Keh-Chyuan Tsai

Improved time integration for pseudodynamic tests

Converting the second-order differential equation to a first-order equation by integrating it with respect to time once as the governing equation of motion for a structural system can be very promising in the pseudodynamic testing. This was originally found and developed by Chang. The application of this time-integration technique to the Newmark explicit method is implimented and investigated in this paper. The main advantages of using the integral form of Newmark explicit method instead of the commonly used Newmark explicit method in a pseudodynamic test are: a less-error propagation effect, a better capability in capturing the rapid changes of dynamic loading and in eliminating the adverse linearization errors. All these improvements have been verified by theoretical studies and experimental tests. Consequently, for a same time step this time-integration technique may result in less-error propagation and achieve more accurate test results than applying the original form of Newmark explicit method in a pseudodynamic test due to these significant improvements. Thus, the incorporation of this proposed time-integration technique into the direct integration method for pseudodynamic testings is strongly recommended.

Cyclic performance of steel beam-column moment joints

The performance of ten seismic beam-to-column moment connection specimens using bolted web-welded flanges (BWWF) detail is critically assessed. Key parameters include: beam flange to entire section plastic modulus ratio, ZFZ, supplemental web bolts, supplemental web welds and column panel zone strength. It was found that the cyclic plastic rotational capacities of the BWWF connections ranged between 0.009 and 0.018 rad, and did not indicate apparent relationships with respect to the beam section modulus ratios. The ultimate beam flange flexural strength, ZFFu, well predicted the ultimate moment capacities of BWWF connections. Using the proposed strength criterion, the supplemental web welds, but not the supplemental web bolts, significantly enhance the strength, ductility and energy absorbing capacity of the BWWF connections.

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....

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.

Coupled Tuned Mass Dampers for the Seismic Control of Asymmetric-Plan Buildings

An innovative tuned mass damper, referred to as a coupled tuned mass damper (CTMD), is proposed for the control of a coupled vibration mode of one-way asymmetric-plan buildings. The CTMD simultaneously translates and rotates almost resonantly with the vibration of the controlled mode, which actually vibrates in translation, as well as rotation. Thus, the CTMD can be viewed as a direct approach for controlling the modal vibration of asymmetric-plan buildings. First, the CTMD is developed from the two-degree-of-freedom modal system, which has one active and one spurious vibration frequency. It is illustrated that the optimum parameter values of the CTMD can be conveniently determined from those of the corresponding tuned mass damper (TMD). Second, in order to apply the CTMD to a building structure, the properties of the CTMD obtained in the modal space are transformed into the physical space. Finally, the effectiveness of the CTMD in reducing the vibrations of asymmetric-plan structures is verified by investigating the frequency response functions and the response histories of three eight-story asymmetric-plan buildings with and without dampers. These three buildings are respectively torsionally stiff, torsionally similarly stiff, and torsionally flexible. This study confirms that the CTMD is an effective alternative for the seismic control of asymmetric-plan buildings.

A Ground Motion Scaling Method Considering Higher-Mode Effects and Structural Characteristics

As nonlinear response history analysis (NLRHA) becomes a frequently used procedure for the seismic demand evaluation of multistory buildings, it becomes increasingly important to develop a ground motion scaling method which properly includes the dominating modes in the seismic demand estimates. This paper proposes a multimode ground motion scaling (MMS) method, which applies the square root of the sum of the squares (SRSS) or complete quadratic combination (CQC) rule in computing peak seismic demands. The aim is to minimize the weighted sum of the square differences between the spectral responses of a given ground motion and the design response spectrum for the first few modes. Using four case studies, this paper compares the effectiveness of the MMS method with the other common scaling procedures. It is illustrated that the MMS method is effective in reducing the scatter in the peak seismic demands computed from both the response spectral analysis (RSA) and the NLRHA. Recommendations for selecting the ground motion records for the application of MMS method are also provided.

Pseudo-Dynamic Test of Full-Scale Rcs Frame: Part II - Analysis And Design Implications

This is the second of a two-part paper, describing an investigation of a full-scale three-story composite RCS frame that was tested in the NCREE laboratory in Taipei, Taiwan. The frame specimen was pseudo-dynamically loaded to represent four earthquake ground motions of varying hazard levels, after which the frame was subjected to a monotonic pushover loading out to interstory drift ratios of 10%. This paper summarizes the analytical studies of the test frame, including comparisons with measured response and design implications. Damage indices are investigated to help interpret the analytical results and relate the calculated engineering demand parameters to physical damage in the frame. In terms of peak displacements and overall response, the analytical and measured frame response agree fairly well up to drift ratios of about 3%. Beyond this, discrepancies occur, which are likely due to degradation effects (e.g., local flange buckling) that are not modeled in the analysis. Comparison between calculated damage indices and observed damage suggest the need for further research to improve the performance simulation tools.

Research Needs and Future Directions for Steel Plate Shear Walls

Steel plate shear walls (SPSWs) are one of the most economical and under-utilized lateral load resisting systems currently available to structural engineers. In comparison with traditional lateral load systems, such as steel braced frames, reinforced concrete walls and moment resisting frames, SPSWs have fewer costly detailing requirements, require less stringent construction tolerances, allow for rapid construction, and result in fewer bays of lateral load resisting framing. Past studies have also shown that SPSWs can exhibit exemplary seismic performance. Despite these advantages, SPSWs are not widely used because: i) traditional SPSW configurations result in large column dimensions and prohibit the use of narrow walls, thereby reducing the system’s economy, ii) numerical models used to analyze SPSW systems are cumbersome and overly time consuming for engineers, iii) SPSW system behavior is not well understood, leading to conservative design requirements and further reduction in economy, and iv) SPSWs have a lower flexural stiffness relative to concrete walls, making their use in taller buildings more challenging. Further, SPSW systems must be studied in the context of performance-based design as this will result in reliable and robust systems. This paper will discusses the issues above, with specific examples and propose solutions for developing the next-generation of steel plate shear walls. These solutions will allow SPSWs to be economically implemented by providing new configurations, new modeling techniques, and a more complete understanding of system behavior. Development of these solutions and performance-based criteria for their design will require a significant, coordinated research initiative. As such, research needs are identified and discussed.

Inelastic Responses of Two-Way Asymmetric-Plan Structures under Bidirectional Ground Excitations—Part I: Modal Parameters

It has been found that any one vibration “mode” of an inelastic multistory two-way asymmetrical building structure can be represented by a three-degree-of-freedom (3DOF) modal system representing two modal translations and one modal rotation. This study introduces the inelastic response spectra constructed from the inelastic 3DOF modal systems, which is specifically useful for multistory two-way asymmetric-plan buildings subjected to bidirectional ground excitations. These spectra for asymmetrical structures (SAS) provide the three-component inelastic peak modal responses of multistory two-way asymmetric-plan buildings subjected to bidirectional ground excitations. In order to construct the SAS, the independent elastic parameters of the 3DOF modal systems were identified and the inelastic 3DOF modal parameters versus the strength ratio relationships were established. The parametric study of the 3DOF modal parameters showed the ranges and the variation trends of these parameters. Two example buildings are analyzed to verify the effectiveness of the above-mentioned investigations.

Inelastic Responses of Two-Way Asymmetric-Plan Structures under Bidirectional Ground Excitations— Part II: Response Spectra

Based on the studies in the companion paper, this paper presents the inelastic response spectra for asymmetrical structures (SAS) under bidirectional ground excitations. Firstly, the constant-strength SAS were constructed and compared with the corresponding conventional constant-strength response spectra. It was found that the modal ductility demands of an asymmetric-plan structure could be significantly overestimated from the conventional constant-strength response spectra as the nonlinear “modal” rotation and translations may not be proportional. Furthermore, the translation-rotation interaction effect is not considered in the conventional constant-strength response spectra. Secondly, the influences of the three-degree-of-freedom (3DOF) modal parameters on the ductility demand were extensively studied. Thirdly, the normalized peak edge translation spectra were also investigated. It was found that the normalized peak edge translation resulting from a single vibration mode may be overestimated twofold by using the single-degree-of-freedom (SDOF) modal systems.