Eun-Taik Lee

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.

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.

Effect of Aggregate Size on Shear Behavior of Lightweight Concrete Continuous Slender Beams

In this paper twelve continuous slender beams were tested to ascertain the effect of the maximum aggregate size on the shear behavior of concrete beams. The typical characteristics of the failure surface along the inclined cracks of the beams tested were compared according to the maximum aggregate size and the type of concrete by using a microphotograph. The test results showed that the shear strength of light weight concrete (LWC) continuous beams increased with the maximum aggregate size, though the increasing rate was lower than that of normal weight concrete (NWC) continuous beams. The microphotograph showed that the inclined crack of mortar beams with an aggregate size of 4 mm (0.16 in.) had a near-linear shape and a smooth failure surface, whereas that of the concrete beams was emboss-shaped with a failure plane partially formed along the pastes around the aggregate particles, regardless of the type of concrete. These characteristics of the failure plane contributed to the enhancement of the shear strength of LWC beams, though the shear force transferred by the aggregate interlock was much lower than that in NWC beams. The increasing rate of shear strength of LWC beams against aggregate size is similar to that predicted from the simplified compression field theory or the empirical formulas proposed by Bazant and Sun. The modification factor for shear strength of LWC specified in ACI 318-08 and EC2 is unconservative in the continuous beams tested, showing an increase of the unconservatism with the maximum aggregate size.

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.

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.

Damage Detection Using Measurement Response Data of Beam Structure Subject to a Moving Mass

The moving load problem is divided into two typical types: moving force and moving mass. The moving mass problem includes the time-varying mass effect. This work considers damage detection of the beam structure subject to a moving load including the inertia effect based on the only measurement data from strain gages and accelerometers without any baseline data. This experiment compares the feasibility of damage detection methods depending on the measurement sensors of strain gages and accelerometers. The measurement data are transformed to the proper orthogonal modes (POMs) in the time domain and the frequency domain, respectively. The magnitude of the moving mass and its velocity are also evaluated as test variables in this experiment. It is shown in the beam tests that the measured strain data can be more explicitly utilized in detecting damage than the acceleration data, and the mass magnitude and its velocity affect the feasibility of damage detection.

Damage identification of a frame structure model based on the response variation depending on additional mass

The structural health evaluation is performed by comparing the responses between the undamaged and damaged states. Because it is impractical to collect the response data at the intact state, it is important to establish the baseline data to be compared at measurement. The measured frequency response functions (FRFs) in the neighborhood of the first resonance frequency are transformed to the proper orthogonal mode (POM) corresponding to the first proper orthogonal value (POV). The POM data set at the first measurement on the damage-expected structure is taken as the baseline datum, and it is compared with another set extracted from the structure to attach a small mass on an element. The POM difference between two states is utilized as an index to detect damage. The FRF variation before and after a small mass attachment for the purpose of detecting the damage is investigated. The validity of the proposed method based on POM variation is illustrated in the damage detection of a two-dimensional frame structure model. It is shown that the damage region in the frame structure can be inferred by gradually narrowing from the global structure to the damage-expected element.

A STUDY ON PERFORMANCE ASSESSMENT OF DAMAGED BEAM BASED ON STATIC APPROACH

Starting from the derivation of an analytical method to expand measured static displacement data to full degrees of freedom, this study proposes a damage detection method to detect the damage of damaged beam by introducing displacement curvature and damage factor (DF). The validity of the proposed method is evaluated in damaged beam system that two simple beams are perpendicularly interconnected at a point.