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Abstract: In the last few decades there has been a significant increase in the design strength and performance of different building materials. In particular, new methods, materials and admixtures for the production of concrete have allowed for strengths as high as 100 MPa to be readily available. In addition, the standard manufactured yield strength of reinforcing steel in Australia has increased from 400 MPa to 500 MPa. -- A perceived design advantage of higher-strength materials is that structural elements can have longer spans and be more slender than previously possible. An emerging problem with slender concrete members is that they can be more vulnerable to loading induced vibration. The damping capacity is an inherent fundamental quantity of all structural concrete members that affects their vibrational response. It is defined as the rate at which a structural member can dissipate the vibrational energy imparted to it. -- Generally damping capacity measurements, to indicate the integrity of structural members, are taken once the structure is in service. This type of non-destructive testing has been the subject of much research. The published non-destructive testing research on damping capacity is conflicting and a unified method to describe the effect of damage on damping capacity has not yet been proposed. -- Significantly, there is not one method in the published literature or national design codes, including the Australian Standard AS 3600-2001, available to predict the damping capacity of concrete beam members at the design stage. Further, little research has implemented full-scale testing with a view to developing damping capacity design equations, which is the primary focus of this thesis. -- To examine the full-range damping behaviour of concrete beams, two categories of testing were proposed. The categories are the 'untested' and 'tested' beam states. These beam states have not been separately investigated in previous work and are considered a major shortcoming of previous research on the damping behaviour of concrete beams. -- An extensive experimental programme was undertaken to obtain residual deflection and damping capacity data for thirty-one reinforced and ten prestressed concrete beams. The concrete beams had compressive strengths ranging between 23.1 MPa and 90.7 MPa, reinforcement with yield strengths of 400 MPa or 500 MPa, and tensile reinforcement ratios between 0.76% and 2.90%. The full- and half-scale beams tested had lengths of 6.0 m and 2.4 m, respectively. The testing regime consisted of a series of on-off load increments, increasing until failure, designed to induce residual deflections with increasing amounts of internal damage at which damping capacity (logarithmic decrement) was measured. -- The inconsistencies that were found between the experimental damping capacity of the beams and previous research prompted an initial investigation into the data obtained. It was found that the discrepancies were due to the various interpretations of the method used to extract damping capacity from the free-vibration decay curve. Therefore, a logarithmic decrement calculation method was proposed to ensure consistency and accuracy of the extracted damping capacity data to be used in the subsequent analytical research phase. -- The experimental test data confirmed that the 'untested' damping capacity of reinforced concrete beams is dependent upon the beam reinforcement ratio and distribution. This quantity was termed the total longitudinal reinforcement distribution. For the prestressed concrete beams, the 'untested' damping capacity was shown to be proportional to the product of the prestressing force and prestressing eccentricity. Separate 'untested' damping capacity equations for reinforced and prestressed concrete beams were developed to reflect these quantities. -- To account for the variation in damping capacity due to damage in 'tested' beams, a residual deflection mechanism was utilised. The proposed residual deflection mechanism estimates the magnitude of permanent deformation in the beam and attempts to overcome traditional difficulties in calculating the damping capacity during low loading levels. Residual deflection equations, based on the instantaneous deflection data for the current experimental programme, were proposed for both the reinforced and prestressed concrete beams, which in turn were utilised with the proposed 'untested' damping equation to calculate the total damping capacity. -- The proposed 'untested' damping, residual deflection and total damping capacity equations were compared to published test data and an additional series of test beams. These verification investigations have shown that the proposed equations are reliable and applicable for a range of beam designs, test setups, constituent materials and loading regimes.
This dissertation, "Effects of Confinement and Small Axial Load on Flexural Ductility of High-strength Reinforced Concrete Beams" by Siu-lee, Chau, 周小梨, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled EFFECTS OF CONFINEMENT AND SMALL AXIAL LOAD ON FLEXURAL DUCTILITY OF HIGH-STRENGTH REINFORCED CONCRETE BEAMS Submitted by CHAU Siu Lee for the Degree of Master of Philosophy at The University of Hong Kong in August 2005 Compared with normal-strength concrete, high-strength concrete has higher strength but is generally more brittle. Its use in a reinforced concrete structure could lead to an undesirable reduction in ductility if not properly controlled. In this thesis, the effects of confinement and small axial load on the flexural ductility of reinforced concrete beams cast of both normal- and high-strength concrete have been evaluated by analyzing the complete moment-curvature behaviour of the beam sections. The results reveal that the use of high-strength concrete would at a constant tension steel ratio increase the flexural ductility, while at a constant tension to balanced steel ratio decrease the ductility. On the other hand, provision of confinement enhances the ductility of both normal- and high-strength concrete sections at both a constant tension steel ratio and at a constant tension to balanced steel ratio. It does this in two ways. Firstly, it increases the balanced steel ratio of the section. So, for a constant steel ratio, the section with higher confinement is more under-reinforced. Secondly, it increases the residual strength and ductility of the concrete such that at the same tension to balanced steel ratio, the ductility of the section increases. From the results of the analysis, it can be concluded that providing confinement to a section is an effective way of improving the ductility of reinforced concrete beam sections, especially those cast of high-strength concrete. However, most codes of practice do not specify a suitable design method for reinforced concrete beams that takes into account the effect of confinement. Therefore, design formulas for the flexural strength and ductility design of high-strength concrete beams with the effects of confinement taken into account have been developed. On the other hand, it is proposed to compensate for the reduction in flexural ductility due to the use of high-strength concrete by adding compression and/or confining reinforcement. A simple design method that correlates the amount of addition reinforcement needed to maintain a constant level of minimum ductility and the concrete strength is developed. Conversely, the presence of compressive axial load, even at a low level, has an adverse effect on flexural ductility. As a portion of concrete is used to resist the axial load, the section becomes less under-reinforced. Therefore, the flexural ductility decreases with the level of axial load applied. From the results obtained, it is found that the presence of axial load mainly affects the degree of the section being under- or over-reinforced. Measures should therefore be taken to maintain the ductility level of sections with applied axial load at an acceptable level. The study recommends the provisions of additional compression reinforcement to resist the applied axial load, and proposes a design method for restoring the ductility of a section with applied axial load to a ductility level attained by an identical section without axial load. DOI: 10.5353/th_b3199766 Subjects: