Dawn E. Lehman

Update to ASCE/SEI 41 Concrete Provisions

A proposed supplement to ASCE/SEI 41 Seismic Rehabilitation of Existing Buildings has been developed for the purpose of updating provisions related to existing reinforced concrete buildings. Based on experimental evidence and empirical models, the proposed supplement includes revisions to modeling parameters and acceptance criteria for reinforced concrete beams, columns, structural walls, beam-column joints, and slab-column frames. The revisions are expected to result in substantially more accurate and, in most cases, more liberal assessments of the structural capacity of concrete components in seismic retrofit projects.

Strength and Stiffness of Circular Concrete-Filled Tubes

Concrete-filled tubes (CFTs) are composite structural members that consist of a steel tube and concrete infill. CFTs optimize the contributions of both components by improving their geometric efficiency and fully using their inherent strengths. The concrete infill is confined by the steel tube, resulting in a triaxial state of compression that increases the strength and strain capacity of the concrete. The perimeter steel is at its optimal location, and the concrete infill delays local and global buckling of the tube. CFTs are easily and rapidly constructed and provide significant compression, bending, and shear resistance. They may be used for bridge piers and building columns. However, current design specifications for CFTs vary significantly, thereby limiting the current understanding and use of these components. This study addresses combined axial and flexural loading and determines the best models for predicting the stiffness and resistance of circular CFT. A database of 122 test specimens was compiled and evaluated. The results indicate that the plastic stress method is a simple yet effective method to predict the resistance of circular CFT components under combined loading. These data show that current specifications provide inaccurate predictions of the flexural stiffness, and a new stiffness expression is proposed. The proposed models permit simple yet accurate predictions of stiffness and resistance and allow engineers to use CFT components routinely in structural design.

REPAIR OF EARTHQUAKE-DAMAGED BRIDGE COLUMNS

Columns supporting bridge structures are likely to respond inelastically during strong earthquakes. Restoration to serviceable conditions may require repair or replacement of damaged regions, with considerable cost and operational delay. This paper describes an experimental study undertaken to identify the performance of reinforced concrete columns repaired by varying techniques. Four spirally reinforced columns designed to meet current seismic detailing practice were damaged, repaired, and retested. The damage levels were classified as either moderate or severe. Four different repair techniques were applied. The performance of each repair technique was determined by comparing the response of the repaired column with the response of the original column as well as with the design intent.

Lumped-Plasticity Models for Performance Simulation of Bridge Columns

In bridge seismic design, a lumped-plasticity model based on a specified plastic-hinge length expression is used to estimate the ultimate displacement capacity. In contrast to current design guidelines, performance-based earthquake engineering (PBEE) requires assessment of damage for multiple demand levels. Implementation of PBEE of bridges therefore requires analytical methods capable of predicting bridge damage for multiple levels of earthquake demand. This paper evaluates models for PBEE of bridge columns, including new expressions for effective elastic stiffness, plastic-hinge length, and strain at onset of bar buckling. Data from 37 tests of large-scale circular bridge columns with post-1980 design details were used for model evaluation and development. The models were used to compute displacements and strains associated with various damage states and the resulting mean values and standard deviations. Findings indicate that the new model provides more accurate and precise predictions of damage in bridge columns than do existing models.

Bond-Slip Response of Reinforcing Bars Grouted in Ducts

The bond behavior of reinforcing bars grouted in ducts and subjected to cyclic loads is addressed. This behavior has a significant influence on the seismic response of precast concrete frames in which the connections depend on these grouted bars. The critical response quantity is the displacement at the loaded end of the bar caused by inelastic strain penetration into the bonded region. A study was carried out using experimental and analytical methods. In the experimental portion of the study, tests were conducted on bars grouted over only a short length to develop a constitutive relationship between local bond stress and slip. That relationship was then incorporated into an inelastic, finite element model of the bar. The model was validated by comparing the measured free end displacement of a fully developed bar with the value predicted by analysis. It was then used to conduct a parametric study that extended the investigation beyond the bounds of the matrix of the physical tests. Results of the parametric study were used to develop a simple design equation for predicting the displacement of the loaded end of the bar.

Seismic Performance Assessment of Concentrically Braced Steel Frames

Concentrically braced frames (CBFs) are stiff, strong structures, which are used for many seismic design applications. The seismic performance of CBFs varies widely because their behavior is a complex combination of resistance, elastic stiffness, and inelastic behavior and deformation. Performance-based seismic engineering (PBSE) is increasingly important. Fragility curves are statistical estimates of performance as a function of an engineering demand parameter. The Applied Technology Councils ATC-58 program is developing fragility curves for a wide range of structural systems, and this paper describes the development of fragility curves for CBF systems using experimental data that accurately simulated the boundary conditions and inelastic response of CBFs. Fragility curves were developed for common brace cross sections and gusset plate geometries at various damage states. In addition, fragility curves were developed for CBFs designed using an improved design procedure and compared to those designed using standard design methods.

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...

Seismic Behavior of a Modern Concrete Coupled Wall

AbstractRC core walls are used commonly in modern building construction as the primary lateral load–resisting system. To meet architectural constraints, including elevator, stair, and doorway openings, common configurations include walls coupled together with heavily reinforced, low-aspect-ratio coupling beams. Numerous studies have focused on coupling beams and improved seismic performance and design of coupling beams. However, far fewer research programs have studied the seismic behavior of the coupled wall system. Coupled walls are typically used in mid- to high-rise construction, and understanding their seismic response requires simulation beyond the coupling beam and must include all important yielding components of the system. A research project was undertaken to investigate the response of a midrise coupled wall designed to meet current codes. The advanced testing capabilities of the University of Illinois at Urbana-Champaign George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES)...

Experimental Performance of Steel Braced Frames Subjected to Bidirectional Loading

Concentrically braced frames (CBFs) are stiff, strong systems frequently used to resist seismic loading. Special CBF (SCBF) behavior is dominated by brace buckling, while buckling restrained braced frames (BRBFs) develop tensile and compressive yielding and avoid brace buckling. Both systems are widely used in seismic design, and both have a number of specific design issues. This paper describes a first of its kind, 2-story, 1-bay by 1-bay frame tested at the University of Minnesota Network for Earthquake Engineering Simulation facility to examine the large-displacement, bidirectional behavior of SCBFs and BRBFs with realistic boundary conditions and to verify the design approach. The SCBF had rectangular hollow steel section (HSS) braces in a single-story X configuration, and the BRBF used a single-diagonal configuration. The design of the gusset plates for the HSS braces followed a previously proposed balanced design procedure with an elliptical clearance to permit out-of-plane rotation caused by brace buckling. The single-story X-brace SCBF concentrated damaged into one-half the brace length, and the first HSS brace fractured at 2% story drift. The BRBF gusset-plate design followed current design standards, and two of the BRB cores fractured at 3.6 and 4.2% story drift prior to any instability in the BRB or system. The SCBF sustained limited damage to the beams and columns; however, the BRBF had much more significant damage to these members because of larger deformations and BRBF behavior. The results indicate that these systems have a stable response to large cyclic deformations and the impact of bidirectional loading on the measured response was minimal.

Seismic Vulnerability of Older Braced Frames

AbstractConcentrically braced frames (CBFs) are broadly used as lateral-load resisting systems in buildings throughout the United States. Current state-of-the-practice is the use of special concentrically braced frames (SCBFs) where ductility under seismic loading is necessary. Prior to modern seismic codes, braced frames were designed without ductile detailing. Here these systems are termed nonseismic braced frames (NCBFs), which are essentially CBFs designed with no special detailing requirements. These may comply with older code requirements in high-seismicity regions or current code requirements in low-seismicity regions. Many are still in service throughout the United States. Prior research has focused on SCBFs, which has improved their seismic performance. In comparison, there is significant uncertainty regarding the seismic performance of NCBFs and they may be vulnerable to collapse. A study was conducted to evaluate this vulnerability. At the start, a pilot experimental test of NCBFs was conducted...