Nahit Kumbasar

COMPRESSIVE BEHAVIOUR OF CARBON FIBRE COMPOSITE JACKETED CONCRETE WITH CIRCULAR AND NON-CIRCULAR CROSS-SECTIONS

This paper describes the concentric compression test results of carbon fibre reinforced polymer (CFRP) composite sheet jacketed concrete specimens with circular, square and rectangular cross-sections. In the experimental program, pre-damaged specimens and repeated compressive loads were considered as well as undamaged specimens and monotonic compressive loads. The contribution of CFRP composite jackets to the compressive behaviour of the specimens is evaluated quantitatively, in terms of strength, longitudinal and lateral deformability and energy dissipation. Simple analytical expressions are proposed for compressive strength and ultimate axial strain that are valid for CFRP composite jacketed concrete with circular, square and rectangular cross-sections. The analytical results obtained by the proposed expressions are in good agreement with the experimental data obtained in this study, as well as the experimental data available in literature.

Behavior of FRP-Retrofitted Joints Built with Plain Bars and Low-Strength Concrete

Two series of tests on eight full-scale exterior beam-column joint subassemblages built with plain bars and low-strength concrete were conducted. No transverse reinforcement was present in the joint cores. In the first series of tests, which included three specimens, the behavior of joints before fiber-reinforced polymer (FRP) retrofitting was investigated. In the second series, which included five specimens, the behavior of the FRP-retrofitted joints was investigated. The six specimens consisted of a column, an in-plane beam, a transverse beam, and a slab part, and two specimens were plane members without transverse beams and slabs. The utilized retrofitting scheme is easily applicable for actual exterior beam-column joints, even in the presence of a transverse beam and a slab. Two types of strength limitation were observed for specimens in the first series. The strength of the specimen with beam longitudinal bars sufficiently anchored to the joint core was limited by the shear strength of the joint. The strengths of the other two specimens were limited by the slip of the beams’ longitudinal bars at their anchorages. In the second series of tests, significantly better performance was obtained both in terms of shear strength and ductility, provided that the slip of the beam bars was prevented. Furthermore, by using a simple theoretical algorithm based on truss analogy, the strength and deformability characteristics of the tested reference and FRP-retrofitted joints are predicted with reasonable accuracy. The same algorithm is used for predicting the joint shear strength of specimens tested by other researchers, and satisfactory agreement is obtained between the predictions and test results.

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.

A finite element formulation for moderately thick shells of general shape

Abstract An expression for the potential energy of a shell of general shape, including thickness shear deformation and without neglecting z R in comparison with unity, is derived. Then a finite element formulation based on this functional is obtained. Furthermore, a finite element shape function presented in this study contains high degree terms to describe the surface geometry better. Accuracy of finite element solutions of moderately thick shells of general shape is shown in examples.

Effects of Reinforcement Corrosion on the Performance of RC Frame Buildings Subjected to Seismic Actions

AbstractCorrosion of reinforcing bars is a common deficiency for existing RC buildings, which may have significant adverse effects on seismic performance of those buildings. The main effects of corrosion of reinforcing bars are strength loss as a result of reduction of the effective cross-sectional area of the bars, reduced bond strength between concrete and bars as a result of the increased volume of the bars because of rust products, and reduced elongation capacity of the bars as a result of the concentration of plastic deformations at sections of corrosion pits. The last item (concentration of plastic deformations of reinforcing bars only at the most heavily corroded sections rather than occurring all through the potential plastic hinge length) has a very remarkable adverse effect on the displacement capacity of existing buildings. It is known that the displacement capacity is as important as strength, particularly for the buildings that are designed to exhibit ductile behavior during earthquakes. In t...

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.

External Confinement of Low-Strength Brittle Reinforced Concrete Short Columns

In this study, FRP jacketed short reinforced concrete columns with low concrete strength and inadequate transverse reinforcement were tested under uniaxial compression. The diameter of the longitudinal bars and spacing of the transverse bars were designed to allow buckling of longitudinal bars. The effects of the jacket thickness, pre-damage, cross-section shape and the usage of FRP jackets - either continuous or as straps like hoops or spirals - were investigated. The test results showed that external confinement of these type of columns was very effective in terms of deformability and strength enhancement. The buckling of the longitudinal bars was delayed significantly by the FRP jackets. The pre-damage did not have an adverse effect on the performance of the jacketed members. It was also observed that for equivalent amount of FRP, continuous jackets and straps provided similar performances. The compressive strengths and the corresponding axial deformations of the columns were also predicted by the empirical equations proposed by the authors before. It was seen that these predictions were in reasonable agreement with experimental results.

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.

Innovative Techniques for Seismic Retrofitting of RC Joints

The main target of this study is to develop simple, realistic and applicable retrofitting techniques by using innovative materials in existing deficient beam-column joints. Retrofitting is targeted at overcoming deficiencies such as usage of low-strength concrete, absence of stirrups in the joint and poor anchorage of beam longitudinal bars at the joint.

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.