Severe Seismic Excitations Research Articles

Effects of connection deformation softening on behavior of steel moment frames subjected to earthquakes

In the years since the 1994 Northridge earthquake, the profession has paid significant attention to the potential effects of various forms of deterioration in connection strength and stiffness that steel moment-resisting frames can experience during severe seismic excitations. The brittle connection fractures that a number of welded steel moment-resisting frame structures experienced during recent earthquakes have been the most extensively studied to date. However, cyclic testing of post-Northridge beam-column connections demonstrates that ductile connections may suffer other forms of deterioration. Negative post-yield tangent stiffness or capping, hereafter referred to as deformation softening, is a behavior of particular interest because it may have significant adverse effects on frame system behavior. The effects of deformation softening on frames subjected to pulse excitations were examined as part of an integrated experimental and analytical investigation of the effect of various forms of hysteretic deterioration on the overall system behavior of moment resisting steel frames. Pulse excitations, and the near-field ground motions they represent, can be highly damaging to structures and are therefore the primary focus of the results presented in this paper. The experimental portion of this study consisted of a series of thirty-two shaking table tests, which were performed on a one-third scale, two-story, one bay, steel moment frame with idealized, mechanical connections. These tests and subsequent analytical studies show that, in general, significant loss of connection strength capacity, whether from deformation softening or other types of deterioration, leads to large residual drifts and, for large pulse excitations with durations longer than the fundamental period of the structure, to collapse. In particular, frames with connections exhibiting negative post-yield stiffness tend to have substantially increased peak and residual displacements when subjected to pulse excitations.

Effective linear damping and stiffness coefficients of nonlinear systems for design spectrum based analysis

A stochastic approach for obtaining reliable estimates of the peak response of nonlinear systems to excitations specified via a design seismic spectrum is proposed. This is achieved in an efficient manner without resorting to numerical integration of the governing nonlinear equations of motion. First, a numerical scheme is utilized to derive a power spectrum which is compatible in a stochastic sense with a given design spectrum. This power spectrum is then treated as the excitation spectrum to determine effective damping and stiffness coefficients corresponding to an equivalent linear system (ELS) via a statistical linearization scheme. Further, the obtained coefficients are used in conjunction with the (linear) design spectrum to estimate the peak response of the original nonlinear systems. The cases of systems with piecewise linear stiffness nonlinearity, along with bilinear hysteretic systems are considered. The seismic severity is specified by the elastic design spectrum prescribed by the European aseismic code provisions (EC8). Monte Carlo simulations pertaining to an ensemble of nonstationary EC8 design spectrum compatible accelerograms are conducted to confirm that the average peak response of the nonlinear systems compare reasonably well with that of the ELS, within the known level of accuracy furnished by the statistical linearization method. In this manner, the proposed approach yields ELS which can replace the original nonlinear systems in carrying out computationally efficient analyses in the initial stages of the aseismic design of structures under severe seismic excitations specified in terms of a design spectrum.

Probabilistic collapse analysis of degrading Multi Degree of Freedom structures under earthquake excitation

Collapse of Multi Degree of Freedom (MDOF) structures under seismic excitation is due to the formation of structure mechanisms caused by local failure of the individual elements amplified by P –Delta effects due to gravity loads. The paper presents an evaluation of the risk of collapse for degrading MDOF structures. A statistical approach based on seismic fragility analysis is adopted to conduct the study. Earlier work focused on developing seismic fragility curves of systems for several values of damage measures. Collapse was expressed as the state of the structure when this damage measure approaches a limiting value. This research work focuses on developing seismic fragility curves for a collapse criterion based on hysteretic degradation. To perform this task, finite element models of a series of generic MDOF structures were developed. These structures are all single-bay frames with multiple stories. The constitutive materials used to represent the elements’ behavior followed energy-based degrading hysteretic models. The degrading structural behavior along with P –Delta effects due to gravity loads proved to successfully replicate explicit collapse. An ensemble of recent earthquake records recorded on stiff soils, and a variety of degrading MDOF structures that cover a wide range of fundamental periods of vibrations, first-mode yield force values, and levels of degradation were considered in the study. For each structure, fragility curves were developed for a collapse criterion, where collapse is simulated explicitly through the degrading constitutive models. The newly developed collapse fragility curves are based on a more direct and efficient approach than the one that utilizes damage measures. This approach is better suited for assessing the risk of collapse of structures subjected to severe seismic excitation.

Fuzzy Optimal Control of Uncertain Dynamic Characteristics in Tall Buildings Subjected to Seismic Excitation

Analytical investigations have been conducted to suppress vibration of tall building structures in the presence of uncertainty in structural dynamic characteristics. Three control algorithms consisting of probabilistic optimal control, fuzzy logic control and optimal control theories are combined to control system fluctuations and severe seismic excitations. A state-space reduced order model is constructed based on dominant observable and controllable Gramians of Lyapunov equations in order to prevent the control matrix singularity and achieve computational efficiency. Both types of active and semi-active control systems are installed in the buildings to reduce the seismic response. In the case of active control systems, both an active tuned mass damper and active mass driver are installed on the top floor. In the case of semi-active systems, such as electric rheological/magnetic rheological dampers, control devices are installed in selected story units. The class of multi-objective control constraints based on minimum control efforts with respect to stochastics evaluation criteria have been defined and satisfied. The fuzzy rule base matrices and fuzzy inference systems are appropriately constructed corresponding to regular forms of optimal control laws and input-output measured data in the control sequences. The performance and robustness of both active and semi-active control systems are investigated through a series of numerical simulations of a multistory building subjected to a wide range of seismic disturbances.

Seismic Behavior of Code-Designed Bidirectionally Eccentric Systems

Extreme seismic vulnerability of bidirectionally eccentric structural systems such as street corner buildings has been demonstrated repeatedly during past severe earthquakes. Inelastic range response of such structural systems may vary considerably depending upon the sense of eccentricities regulated by relative quadrant wise position of center of stiffness with respect to center of mass particularly for reinforced concrete (RC) structures with strength and stiffness degrading behavior. Recognizing the same, an effort has been made in the present paper to analyze the inelastic range response of bidirectionally asymmetric structures designed as per the existing codal standards. Such response helps to perceive the adequacy of the code provisions to minimize the progressively damaging effects expected to arise in RC bidirectionally eccentric structural systems due to strength and stiffness degradation under severe seismic excitation. The effect of change in stiffness eccentricity due to cracking of reinforce...

Seismic energy dissipation in deteriorating systems through low‐cycle fatigue

AbstractEnergy dissipation characteristics of structural members which exhibit both strength and stiffness deterioration under imposed displacement reversals are investigated. In the experimental part, 17 reinforced concrete beam specimens were tested under constant and variable amplitude inelastic displacement cycles. The constant‐amplitude tests were employed to determine the low‐cycle fatigue behaviour of specimens where the imposed displacement amplitude was the major variable. A two‐parameter fatigue model was developed in order to express the variation of dissipated energy with the number of displacement cycles. This model was then used to predict the energy dissipation of test specimens subjected to variable‐amplitude displacement cycles simulating severe seismic excitations. It has been demonstrated that the remaining energy dissipation capacity in a forthcoming displacement cycle is dependent on the energy dissipated along the completed displacement path. Moreover, it is observed that total energy dissipation is dependent on the length of the displacement path. Copyright © 2003 John Wiley & Sons, Ltd.

Instability of buildings during seismic response

The issue of gravity induced instability during response to severe seismic excitation is examined. It is contended that while static instability is fully determined by the existence of at least one negative eigenvalue in the second-order tangent stiffness, this condition is necessary, but not sufficient for instability during dynamic response. It is noted that the likelihood of collapse is strongly dependent on the shape of the mechanism that controls during the critical displacement cycle and that this shape can be reasonably identified using a pushover analysis with an appropriately selected lateral load distribution. A characterization of the instability limit state based on reduction of a multistorey building to an equivalent SDOF system is presented.

Earthquake-resistant steel frames with energy dissipating knee elements

Moment-resisting and concentrically braced frames have been widely used for earthquake-resistant steel buildings. However, neither of these structural systems alone, can efficiently provide the required stiffness and ductility simultaneously, which are required for structures subjected to severe seismic excitations. This paper presents the results of an investigation into a new alternative structural system for earthquake-resistant steel buildings, which aims to overcome the limitations of existing systems. This system employs a diagonal brace with one end anchored to a knee element. Stiffness is derived from the nonbuckling brace, while ductility under severe lateral loading is achieved through flexural yielding of the knee element. Large-scale model tests were conducted to assess the performance of this system. A nonlinear analytical model of the knee element has been presented for application in the dynamic analysis of the new structural system. Using this analytical model, the dynamic response of a multistorey frame with energy dissipating knee elements is compared with the corresponding eccentric braced frames.

Seismic Response Control of Buildings Using Friction Dampers

In the last decade, many energy dissipating systems have been proposed to raise the seismic design of structures beyond the conventional ductility design approach. Among these new systems, friction damping has shown some great potential. In a friction damped system, friction damping devices are inserted in a structure and slip at a predetermined optimum load during severe seismic excitations, before any yielding of the structural members has occurred. Slipping of the devices allows the structure to dissipate the input seismic energy mechanically by friction rather than by inelastic deformation of the structural elements. This paper presents an overview of the recent research and development in Canada on a particular type of friction damped bracing. Analytical and shake table test results are first summarized to illustrate the earthquake performance of friction damped structures compared to the performance of conventional building systems. The development of a design slip load spectrum for the rapid estimation of the optimum slip load distribution is then presented. Finally, two practical examples of the implementation of this system are described: (1) the design of a reinforced concrete library building; and (2) the retrofit of a precast concrete school building.

Nonlinear seismic response of antenna-supporting structures

In the event of a severe seismic excitation, preservation of essential infrastructures, such as telecommunication facilities, is of high priority. The objective of this paper is to investigate the geometrically nonlinear response of antenna-supporting guyed towers under earthquake loading. Two guyed towers are analysed: a 350 ft (107 m) tower with six stay levels and an 80 ft (24m) mast with only two stay levels. Two horizontal accelerograms are used, El Centro and Parkfield, with each record being scaled to match the elastic design spectra of the 1990 National Building Code of Canada. Elements of response analysed are: guy tensions, horizontal shears, and displacements and rotations at the tip of the mast. Results indicate that although the absolute values of the dynamic amplifications are well below the limit strength and serviceability criteria for such towers, dynamic interactions between the guywires and the mast are important, especially in the vertical direction. Multiple support excitation of the tallest tower also causes additional dynamic effects that are not present when only synchronous ground motion is studied.

Preliminary studies into the behaviour of knee braced frames subject to seismic loading

A recently proposed structural system for earthquake-resistant steel-structures is investigated. This new framing system, called the knee braced frame (KBF), dissipates energy during severe seismic excitations through flexural yielding of the knee element. The lateral stiffness is provided by a conventional diagonal brace with at least one end connected to the knee element. As the performance of this framing system depends on the ductile behaviour of the knee element, this critical element was tested to confirm its behaviour under cyclic loading. An analytical model for the moment-rotation relationship of the knee element, derived from the bilinear stress-strain relationship obtained through tensile test, is found to predict the cyclic response of the knee element accurately. Using this analytical model, the energy dissipation characteristics of the KBF are presented.

Diagonal brace with ductile knee anchor for aseismic steel frame

AbstractA new structural system for earthquake resistant steel structures is investigated in this paper. This new framing system, called the knee‐brace‐frame (KBF), is a braced frame with diagonal braces connected to ductile knee members. The diagonal braces provide the lateral stiffness, whereas the knee anchors yield in flexure to dissipate energy during severe seismic excitation. To assess the inelastic characteristics of the KBF, a large scale model of a KBF was tested using the pseudodynamic test procedure. The experimental results are compared with analytical results obtained from a DRAIN‐2D model. It is found that, with an appropriate design of knee anchors, the KBF can be made to be ductile to dissipate energy during severe seismic excitation.

Local Bond of Deformed Bars With Different Diameters in Confined Concrete

Slippage of the beam reinforcement crossing the interior joints in reinforced concrete frames can lead to large fixed-end rotations and overall beam deformations, causing major structural damage under severe seismic excitations. It is thus important to anchor adequately deformed bars inside the confined core of earthquake-resistant joints. The key factor governing the anchored-bar behavior in confined concrete is the local bond stress-slip relationship. This investigation addreses the effects of bar diameter on this relationship. Experimental data have been generated on the local bond behaviour for deformed bars of different diameters partially embedded in confined concrete. The results are used to assess the anchored bar-diameter effects of the ultimate local bond strenth and local bond stress-slip relationship in confined concrete. Based on the test data, an empirical local bond constitutive model has been generated that accounts for the bar diameter effects.

Developments in the design of ductile reinforced concrete frames

A condensed step by step summary of the application of a recently published capacity design philosophy, as applied to earthquake resisting ductile reinforced concrete frames, is presented. The theoretical inelastic dynamic response of three prototype frames, so designed and subjected to particularly severe seismic excitations, is then reported. It is shown how the predicted maximum actions compare with those used in the design. The design quantities, derived from a modified conventional elastic frame analysis for a code specified lateral static loading, were found to ensure a very high degree of, and yet economical and practical, protection against hinging in columns at and above the first floor.