Kazuhiko Kasai

A Spectral Difference Method to Estimate Building Separations to Avoid Pounding

A method to estimate the likely minimum building separation necessary to preclude seismic pounding is presented. The method is based on random vibration theory. The accuracy is demonstrated by numerical experiments using 9 artificial and 6 actual earthquake records. The relation between the minimum separation, period, height, damping and earthquake type are discussed. Example applications of the theory are presented. The effects of inelastic hysteretic building behavior are also discussed.

Seismic Retrofit Using Rocking Walls and Steel Dampers

A retrofit system of prestressed concrete rocking walls and steel dampers is used to control the seismic damage mode and increase the strength and energy dissipating capacity of an 11 story steel reinforced concrete frame in Japan. Important details of the retrofit design are introduced. The seismic performance of the structure before and after the retrofit is evaluated through extensive nonlinear time history analysis. Results show that the rocking system can significantly reduce both the seismic responses to different earthquake ground motions and their scattering. This makes the damage mode and the seismic performance of the retrofitted building more predictable, leading to a possibility of more reliable performance-based seismic design.

Behavior of Interior Narrow and Wide Beams

Narrow as well as wide beams are used in moment-resisting reinforced concrete frames in seismic zones; typically the logitudinal beam bars in such frames are placed inside the column. Experimental investigations into the behavior of reinforced concrete beam-column-slab cruciform interior joint subassemblages studied include narrow and wide beams with some of the top longitudinal reinforcing bars placed outside the columns, thereby reducing the bars in the columns. This detailing is not permitted in U.S. seismic codes. However, test assessment indicates that placement of some of the longitudinal bars outside of the columns is permissible, leading to lesser congestion of reinforcement.

Performance of Seismic Protection Technologies during the 2011 Tohoku-Oki Earthquake

Many buildings in the Tohoku and Kanto area were strongly shaken during the Tohoku-oki earthquake on 11 March 2011. Soon after the earthquake, the Japan Society of Seismic Isolation (JSSI) established committees to investigate the performance of structures where modern seismic protection methods were used. Since response-control technologies are relatively new, few systems had been validated with actual earthquakes. Generally, the various buildings performed well, and there were no failures of superstructures of base-isolated or supplementally damped buildings. Failures of numerous expansion joints and lead dampers in base-isolated buildings are described. Acceleration records of both base-isolated and supplementally damped buildings are analyzed and their seismic performance characteristics are discussed.

Large-Scale Testing of Steel Unbonded Braces for Energy Dissipation

This paper outlines large-scale tests of tension/compression yielding braces (also called “unbonded braces”) in support of their first applications in the United States. The core steel in these braces provides stable energy dissipation by yielding under reversed axial loading, while the surrounding concrete-filled steel tube resists compression buckling. The paper summarizes a series of tests on large-scale unbonded braces, having yield forces of 1200, 1600, and 2100 kN (270, 360, and 470 kips). Each brace was subjected to a cyclic loading pattern consistent with that used widely for testing steel beam-column connections. Additional tests explored the behavior of the braces under a near-field loading history, a displacement time history derived from a seismic analysis of an idealized 5-story building, and a low-cycle fatigue test.

E-Defense Tests on Full-Scale Steel Buildings Part 2 - Collapse Experiments on Moment Frames

A major research project on steel buildings utilizing the E-Defense three-dimensional shake table facility is underway in Japan. It involves several studies on moment frames, innovative methods for new or existing buildings, protective systems, and nonstructural elements. As one of the studies, a challenging seismic experiment to simulate collapse of a full-scale bulding comprising the moment frame will be conducted in September 2007. Collapse is defined as the state where the structure loses its ability to sustain gravity loads. To-date, only a few full-scale tests have been conducted for the multi-story steel moment frames, and none of those simulated collapse. Two decades ago, a 6-story moment frame was pseudo-dynamically tested by static actuators simulating severe earthquakes. Recently, a 3-story moment frame including exterior walls was tested up to an overall drift angle of 1/15 rad, by static loads to reflect the effects of the ground motion that is greater than considered in the current seismic codes. These full-scale tests employed quasi-static loading, since a dynamic loading facility capable of much more realistic simulations was not available. The E-Defense shake table which commenced operation in 2005 is capable of subjecting a full-scale structure to the strongest ground motion recorded in the world. With a 12,000 kN specimen placed on 15m x 20m table, the table can produce a maximum velocity of ±2.0m/s and a maximum displacement of ±1.0m. With the E-Defense shake table, we have the first opportunity ever to simulate the behavior of full-scale structures up to collapse, and to obtain detailed response data. This paper outlines the test plan of collapse simulation, as well as technical issues being addressed during the process of test preparation.

Analysis of Northridge damaged thirteen-story WSMF building

The case study building experienced moment connection fractures and has instructure recorded response. Correlative analyses are performed using a probability approach to describe the connection rotational capacities. The Northridge calibrated model is then subjected to other scenario earthquake records. The main points include: -Northridge connection damage can be reasonably simulated using models that idealize the rotation capacities as random variables, -Many of the connection fractures occurred from beam moment demands that were less than the beam yield moments, -Vertical earthquake excitation and gravity loads did not play a major role in the connection fractures, -The damaged building can withstand another Northridge intensity quake, -Near-source ground motions from great quakes have the potential to collapse the building with pre-Northridge type connection details, and -Connection rotation capacity can make the difference between building survival or collapse during severe quakes.

ASCE-41 and FEMA-351 Evaluation of E-Defense Collapse Test

A welded steel moment-frame building is used to assess performance-based engineering guidelines. The full-scale four-story building was shaken to collapse on the E-Defense shake table in Japan. The collapse mode was a side-sway mechanism in the first story, which occurred in spite of a strong-column and weak-beam design. Computer analyses were conducted to simulate the building response during the experiment. The building was then evaluated using the Seismic Rehabilitation of Existing Buildings (ASCE-41) and Seismic Evaluation and Upgrade Criteria for Existing Welded Steel Moment-Frame Buildings (FEMA-351) for the collapse prevention performance level via linear and nonlinear procedures. The guidelines had mixed results regarding the characterization of collapse, and no single approach was superior. They mostly erred on the safe side by predicting collapse at shaking intensities less than that in the experiment. Recommendations are made for guideline improvements.

Rigid-bolted repair method for damaged moment connections

Historically, bolted connections have been relegated to providing only gravity or semi-rigid moment connections in seismic steel frame buildings. Failure of welded connections in the Northridge earthquake to provide expected design capacities gave impetus for renewed research into the use of bolted connections for rigid moment frame connections in high seismic zones. Tests of full size beam-column subassemblies employing new types of bolted connections were conducted at Lehigh University as a joint industry study with ICF Kaiser Engineers. The results have shown that bolted connections are capable of providing rigid moment connections with cyclic plastic rotational capacities in excess of equivalent welded joints, but with the same rigidity as welded connections. The beam depths ranged from 406 mm (16 in) to 914 mm (36 in). Connections were designed to allow easy installation for retrofit of existing connections.

RATIONAL POLYNOMIAL APPROXIMATION MODELLING FOR ANALYSIS OF STRUCTURES WITH VE DAMPERS

The objective of this paper is to introduce a Rational Polynomial Approximation (RPA) method for modelling the response of structures that contain discrete elements with linear frequency-dependent stiffness and damping characteristics. The RPA method consists of two steps: First, system identification is performed to obtain a rational polynomial approximation for the systems transfer functions. Then, a time-domain model for the system is realised. The main advantage of the RPA method is that the resulting model is a system of ordinary differential equations, facilitating time history analysis of both linear and nonlinear structures using standard time-step integration algorithms and procedures. Viscoelastic (VE) dampers comprise one of the primary classes of frequency-dependent dampers with both frequency-dependent stiffness and damping. VE dampers are used for mitigation of seismic- and wind-induced structural vibration. When using VE dampers in analysis, effective modelling of the frequency-dependent characteristics of the VE damper plays a key role in accurate simulation of structural responses. Following a description of the theory behind the RPA method, the efficacy of the method is verified through several numerical examples employing VE damped structures. The results are compared with Kasais fractional derivative model for VE dampers. Application of the RPA method to nonlinear structures is also given. The RPA method is shewn to be effective and efficient for modelling VE damped structure.