Masayoshi Nakashima

Real-time on-line test for MDOF systems

This paper presents a test system for conducting on-line tests in a real time and a series of real-time on-line tests conducted to verify the effectiveness of the system. The proposed system is characterized by (1) use of a Digital Signal Processor (DSP) now readily available, (2) adoption of the C language to ensure easy programming, and (3) separation of response analysis and displacement signal generation to apply the system for tests with complex structures. To create displacement signals successively without being interrupted by the computation of equations of motion, extrapolation and interpolation procedures using present and past target displacements are developed. Base-isolated building models were chosen for the real-time on-line test. The effectiveness of the extrapolation and interpolation procedures was demonstrated through a series of real-time on-line tests applied to the models treated as SDOF structures. A five-storey base-isolated building model (treated as a six DOF structure) was tested for various ground motions, and it was verified that the system is able to simulate earthquake responses involving large displacements and large velocities. The number of DOFs that can be handled in the proposed system was investigated, and it was found that the system is capable of performing the test with reasonable accuracy for up to 10 DOF structures with a range of response frequency not greater than 3·0 Hz, or 12 DOF structures with a range of response frequency not greater than 2·0 Hz. Copyright

CLASSIFICATION OF DAMAGE TO STEEL BUILDINGS OBSERVED IN THE 1995 HYOGOKEN-NANBU EARTHQUAKE

This paper presents an overview of the damage to steel buildings observed in the 1995 Hyogoken-Nanbu earthquakes. It summarizes the Japanese seismic design concept and the level of earthquake forces exerted on steel buildings built in severely shaken regions. Statistical data on the damage are presented with respect to the building height and type and the location of damage and typical damage is highlighted. The damage to welded beam-to-column connections is explained from the perspectives of materials, welding, connection details and plastic rotation demand. Finally, the damage was classified with respect to the previous awareness and the difficulty in providing solutions.

BASE-ISOLATION DESIGN PRACTICE IN JAPAN: INTRODUCTION TO THE POST-KOBE APPROACH

Japan has twenty years of experience in designing and constructing base-isolated building structures. Construction has increased significantly since the 1995 Hyogoken-Nanbu (Kobe) earthquake, having reached over 150 annual construction projects. Many new developments and refinements have been made in the material, device, design, and construction of these structures. This paper summarises recent design and construction of base-isolated building structures in Japan, including statistical data with respect to the common usage as well as the number of new projects. It is notable that the size, height, and fundamental natural period of new base-isolated buildings increase steadily with time, indicating that base-isolation in Japan is reaching maturity. Base-isolators and dampers commonly adopted in Japan are also introduced, with emphasis on recent design efforts to enlarge the natural period of base-isolated structures and reduce the lateral forces induced in the superstructure. Basic design procedures are presented, including determination of design earthquake forces, modelling of base-isolation layers, modelling of the superstructure, selection of ground motions, time-history analyses, and performance criteria. A mandated peer-review system, unique for design of base-isolated structures, is also noted. Several characteristic issues in the design of base-isolated structures are discussed: Variation of base-isolation material properties, applications to high-rise buildings, effects of vertical ground motions, and response when subjected to near-fault ground motions.

ENERGY INPUT AND DISSIPATION BEHAVIOUR OF STRUCTURES WITH HYSTERETIC DAMPERS

This paper presents qualitative investigations on the energy behaviour of structures into which hysteretic dampers are incorporated. Emphasis was given to the ratio of the structural stiffness after the yielding of hysteretic dampers to the initial elastic stiffness, with a premise that this ratio, termed α in this study, tends to be large for structures with hysteretic dampers. Structures concerned were represented by discrete spring-mass systems having bilinear restoring force behaviour, in which the second stiffness relative to the initial stiffness is α. It was found that with the increase of α the total input energy tends to increase, but the increase is confined to a narrow range of natural periods. Both the total input energy and hysteretic energy were found to become less sensitive to the yield strength with the increase of α. A simple formula was also proposed to estimate the maximum deformation given the knowledge of the hysteretic energy. Analysis of MDOF systems revealed that, even when α is large, the total input energy and hysteretic energy for MDOF systems are approximately the same as those of the equivalent SDOF system, and the hysteretic energy can be distributed uniformly over the stories if α is large.

Seismic Damage Detection of a Full-Scale Shaking Table Test Structure

A series of full-scale tests was conducted on the E-Defense shaking table facilities in Japan to simulate various levels of realistic seismic damage in a high-rise structural steel building. During the shaking table tests, the specimen experienced damage of the concrete slabs, beam-to-column connections, and nonstructural walls. The densely recorded test data of global and local structural deformation and the extensive acceleration records provide a unique benchmark case for evaluating the effectiveness of vibration-based damage diagnosis methods. Dynamic properties of the specimen were extracted from floor accelerations under the white noise excitations by the frequency response function curve-fitting method and autoregressive with exogenous term method. The natural frequencies of the structure decreased on average 4.1, 5.4, and 11.9% after three levels of seismic excitation, respectively, because of increasing extent of structural and nonstructural damage. The analysis of the vibration data shows that t...

Out-of-Plane Stability of Buckling-Restrained Braces Placed in Chevron Arrangement

AbstractLarge-scale shake table tests were performed at E-Defense to examine the out-of-plane stability of buckling-restrained braces (BRBs). Two specimens were subjected repeatedly to a near-fault ground motion with increasing amplification. The test specimens comprised a single-bay, single-story steel frame and a pair of BRBs placed in a chevron arrangement. The specimens were not braced at the brace-to-beam intersection in order to produce a condition where the BRBs were susceptible to out-of-plane instability. Standard BRBs were used in the first specimen, while BRBs with a flexible segment at each end of the steel core were used in the second specimen. A simple stability model predicted the BRBs in the second specimen to fail because of out-of-plane buckling. The first specimen exhibited excellent ductility during the shake table tests, while the second specimen developed severe out-of-plane deformation that compromised the ductility of the BRBs. Based on the experimental observations and the stabili...

Hysteretic Behavior and Strength Capacity of Shallowly Embedded Steel Column Bases

A series of shallowly embedded steel column base consisting of an exposed column base and a covering reinforced concrete floor slab were tested under horizontal cyclic loading to very large deformation. By adjustments to the floor slab thickness, shape, and reinforcing bars in the slab, the initial stiffness, maximum strength, and dissipated energy of the shallowly embedded column base increase significantly with respect to those of the exposed column base. It is found to be practical to strengthen the shallowly embedded column base so that it would behave like a fully embedded column base. Punching shear failure in the floor slab around the column due to the uplift of the base plate occurs when the shallowly embedded column base fails. Based on the plastic theory, a mechanical model that considers the contributions of the anchor bolts and the bearing and punching shear of the floor slab is proposed to evaluate the maximum strength. The evaluated results have good agreement with the test results, with errors not greater than 20%.

EFFECT OF COLUMN BASE BEHAVIOUR ON THE SEISMIC RESPONSE OF STEEL MOMENT FRAMES

The plastic deformation behaviour of embedded-type column bases was experimentally examined. The primary parameters were width-to-thickness ratio and axial load ratio, and results showed significant strength deterioration induced by local buckling that was largely affected by the test parameters. Analytical models of the test specimens were developed. The developed models were applied for nonlinear dynamic analysis of low- and mid-rise steel moment frames. The analysis results showed the importance of considering deterioration behaviour of column bases as it significantly affected the response and the collapse limit. Large dependency of the collapse limit on the width-to-thickness ratio of the column in addition to the strength itself is also noted from the analysis.

Experimental Behavior of Concrete-Filled Steel Tube Columns Using Ultrahigh-Strength Steel

Concrete-filled steel tube (CFT) columns offer significant advantages over columns made of either steel or concrete alone, such as large energy dissipation and increased strength and stiffness. To further improve the seismic performance of these columns, an experimental investigation was conducted into CFT columns using ultrahigh-strength steel. More specifically, seven square and circular specimens made with high-strength and conventional steel were subjected to constant compressive axial load and cyclic flexural load protocols with two and 20 cycles imposed at each drift level. Based on the test results, the influence on the CFT’s cyclic behavior of the high-strength steel, crosssectional shape, axial load, and number of cycles in lateral loading history was studied. In comparison with the conventional steel specimens, larger elastic deformation, higher strength, and delay of local buckling were observed in the high-strength steel specimens, while compared with the circular specimens, the square specimens sustained larger drift angles without fracture of their steel tubes because of the development and progress of serious local buckling. Furthermore, a simple analytical model based on the concept of the superposed strength method was proposed. The accuracy of this model was confirmed with the experimental results.

Development of a Steel Brace with Intentional Eccentricity and Experimental Validation

AbstractConventional buckling braces are the most commonly used steel bracing systems but are characterized by intense local midlength buckling that leads to unstable energy dissipation and finally...