Sungjin Bae

Plastic hinge length of reinforced concrete columns

Severe damage can be observed in regions subjected to large moments when a column experiences earthquake-induced lateral displacements while supporting gravity loads. These regions, which experience large inelastic curvatures, are referred to as plastic hinges. The inelastic curvatures in plastic hinges are typically assumed to be constant over the plastic hinge length (lp) to simplify the estimation of the tip displacement of a column. Therefore, if the plastic hinge length is known, the tip displacement of a column can easily be obtained by integrating curvatures, and vice versa. In this paper, the effects of axial load and shear span-depth ratio on lp are evaluated experimentally. Results from experimental data on four full-scale concrete column tests indicate that ACI 318-05 provisions for the length of potential plastic hinge rations are slightly unconservative for columns supporting high axial loads. The level of axial load included the length of the plastic hinges that formed in the full-scale column specimens. Specimens tested under high axial loads developed longer plastic hinges than those tested under low axial loads. An equation is developed to estimate the length of the plastic hinges forming in columns supporting a wide range of axial loads.

Seismic performance of full-scale reinforced concrete columns

This paper presents results of simulated seismic tests conducted on five full-scale reinforced concrete columns to investigate the influence of P-Δ effect on the relationships between curvature ductility and displacement ductility or drift capacity. Experimental results show that the shear span-depth ratio at high axial load levels can significantly reduce the member performance of columns due to the P-Δ effect. Findings also indicate that axial load applied on a column plays an important role in determining the plastic hinge length. As the plastic hinge length depends on the axial load, the relationship between the sectional and member behaviors is affected by the axial load. On unexpected and undesirable finding was that the 135-degree hooked anchorages used in the first specimen opened during the scaled column tests, necessitating the need for longer hook lengths in other specimens.

Stress Block Parameters for High-StrengthConcrete Members

Early spalling of cover concrete has been reported by researchers in high-strength concrete (HSC) column tests. A rational way to incorporate this phenomenon into structural building codes has yet to be introduced. Moreover, use of ACI 318-02 stress block parameters to estimate the strength of HSC columns results in unconservative predictions for moment capacity in some cases. Hence, modification of ACI 318-02 stress block parameters to incorporate early cover spalling in HSC columns is of great significance. With this in mind, experimental results obtained during the course of this research and those available from the literature were used to study the accuracy and conservativeness of the ACI 318-02 stress block parameters. Based on experimental evidence, it was determined to reduce the ultimate compressive strain limit of ACI 318-02 to 0.0025 for HSC members such that the capacity of these members can be evaluated prior to cover spalling. A new set of stress block factors was analytically derived and validated with experimental data.

Drift Capacity of Reinforced Concrete Columns

An important parameter in performance-based earthquake engineering is the deformation capacity of concrete columns. This paper introduces simple closed-form equations that can be used to estimate the lateral deformation capacity of reinforced concrete columns. The proposed equations are derived from an idealized lateral load-displacement behavior of a column and by taking the P-Δ effect into account. Findings from 135 column tests are used to validate the proposed equations. Estimated drift capacities are compared with the experimentally-observed drift capacities. The results show that the proposed equations provide conservative lower-bound estimates of lateral drift capacities of columns that satisfy the provisions of ACI 318-08 Chapter 21. Findings also show that for slender columns supporting high axial loads, a satisfactory lateral load performance can not be expected due to pronounced secondary moments, even though the detailing requirements of ACI 318-08 Chapter 21 are met.

Effect of Alkali-Silica Reaction/Delayed Ettringite Formation Damage on Behavior of Deeply Embedded Anchor Bolts

In Texas, many drilled-shaft concrete foundations of high-mast illumination poles (HMIPs) constructed in the late 1980s were found to have premature concrete deterioration due to alkali-silica reaction (ASR) and delayed ettringite formation (DEF). To investigate the effect of ASR/DEF damage on the structural performance of deeply embedded bolts anchoring the pole to the HMIP foundation, six full-scale field tests were conducted in Houston, TX. In this study, two types of drilled-shaft foundations were examined: drilled shafts with 16 anchor bolts supporting 150 ft (46 m) poles, and shafts with 20 anchor bolts supporting 175 ft (53 m) poles. One of the tested drilled shafts with 20 anchor bolts was repaired by wrapping it with carbon fiber-reinforced polymer (CFRP) sheets. Failure modes and related load-transfer mechanisms were investigated and design refinements were developed to current ACI requirements for these deep anchor bolts.

Performance-Based Design of Confining Reinforcement: Research and Seismic Design Provisions

In performance-based seismic design, evaluation of the deformation capacity of reinforced concrete columns is of paramount importance. The deformation capacity of a column can be expressed in several different ways: (1) curvature ductility, (2) displacement ductility, or (3) drift. Even though several performance-based confining reinforcement design procedures have been proposed, the relationship between different ductility factors is not clearly understood. The effect of concrete strength, longitudinal reinforcement ratio, volumetric ratio of confining reinforcement, shear span-to-depth ratio, and axial load level on the relationship between different ductility factors was studied. Finally, the confinement reinforcement design requirements of current design codes and recently proposed performance-based design methods were compared and critically examined.