Charles W. Roeder

Overview of hybrid and composite systems for seismic design in the United States

An overview of the use of composite structural systems for seismic design in the United States. Provides a brief summary of the types of buildings constructed from different composite systems including normal composite slabs, concrete filled tubes, encased steel sections, and connections between concrete walls and steel framing. The present understanding of the behavior of these systems is described. It will be shown that the construction practice has frequently gone beyond the research and theoretical understanding of the behavior of these complex elements. Present ongoing research efforts are, therefore, briefly noted.

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.

STATE-OF-THE ART ELASTOMERIC BRIDGE BEARING DESIGN

The paper summarizes research and proposed design provisons for elastomeric bridge bearings and correlates them to earlier design methods. Proposed provisions for controlling fatigue, delamination, yield and rupture of the reinforcement, stability, and excessive rotation are summarized. Quality control measures needed to sustain these proposed provisions are reviewed, and the effect of low-temperature stiffness on bearing performance is noted. It is shown that the proposed provisions will give a significant increase on the load capacity and height limitations for elastomeric bridge bearings, and that these increased limits will allow elastomeric bearings to accommodate much larger movements and rotations, resulting in the transmission of smaller forces between the bridge substructure and superstructure. This may afford more economical bridge design and maintenance, and permit the use of the less expensive elastomeric bearings.

Advantages and limitations of seismic isolation

Under many circumstances, seismic isolation is an effective way of reducing the impact of earthquakes on structures. Dynamic forces in the structure itself are reduced at the expense of relatively large displacements in the isolators. These displacements can generally be predicted adequately by simple methods and accommodated without difficulty. However, some circumstances appear to exist where the isolator displacements might be significantly larger than conventional analysis would suggest, or where simplified methods of analysis may prove inadequate and fail to predict the response properly. The paper explores the limits of the applicability of equivalent linear analyses and the response to ground motions which might lead to large displacements isolation that can be achieved.

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...

Experimental Investigation of Chevron Concentrically Braced Frames with Yielding Beams

AbstractSteel concentrically braced frames designed prior to the implementation of capacity design principles in seismic design provisions may exhibit poor inelastic response under seismic excitation. These older, nonductile concentrically braced frames (NCBFs) used several configurations, with the chevron configuration being one of the most common. The response of chevron-configured NCBFs is unknown, as relatively large axial and flexural demands are imposed on the beam after brace buckling. Current code requirements for special concentrically braced frames (SCBFs) promote full yielding of the braces while the beam remains elastic, but NCBFs develop a mechanism in which the beam yields, deforms plastically, and limits tensile elongation of the brace. However, if ductile, this plastic mechanism may meet current performance limits and not require retrofitting. To examine this issue, four tests of two-story NCBFs were conducted at the National Center for Research on Earthquake Engineering in Taipei, Taiwan....

Experimental Evaluation of the Seismic Vulnerability of Braces and Connections in Older Concentrically Braced Frames

AbstractConcentrically braced frames (CBFs) have been used as seismic force resisting systems in steel structures for many decades. CBFs designed prior to about 1988 were not capacity designed for the expected brace strength, and many are therefore prone to undesirable failure modes when subjected to large earthquakes. The consequences of other factors, such as weld toughness, gusset plate clearance to permit brace end rotation, and local slenderness of the brace, were also not fully understood, so older CBFs may also exhibit premature failure relative to modern, special CBFs (SCBFs). Thus, these older CBFs are expected to exhibit low inelastic deformation capacity and are termed nonductile CBFs (NCBFs) here. An infrastructure review showed that many NCBFs are deficient by SCBF standards and an experimental program was undertaken to explore the impact of brace and connection deficiencies in particular. Brace local slenderness and brace-to-gusset plate weld deficiencies were found to be highly detrimental ...

Seismic Design and Hybrid Tests of a Full-Scale Three-Story Concentrically Braced Frame Using In-Plane Buckling Braces

This research investigates the brace-to-gusset connection designs to allow the braces buckle in the plane (IP) of the frame. In order to study the performance of the IP buckling brace connections with different design details, five 3,026 mm–long A36 H 175 × 175 × 7.5 × 11 mm braces were tested using cyclically increasing axial displacements. All specimens failed at an average axial strain less than 0.025 due to the brace fracture at the mid-length where severe local buckling had occurred. Pseudo-dynamic tests on a three-story special concentrically braced frame (SCBF) using the proposed brace end connection details for six A36 H 150 × 150 × 7 × 10 mm braces were conducted using the PGA = 597 cm/s2 LA03 record to confirm with the component tests. The knife plates and IP buckling braces sustained a peak 0.049 rad interstory drift under the design base earthquake without fracture. The highly nonlinear responses were satisfactorily predicted by OpenSees. Recommendations on the seismic design of the IP buckling brace connections are provided.

Improved Seismic Design of Concentrically Braced Frames and Gusset Plate Connections

Special concentrically braced frames (SCBFs) are economical and provide substantial lateral resistance and stiffness. They are frequently used in seismic design, since recent issues with steel moment resisting frames have increased their cost and raised doubts about their performance. Unfortunately SCBFs are not as well understood as many other structural system, because they rely on inelastic post-buckling of the brace during extreme seismic events. As a result, several research studies have been undertaken to better understand and improve SCBF seismic behavior. A research study has been in progress at the University of Washington (UW) to improve the seismic performance of SCBF gusset plate connections. To date more than 20 large-scale SCBF frames with gusset plate connections have been tested, and extensive analysis has been performed. This work is summarized, and the broad general conclusions from the experimental work are noted and demonstrated. The importance of connection stiffness and gusset plate welding and the relative performance of bolted vs. welded and tapered vs. rectangular gusset plates are shown. The results lead to recommended changes in the current design methods. In particular, a modified method for assuring end rotational capacity for brace buckling is proposed. This method provides lighter more compact gusset plates, which develop greater inelastic deformation capacity from the frame and less secondary damage to framing members.