being jacketed externally with carbon fiber reinforced polymer (CFRP) sheets. CFRP layer thickness, cross-section shapes and concrete strength were the test parameters. External confinement of columns with CFRP sheets resulted in an increase in the strength and ductility. The behavior of the CFRP jacketed columns was also predicted by the proposed stress-strain model. There was reasonable agreement between analytical behavior and experimental data.

Evaluation of FRP Confinement Models for Substandard Rectangular RC Columns Based on Full-Scale Reversed Cyclic Lateral Loading Tests in Strong and Weak Directions

Although many theoretical and experimental studies are available on external confinement of columns using fiber-reinforced polymer (FRP) jackets, as well as numerous models proposed for the axial stress-axial strain relation of concrete confined with FRP jackets, they have not been validated with a sufficient amount and variety of experimental data obtained through full-scale tests of reinforced concrete (RC) columns with different geometrical and mechanical characteristics. Particularly, no systematical experimental data have been presented on full-scale rectangular substandard RC columns subjected to reversed cyclic lateral loads along either their strong or weak axes. In this study, firstly, test results of five full-scale rectangular substandard RC columns with a cross-sectional aspect ratio of two (300 mm × 600 mm) are briefly summarized. The columns were tested under constant axial load and reversed cyclic lateral loads along their strong or weak axes before and after retrofitting with external FRP jackets. In the second stage, inelastic lateral force-displacement relationships of the columns are obtained analytically, making use of the plastic hinge assumption and different FRP confinement models available in the literature. Finally, the analytical findings are compared with the test results for both strong and weak directions of the columns. Comparisons showed that use of different models for the stress-strain relationship of FRP-confined concrete can yield significantly non-conservative or too conservative retrofit designs, particularly in terms of deformation capacity.

Material Characterization of the Historical Unreinforced Masonry Akaretler Row Houses in Istanbul

The determination of built-in material properties is a difficult step during the structural assessment of historical structures. Large variations of material characteristics and the difficulty of obtaining an adequate number and type of material samples are among the major problems. In this study, the material characterization of the approximately 135 years old historical Akaretler Row Houses was carried out. Since several structural walls of the row house complex were to be removed according to the restoration design, a large number of different types of specimens could be collected for laboratory tests and a considerable amount of in situ destructive and nondestructive tests was carried out. The laboratory tests included mechanical, physical, and chemical tests on original materials such as bricks, mortar, brick prisms, cores consisting of two layers of bricks and one layer of mortar between bricks, and wallets. The in situ tests included destructive shear tests and nondestructive rebound hammer tests. At the end of the extensive experimental study, the basic material characteristics of the late period Ottoman construction system were obtained. In addition relations between various mechanical characteristics of the construction materials, as well as the relations between the results of the laboratory and in situ tests, and destructive and nondestructive tests are presented.

Assessing Seismic Vulnerability of Unreinforced Masonry Walls Using Elasto-Plastic Damage Model

In this paper, the seismic vulnerability of unreinforced masonry walls is assessed by conducting a numerical study on the interaction of axial and lateral resistance. The walls are modeled in an ABAQUS environment, using a plastic damage model originally developed by Lubliner et al. (Int J Solids Struct 25(3):299–326, 1989) and further extended by Lee and Fenves (J Eng Mech ASCE 124(8):892–900, 1998). The model yields interesting interactive collapse mechanisms that occur as the axial loading on the wall is increased. The different modes of failure identified as the axial load is increased include (i) rocking mode, (ii) sliding mode, (iii) staggered head/bed joint failure, (iv) diagonal cracks through wall blocks accompanied by staggered head/bed joint cracking, and (v) crushing of wall blocks or bricks.

Performance Based Rapid Seismic Assessment Method (PERA) for Reinforced Concrete Frame Buildings

Recent destructive earthquakes have shown that many existing buildings, particularly in developing countries, are not safe against seismic actions. Since code-based seismic safety evaluation methods generally require detailed and complex structural analysis, the necessity for simplified, yet sufficiently accurate evaluation methods emerges for reducing cost and duration of assessment procedures. In this study, a performance based rapid seismic safety assessment method (PERA) is proposed for reinforced concrete buildings. The overall structural performance is determined based on the demand/capacity ratios of individual columns, as well as their failure modes (brittle/ductile), confinement characteristics, and levels of axial and shear stresses. The lateral drift of the critical story, calculated through a simplified approach, is also taken into account during determination of the global structural performance. The predictions of this method are compared with the results of conventional detailed seismic safety assessment analyses carried out for 672 different cases representing typical reinforced concrete frame buildings in Turkey. Good agreement is obtained between the predictions of the proposed algorithm and code-based structural performance assessment procedures. Finally, predictions of the proposed approach are compared with actual damages observed in 21 existing buildings in Turkey after destructive earthquakes that have occurred during the last two decades. These comparisons also point to an acceptable level of accuracy and sufficient conservatism for the methodology proposed.

RETROFIT OF CONCRETE MEMBERS WITH EXTERNALLY BONDED PREFABRICATED SFRCC JACKETS

Many existing reinforced concrete structures, built before recent seismic design codes, can not exhibit a ductile behavior during earthquakes, and may experience severe structural damage. The main reason of nonductile behavior is insufficient and inadequately detailed transverse reinforcement. In developing countries, concrete quality of structural members is not sufficient as well. External confinement of structural members can overcome this deficiency. In this study, a retrofit technique in terms of external confinement of concrete members by using prefabricated panels of steel fiber reinforced cementitious composites (SFRCC) is investigated. For this purpose, concrete members with rectangular cross-section were externally confined by using prefabricated SFRCC panels and tested under axial loads. The concrete compressive strength of original specimens was around 10 MPa, while the compressive and tensile strengths of the prefabricated SFRCC panels were around 100 and 14 MPa, respectively. Test results showed that external confinement with prefabricated panels provided significant enhancement in ductility, as well as compressive strength enhancement. The investigated technique seems to be a promising method due to ease in application, relatively less hindrance to the building, high efficiency in the case of non-circular members, better quality control due to prefabrication and relatively less cost with respect to its alternatives.

Code Based Performance Prediction for a Full-Scale FRP Retrofitted Building Test

Lack of adequate ductility in substandard reinforced concrete buildings is a major reason for significant amount of life and economic losses experienced during major earthquakes. Laboratory tests realized at member level (i.e. column and beam tests) show that external wrapping of potential plastic hinging regions of columns with Fiber Reinforced Polymer (FRP) sheets, can significantly enhance the ductility capacity. Thus, major seismic retrofitting documents available worldwide interpret this retrofit approach as a viable alternative and lay the rules for retrofit design with these materials. In this study, a very recent experimental activity that included the full-scale testing of a substandard building retrofitted with the above mentioned approach is briefly presented. Then, the performance prediction of the Turkish Seismic Design Code (2007) for this building is evaluated. Finally, a revision is proposed for the FRP effective rupture strain value defined by this code.