Senin, Oktober 29, 2007

REINFORCED CONCRETE DEEP BEAMS


This book is designed as an international reference work on the behaviour, design and analysis of reinforced concrete deep beams. It is intended to meet the needs of practising civil and structural engineers, consulting engineering and contracting firms, research institutes, universities and colleges.

Reinforced concrete deep beams have many useful applications, particularly in tall buildings, foundations and offshore structures. However, their design is not covered adequately by national codes of practice: for example the current British Code BS 8110, explicitly states that ‘for design of deep beams, reference should be made to specialist literature’. The major codes and manuals that contain some discussion of deep beams include the American ACI Building Code, the draft Eurocode EC/2, the Canadian Code, the CIRIA Guide No. 2, and Reynolds and Steedman’s Reinforced Concrete Designer’s Handbook. Of these, the CIRIA Guide No. 2: Design of Deep Beams in Reinforced Concrete, published by the Construction Industry Research and Information Association in London, gives the most comprehensive recommendations.

The contents of the book have been chosen with the following main aims: (i) to review the coverage of the main design codes and the CIRIA Guide, and to explain the fundamental behaviour of deep beams; (ii) to provide information on design topics which are inadequately covered by the current codes and design manuals: deep beams with web openings, continuous deep beams, flanged deep beams, deep beams under top and bottom loadings and buckling and stability of slender deep beams; (iii) to give authoritative reviews of some powerful concepts and techniques for the design and analysis of deep beams such as the softened-truss model, the plastic method and the finite element method.

The contributing authors of this book are so eminent in the field of structural concrete that they stand on their own reputation and I feel privileged to have had the opportunity to work with them. I only wish to thank them for their high quality contributions and for the thoroughness with which their chapters were prepared.

I wish to thank Mr A.Stevens, Mr J.Blanchard and Mr E.Booth of Ove Arup and Partners for valuable discussions, and to thank Emeritus Professor R.H.Evans, C.B.E., of the University of Leeds for his guidance over the years. Finally, I wish to thank Mrs Diane Baty for the much valued secretarial support throughout the preparation of this volume.

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Discussion of “Unloading and Reloading Stress-Strain Model for Confined Concrete” by Junichi Sakai and Kazuhiko Kawashima

by Asad Esmaeily, A.M.ASCE; Steven D. Hart; and`Brandy Gaitan

In this paper, the authors, Sakai and Kawashima, propose a “comprehensive . . . model . . . that takes into account the effect of repeated unloading and reloading and partial loading.” This model was evaluated by conducting several tests on concrete cylinders confined by carbon-fiber-reinforced polymer CFRP material.

Three standard 150 mm300 mm 6 in.12 in. concrete cylinders were cast and cured for 28 days in a moist curing room. Two strain gauges with a length of 50 mm 2 in. were placed
longitudinally on the central part of the specimens on opposite sides. The unconfined compressive strength of the specimens was 40.7 MPa 5.9 ksi at the time of testing. Confinement was provided by two layers of CFRP attached by an epoxy adhesive, which is different from conventional confinement by steel as used in the authors’ original research. Testing was conducted by using a closed loop servocontrolled material testing system with a maximum capacity of 667 kN 150 kips. Since the envelope for CFRP confined concrete does not have the descending branch after a peak point as observed for conventionally reinforced cases, all specimens were initially loaded to 614 kN 138 kips at a rate of 62 kN 14 kips per minute to achieve sufficient plastic strain for a reasonable evaluation of the unloading/reloading paths.

There was a creep-hold of 108 min at 133 kN30 kip load level for one of the specimens. At the load level of 614 kN 138 k, the specimen with a creep-hold was subjected to three complete unloading and reloading cycles at a rate of 124 kN 28 kips per minute, the next specimen had similar cycles but at a rate of 186 kN 42 kips per minute, and the last one was loaded monotonically to 614 kN 150 kips to establish the envelope curve.

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Analytical Sensitivity of Plastic Rotations in Beam-Column Elements

by Michael H. Scott

Abstract: Analytical sensitivity equations for the plastic rotation of beam-column finite elements are derived for reliability and optimization algorithms in structural engineering and for the assessment of plastic rotation sensitivity to uncertain design parameters and modeling assumptions. The plastic rotation is defined by elastic unloading of element forces in a basic system, which makes the corresponding sensitivity computations applicable to most material nonlinear beam-column formulations available in the literature. The analytical response sensitivity is verified by finite differences then applied to a first-order reliability analysis of a steel subassemblage where the performance function places a limit on plastic rotation.

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Kamis, Oktober 04, 2007

Controlling All Interstory Displacements in Highly Nonlinear Steel Buildings Using Optimal Viscous Damping

by T. L. Attard, M.ASCE

Abstract:
A gradient-based optimization algorithm is used to simultaneously control all interstory displacements in nonlinearly degrading steel buildings using optimal viscous dampers. Optimal damping ratios are computed in each mode of vibration such that the sum of the errors between the interstory displacements and the “just-yielded” performance objectives is minimized. A representative damping formulation is used to determine the sizes and locations of the damper devices. The members of the buildings are assumed to degrade smoothly according to a constitutive rule that was developed to model the behavior of kinematically strain-hardened materials. Numerical examples are used to demonstrate the ability of the algorithm to control potential damages in a 10-story building and also in an 8-story building responding at significant higher modes of vibration. It is found that the interstory displacements in the 10-story building are very adequately controlled. Although demands in the 8-story building are significantly reduced, some modes remain overdamped and not all performance levels are exactly met as some stories remain marginally damaged. Finally, the algorithm is applied in a 20-story benchmark building, and it is shown that the interstory displacements, postyield curvatures, and plastic damages are very adequately controlled.

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Classification and Seismic Safety Evaluation of Existing Reinforced Concrete Columns

by L. Zhu; K. J. Elwood; and T. Haukaas

Abstract:
This study contributes to the critical need for safety assessment tools for existing reinforced concrete structures. Of particular concern is the possibility of collapse due to shear failure followed by axial failure of columns supporting gravity loads. This is a potential threat to a number of existing buildings in seismically active regions. Due to unavoidable uncertainties, drift capacity predictions can only be made in a probabilistic manner. This is addressed by the development of probabilistic drift capacity models at two performance levels:
lateral strength degradation and axial load failure. First, a classification method is proposed to approximately distinguish between shear-dominated columns and flexure-dominated columns. Second, for each type of column, a probabilistic shear capacity model is developed by applying an existing Bayesian methodology to an experimental database. The focus of the presentation is on the physical insight gained from the model development. Third, a probabilistic model is developed for the drift capacity at axial load failure. Finally, the probabilistic drift capacity models are employed to develop fragility curves—with confidence bounds—that are utilized to assess the probability of failure implied by current seismic rehabilitation guidelines.

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