Hak Eun Lee

Strength of Circular Concrete-Filled Tubes with and without Internal Reinforcement under Combined Loading

AbstractConcrete-filled tubes (CFTs) have been used in civil engineering practice as piles, caissons, columns, and bridge piers. Relative to conventional structural steel and reinforced concrete components, CFTs have several advantages. The steel tube serves as both reinforcement and formwork, eliminating the need for both, and provides large tensile and compressive capacities; the concrete fill restrains buckling of the steel tube, which increases the strength, stiffness, and deformability of the section. In some cases, internal reinforcement is used to enhance the strength and facilitate connection to adjacent members. Although these properties are well accepted, the use of CFTs in practice is awkward because design provisions among codes vary significantly and previous research has not considered internal reinforcement. An analytical research study was undertaken to evaluate and improve design provisions for CFTs with and without internal reinforcement under combined axial load and bending. A continuum...

Investigation of Cement Matrix Compositions of Nanosilica Blended Concrete

The use of pozzolanic materials in concrete mixtures can enhance the mechanical properties and durability of concrete. By reactions with pozzolanic materials and calcium hydroxide in cement matrix, calcium-silicate-hydrate (C-S-H) increases and calcium hydroxide decreases in cement matrix of concrete. Consequently, the volume of solid materials increases. The pozzolanic particles also fill spaces between clinker grains, thereby resulting in a denser cement matrix and interfacial transition zone between cement matrix and aggregates; this lowers the permeability and increases the compressive strength of concrete. Moreover, the total contents of alkali in concrete are reduced by replacing cements with pozzolanic materials; this prevents cracks due to alkali-aggregate reaction (AAR). In this study, nanosilica is incorporated in cement pastes. The differences of microstructural compositions between the hydrated cements with and without nanosilica are examined using nanoindentation, XRDA and 29 Si MAS NMR. The results can be used for a basic

Analytical Evaluation of Reinforced Concrete Pier and Cast-in-Steel-Shell Pile Connection Behavior considering Steel-Concrete Interface

The seismic design of bridges may require a large-diameter deep pile foundation such as a cast-in-steel-shell (CISS) pile where a reinforced concrete (RC) member is cast in a steel casing. In practice, the steel casing is not considered in the structural design and the pile is assumed to be an RC member. It is partially attributed to the difficulties in evaluation of composite action of a CISS pile. However, by considering benefits provided by composite action of the infilled concrete and the steel casing, both the cost and size of CISS pile can be reduced. In this study, the structural behavior of the RC pier and the CISS pile connection is simulated by using an advanced 3D finite element (FE) method, where the interface between the steel and concrete is also modeled. Firstly, the FE model is verified. Then, the parametric study is conducted. The analysis results suggest that the embedment length and the friction coefficient between the steel casing and the infilled concrete affect the structural behavior of the RC pier. Finally, the minimum embedment length with reference to the AASHTO design guideline is suggested considering the composite action of the CISS pile.

Simulation of Accordion Effect of I-Girder with Corrugated Steel Webs

The axial stiffness of the corrugated steel plate is negligible by nature of unique geometric characteristics of the plate called as the accordion effect. This unique effect results in high efficiency on post-tensioning. Thus, corrugated steel plate is very suitable for a web of PC-box girder. Recent researches show that strain and stress in sub-panel of corrugated steel webs, that are induced by local bending of the sub-panel, exist even if the axial stiffness is negligible. These strain and stress in sub-panel are important since it might cause fatigue failure of the structure under repeated loading. This study presents the analytical simulation of the accordion effect of the I-girder with corrugated steel webs under pure axial load or bending including the effect of local bending of sub-panel of the corrugated steel webs. Theoretical study and numerical results were combined to develop the simplified equation to evaluate the accordion effect including the effect of local bending of the sub-panel.

Seismic Repair of Damaged RC columns with Steel and CFRP Jackets

In this paper, the comparative performance of repaired RC columns using steel and CFRP is presented. Also, the effect of transverse reinforcement ratio on the behavior of the steel and the CFRP repairing is investigated. Monotonic and cyclic load tests are conducted on nine RC column specimens with different repairing strategies and transverse reinforcement ratios to compare the load-displacement curves and the hysteretic behaviors. From the tests, it is observed that both steel and CFRP jacket repairings can significantly increase the displacement ductility and the ultimate load capacity of damaged columns.ࠀĀࠀ會Āࠀぅ?⨀꣈Ā᐀會Ā᐀衅?⨀჈ༀĀ저會Ā저?⨀鎝Ā᐀會Ā᐀㡆?⨀衬鎝Ā᐀會Ā᐀遆?⨀偮鎝瀀ꀏ會Ā?⨀꣈Ā᐀會Ā᐀䁇?⨀偒悗Ā㰀會Ā㰀顇?⨀悗Ā㈀會Ā㈀?⨀聓悗଀Ā᐀會Ā᐀䡈?⨀ᡔ悗Ā᐀會Ā᐀ꁈ?⨀끔悗ကĀ저會Ā저?⨀䣆Ā저會Ā저偉?⨀Ā저會Ā저ꡉ?⨀磇

Prediction of Wind Velocity to Raise Vortex-Induced Vibration through a Road-Rail Bridge with Truss-Shaped Girder

Vortex-induced vibration (VIV) of bridges, related to fluid-structure interaction and maintenance of bridge monitoring system, causes fatigue and serviceability problems due to aerodynamic instability at low wind velocity. Extensive studies on VIV have been performed by directly measuring the vortex shedding frequency and the wind velocity for indicating the largest girder displacement. However, previous studies have not investigated a prediction of wind velocity to raise VIV with a various natural frequency of the structure because most cases have been focused on the estimation of the wind velocity and peeling-off frequency by the mounting structure at the fixed position. In this paper, the method for predicting wind velocity to raise VIV is suggested with various natural frequencies on a road-rail bridge with truss-shaped girder. For this purpose, 12 cases of dynamic wind tunnel test with different natural frequencies are performed by the resonance phenomenon. As a result, it is reasonable to predict wind velocity to raise VIV with maximum RMS displacement due to dynamic wind tunnel tests. Furthermore, it is found that the natural frequency can be used instead of the vortex shedding frequency in order to predict the wind velocity on the dynamic wind tunnel test. Finally, curve fitting is performed to predict the wind velocity of the actual bridge. The result is shown that predicting the wind velocity at which VIV occurs can be appropriately estimated at arbitrary natural frequencies of the dynamic wind tunnel test due to the feature of Strouhal number determined by the shape of the cross section.

Seismic Performance Evaluation of Footing-to-Circular RC Pier Connection Reinforced by High-Manganese Steel Bars (HMSBs)

A footing-to-reinforced concrete (RC) pier connection resists the lateral load induced by earthquakes as well as the gravity load. The footing-to-RC pier connection is the vulnerable part to strong earthquake loading. Several studies have been conducted on improving the seismic performance of the connection by using high-strength reinforcing bars and by adding special structural components, such as steel tube and fiber-reinforced polymer sheet. In this study, reinforcing bars made of high-manganese steel (HMSBs) with high strength and ductility were installed in the connection instead of conventional reinforcing bars to improve the seismic performance. Test specimens were fabricated with HMSBs, and the strength, ductility, and dissipated energy of the connection were evaluated through a cyclic loading test. Three-dimensional finite-element analysis was also performed to investigate the effects of various axial forces on the behavior of the connection with HMSBs. The results show that the connection with HMSBs exhibits better seismic performance, represented by flexural strength, ductility, and energy dissipation, than that with ordinary reinforcing bars.

Analytical and Experimental Evaluation of Flexural Capacity Improvement Method for Reinforced Concrete Pier and Case-in-Steel-Shell Pile Foundation Connection

Recently, Cast-In-Steel Shell(CISS) pile has been used as the foundation of bridges and a pier is directly connected to the pile. In this case, plastic hinge is generally formed at the connection between the pile and pier. To increase the flexural capacity of such structure, a proper improvement method is necessary for the connection. In this study, a steel tube has been used to enhance flexural capacity of the connection and the effect by the such method was evaluated through a series of 3-dimensional finite element method analysis and experimental studies. From the analysis and test results, it can be found that the flexure capacity is considerably increased by applying the steel tube at the connection.

Analytical Simulation of Axial Behavior of RCFT Wall

Rectangular concrete-filled tubes (RCFTs) have been widely used as columns of building and bridge piers due to several advantages such as their strength-to-size efficiency and facilitation of rapid construction. Recently, some researchers have tried to use RCFT as a wall system in a building. RCFT wall have a high aspect ratio while the aspect ratio of the RCFT column is usually one. Thus, the behavior of the RCFT wall is clearly different from that of RCFT column and it needs to be investigated. In this study, the axial behavior of the RCFT wall was investigated through analytical simulation, and the effects of the aspect ratio, internal stud, and through rebar on axial behavior of the RCFT wall were examined. From the results, it was found that axial load capacity is decreased with increasing aspect ratio due to local buckling of the steel tube, and this local buckling can be efficiently prevented by using internal through rebar.

Flexural Ductility of High Strength Steel Girder in Negative Moment Region

For I-girder with high strength steel, it is known that the flexural ductility is considerably decreased by increasing the yield strength of material. Thus, it is necessary to conduct a study for guaranteeing proper flexural ductility of I-girder with high-strength steel. In this study, the evaluation of flexural ductility of negative moment region of I-girder with high strength steel where yield stress of steel is 690Mpa is presented based on the results of finite element analysis and experiment. From the results, it is found that the flexural ductility of the I-girder is significantly reduced due to the increase of elastic deformation and the decrease of plastic deformation ability of the material when the yield strength increases. This study proposed equation predicted flexural ductility of high strength steel in negative moment region. It is also proposed the method to improve the flexural ductility by an unequal installation of cross beam and an optimal position of cross beam. Finally, the effects of the unequal installation of cross beam on the flexural ductility are discussed based on the finite element results and the experimental results.