Hee-Chang Eun

Shear characteristics of high-strength concrete deep beams without shear reinforcements

Based on the strength at the first diagonal crack of normal-strength concrete and normal beams without the consideration of size effects, the ACI code specifies the shear strength of deep beams. It is necessary to evaluate whether the ACI equation for deep beams is applicable to high-strength concrete deep beams with reinforcement ratio less than 1% and to consider size effects. Twenty-one beam specimens were tested to investigate their shear characteristics with the variables of concrete strength, shear span/depth ratio, and overall depth. The decrease in shear span/depth ratio and the increase in overall depth under the same shear span/depth ratio led to more brittle failure, with wide diagonal cracks and high energy release rate related to size effects. The high-strength concrete deep beams exhibited more remarkable size effects with regard to brittle behavior. It was also shown that the ACI code gives similar safety factors on the shear strength at the first diagonal crack of high-strength concrete deep beams, but do not specify a high enough safety factor on their ultimate strength due to the size effects.

Stress–strain curve of laterally confined concrete

Abstract The objective of this study is to present a stress–strain relation of confined concrete from an empirical study of 65 columns. Experimental parameters include the strength of concrete, the volumetric ratio, strength, and confinement type of rectilinear ties, and the distribution of longitudinal bars. For the purpose of investigating confinement effects, an effectively confined distance ratio was introduced and the effects according to each parameter were analyzed. This paper provides an equation to determine the tie stress caused by lateral concrete pressure as an important index to measure confinement degree. Analyzing the experimental data by nonlinear multiple regression method, this study provides the empirical equations to determine the peak stress and its corresponding strain of confined concrete expressed by the tie stress, the effectively confined distance ratio, the strength of concrete, and the configuration of ties. Starting from the model proposed by Popovics, an empirical model for stress–strain curve of laterally confined concrete is developed by three coordinates, (fcc,ecc), (0.85fcc,e0.85), and (0.3fcc,e0.3). Comparison with other stress–strain curves illustrates the validity of the proposed relation.

A damage detection approach based on the distribution of constraint forces predicted from measured flexural strain

Changes in flexural strain are good indicators for locating damage in structures. Despite such promise, flexural strain data measured at a few positions do not provide enough information to detect damage. However, they do become more effective if converted to vertical deflection data. Considering the relationship between measured flexural strain and vertical deflection as a constraint, this work provides a method to detect damage based on the distribution of flexural curvatures and constraint forces along a structural beam-member. The constraint forces are forces required for describing the flexural shape of the damaged beam under static and dynamic loadings. It is illustrated in two applications that the proposed constraint force method is more effective than the displacement curvature method in detecting multiple areas of low-level damage and when considering noisy measured data.

Damage detection by mixed measurements using accelerometers and strain gages

This work investigates the relationship between the FRFs (frequency response functions) measured by accelerometers and strain gages utilized widely for investigating structural performance. Modifying the GDM (global-deviation method), this study examines the possibility of damage detection in utilizing both sensors together. The experimental results on the mixed utilization of two sensors show that the SFRF (strain frequency response function) data measured by strain gages in the neighborhood of end supports are more practical in establishing the baseline curve than the DFRF (displacement frequency response function) data measured by accelerometers. It is shown that the modified GDM can be effectively utilized in detecting damage based on the mixed measurements of accelerometers and strain gages.

Analytical method for constrained mechanical and structural systems

The objective of this study is to present an accurate and simple method to describe the motion of constrained mechanical or structural systems. The proposed method is an elimination method to require less effort in computing Moore-Penrose inverse matrix than the generalized inverse method provided by Udwadia and Kalaba. Considering that the results by numerical integration of the derived second-order differential equation to describe constrained motion veer away the constrained trajectories, this study presents a numerical integration scheme to obtain more accurate results. Applications of holonomically or nonholonomically constrained systems illustrate the validity and effectiveness of the proposed method.

Structural and Mechanical Systems Subjected to Constraints

The characteristics of dynamic systems subjected to multiple linear constraints are determined by considering the constrained effects. Although there have been many researches to investigate the dynamic characteristics of constrained systems, most of them depend on numerical analysis like Lagrange multipliers method. In 1992, Udwadia and Kalaba presented an explicit form to describe the motion for constrained discrete systems. Starting from the method, this study determines the dynamic characteristics of the systems to have positive semidefinite mass matrix and the continuous systems. And this study presents a closed form to calculate frequency response matrix for constrained systems subjected to harmonic forces. The proposed methods that do not depend on any numerical schemes take more generalized forms than other research results.

An analytical approach for structural synthesis of substructures

A structure is broken down into a number of substructures by means of the finite element method and the substructures are synthesized for the complete structure. The divided substructures take two types : fixed-free and free-free elements. The flexibility and stiffness matrices of the free-free elements are the Moore-Penrose inverse of each other. Thus, it is not easy to determine the equilibrium equations of the complete structure composed of two mixed types of substructures. This study provides the general form of equilibrium equation of the entire structure through the process of assembling the equilibrium equations of substructures with end conditions of mixed types. Applications demonstrate that the proposed method is effective in the structural analysis of geometrically complicated structures.

A Model-Based Substructuring Method for Local Damage Detection of Structure

It is impractical to collect the full set of data in finite element model for investigating the structural performance. This study presents a model-based substructuring method to examine it by the data measured at a few positions. Dividing a global structure into several substructures and regarding the frequency response functions (FRFs) measured at the interfaces between the adjacent substructures as the constraints, the constraint forces at the substructure nodes are calculated to decide the resultant responses. The damage-expected substructure is traced by the distribution curve of constraint forces. Modeling the damage-expected substructure subjected to the predicted constraint forces and expanding the displacement data measured at several locations in the substructure, the local damage is detected by the displacement curvature method. A numerical application illustrates the validity of the proposed method.

Structural Damage Detection by Power Spectral Density Estimation Using Output-Only Measurement

Most damage detection methods have difficulty in detecting damage using only measurement data due to the existence of external noise. It is necessary to reduce the noise effect to obtain accurate information and to detect damage by the output-only measurement without baseline data at intact state and input data. This work imported the power spectral density estimation (PSE) of a signal to reduce the noise effect. By estimating the PSE to characterize the frequency content of the signal, this study proposes a damage detection method to trace the damage by the curvature of the PSE. Two numerical applications examine the applicability of the proposed method depending on a window function, frequency resolution, and the number of overlapping data in the PSE method. The knowledge obtained from the numerical applications leads to a series of experiments that substantiate the potential of the proposed method.

Damage identification through the comparison with pseudo-baseline data at damaged state

Without baseline information prior to damage, the damage should be detected by only measured data. Most non-baseline damage detection methods can be sensitive to the external noise and have difficulty in detecting damage when the sensor is not close to it. The purpose of this study is to propose a non-baseline damage detection method using only a few measurements and being less sensitive to the noise. A set of pseudo-reference data to be established at measurement instant is compared with another set of measurement data on the structure with the additional damage at a known location. The damage is found at the location to represent the abrupt change of the difference in the two response data sets. Compared to the global-deviation method, this method has merits to reduce the noise effect and to be able to detect damage by a few sensors. The experimental work also investigates the sensitivity of accelerometer and strain gage in detecting damage. The proposed method is verified in a numerical application and an experiment.