Hyo-Gyoung Kwak

Nonlinear analysis of RC beams based on moment–curvature relation

Abstract Material nonlinear analyses of reinforced concrete (RC) beams considering the tension softening branch and bond slip have been conducted. Instead of adopting the sophisticated layer approach which has some limitations in application to large structures with many degrees of freedom, we have used the moment–curvature relationships of RC sections previously constructed through section analysis. To reduce numerical instability according to the finite element mesh size used, a relation simulating the tension softening branch has been taken into consideration. For the purpose of removing the imprecision in calculation of ultimate resisting capacity, we have included the plastic hinge length in finite element modeling. In addition, governing equations describing the bond-slip behavior in beams have been derived. Unlike the conventional bond elements using double nodes, the proposed model has used beam elements representing the structural response by two nodes at both ends, and has simplified the finite elements modeling and analytical process, besides effectively describing the bond-slip behavior. Moreover, the developed algorithm has been reflected in the moment–curvature relationship of RC section. Finally, correlation studies between analytical and experimental results have been conducted with the objective to establish the validity of the proposed algorithms.

Long-term behavior of composite girder bridges

Abstract This paper deals with the development of an analytical model to predict the long-term behavior of composite girder bridges. The proposed model accounts for the effects of creep, the shrinkage of concrete, and the cracking of concrete slabs in the negative moment regions. Based on the equilibrium of forces and compatibility of strains with time, the prediction of stresses and strains in the constitutive materials at any time is achieved. To consider the different material properties across the sectional depth, the layer approach in which a section is divided into imaginary concrete and steel layers is adopted. The element stiffness matrix is constructed according to the assumed displacement field formulation, and the creep and shrinkage effects of concrete are considered in accordance with the first-order algorithm based on the expansion of compliance function. Correlation studies between analytical and experimental results are conducted with the objective to establish the validity of the proposed model. Besides, many composite girder bridges made of more than one type of concrete or of concrete and structural steel are adopted and analyzed to investigate the effects of creep and shrinkage of concrete slabs on structural behavior.

CRACKING ANALYSIS OF RC MEMBERS USING POLYNOMIAL STRAIN DISTRIBUTION FUNCTION

Abstract In this paper, an analytical model which can simulate the post-cracking behavior and tension stiffening effect in a reinforced concrete (RC) tension member is proposed. Unlike the classical approaches using the bond stress–slip relationship or the assumed bond stress distribution, the tension stiffening effect at the post-cracking stage is quantified on the basis of polynomial strain distribution functions of steel and concrete, and its contribution is implemented into the reinforcing steel. The loads carried by concrete and by reinforcing steel along the member axis can be directly evaluated on the basis of the introduced model. The prediction of cracking loads and elongations of reinforcing steel using the introduced model shows good agreement with results from previous analytical studies and experimental data. Through extension of the introduced tension stiffening model defined for tension member, a descending branch in the tension region of the concrete stress–strain relation is constructed to simulate the tension stiffening effect in RC members subjected to bending moments. Finally, correlation studies between analytical results and experimental values from idealized RC slab tests are conducted to verify the validity of the proposed model.

An integrated genetic algorithm complemented with direct search for optimum design of RC frames

This paper presents an improved optimum design method for reinforced concrete (RC) frames using an integrated genetic algorithm (GA) with a direct search method. A conventional genetic algorithm occasionally has limitations due to a low convergence rate in spite of high computing times. The proposed method in this research uses a predetermined section database (DB) when determining trial sections for the next iteration. From an initial section determined by substituting calculated member forces into a regression formula, a direct search that determines a final discrete solution is followed within a limited range in the section database. Due to the fast convergence and the sequential determination of feasible trial sections close to the final optimum solution, an introduction of the search procedure at each iteration allows difficulties to be solved during the application of a conventional GA to large RC structures. Finally, the effectiveness of the introduced design procedure is verified through correlative tests of the introduced design procedure.

Optimum design of reinforced concrete plane frames based on predetermined section database

For the optimum design of reinforced concrete (RC) structures, predetermined section databases of RC columns and beams are constructed and arranged in order of resisting capacity. Because all the design variables of an RC section are interconnected by a representative design variable of the section identification number, regression equations representing the relation between the section identification number and section resisting capacity are derived to effectively handle all the design variables and to use in determining a continuous solution. An introduction to effective discrete optimization algorithms, which can search for an optimum solution quickly using a direct search method, is followed. Moreover, the investigation for the applicability and effectiveness of the introduced design procedure is conducted through a correlation study for typical example structures. Because of an absence of restrictions on the construction of objective functions, together with very simple optimization processes and fast convergence, the introduced method can effectively be used in the preliminary design of RC frame structures. Especially, the obtained solutions selected from the section database can be applied applicable in practice, because these sections are constructed to satisfy all design code requirements and practical limitations.

Nonlinear analysis of RC shear walls considering tension-stiffening effect

The research reported in this paper was made possible by the ®nancial supports from the National Research Laboratory funded by the Ministry of Science and Technology of Korea and BK21 project funded by the Ministry of Education of Korea. The authors would like to express their gratitude to both organizations for their support.

Nonlinear FE analysis of RC structures under monotonic loads

Abstract This paper deals with the finite element analysis of the monotonic behavior of reinforced concrete ( R C ) beams and beam-column subassemblages. It is assumed that the behavior of these members can be described by a plane stress field. Concrete and reinforcing bars are represented by separate material models which are combined together with a model of the interaction between reinforcing bar and concrete through bond-slip to describe the behavior of the composite reinforced concrete material. Using the rotating crack model among the smeared crack model, the structural behavior is simulated and a relation which can consider the tension stiffening effect in finite element analysis is proposed based on an improved cracking criterion derived from fracture mechanics principles. A new reinforcing steel model which is embedded inside a concrete element is developed to cope with the difficulty in modeling of complex geometry. Correlation studies between analytical and experimental results show the validity of the proposed models and identify the significance of various effects on the local and global response of reinforced concrete members.

Wave attenuation measurement technique for nondestructive evaluation of concrete

Nondestructive testing (NDT) that uses ultrasonic waves is a reliable experimental technique for characterisation of materials in existing concrete structures and for safety evaluations of these structures. The wave velocity is related to the stiffness of material and the wave attenuation can be used to evaluate damaged concrete structures. This paper proposes an experimental technique to quantitatively measure the wave attenuation using contact ultrasonic transducer for NDT. The proposed technique is subject to interference as a result of the transducer-coupling condition. Unbiased attenuation can be obtained by self-compensating the signals measured in a lead–zirconate–titanate ceramic material inserted between a conventional transducer and a sample. The reproducibility and relevancy of the proposed technique are demonstrated using examples of the technique applied to cement paste and concrete. Also, the relevancy of the attenuation measurements for damage evaluation of cement-based material is demonstrated based on a theoretical attenuation model.

Nondestructive Evaluation of Elastic Properties of Concrete Using Simulation of Surface Waves

Nondestructive evaluation using the propa- gation of an impact-induced surface wave can be effec- tively applied in estimating in situ material properties. In this study, to evaluate information of a surface waveform beyond the simple wave velocity, artificial intelligence en- gines are employed to estimate simulation parameters, that is, the properties of elastic materials. The developed artificial neural networks are trained with a numerical database having secured its stability. In the process, the appropriate shape of the force-time function for an im- pact load is assumed so as to avoid Gibbs phenomenon, and the proposed principal wavelet-component analysis accomplishes a feature extraction with a wavelet trans- formed signal. The results of estimation are validated with experiments focused on concrete materials.

Numerical analysis of time-dependent behavior of pre-cast pre-stressed concrete girder bridges

Abstract This paper deals with the analysis of uncertainties associated with the behavior of bridges constructed with pre-cast pre-stressed girders and a continuous deck. To analyze the long-term behavior of bridges, an analytical model which can simulate the effects of creep, the shrinkage of concrete, and the cracking of concrete slabs in the negative moment regions is introduced. Based on the equilibrium of forces and the compatibility of strains with time, the prediction of stresses and strains in the constitutive materials at any time is achieved. To consider the different material properties across the sectional depth, the layer approach in which a section is divided into imaginary concrete and steel layers is adopted. An element stiffness matrix is constructed according to the assumed displacement field formulation, and the creep and shrinkage effects of concrete are considered in accordance with the first-order recursive algorithm based on the expansion of the compliance function. Correlation studies between analytical and experimental results are conducted with the objective of establishing the validity of the proposed model. Also, many parameters related to the continuity of spans are analyzed to minimize deck cracking at the interior supports.