Sashi K. Kunnath

Adaptive Spectra‐Based Pushover Procedure for Seismic Evaluation of Structures

The estimation of inelastic seismic demands using nonlinear static procedures, or pushover analyses, are inevitably going to be favored by practicing engineers over nonlinear time-history methods. While there has been some concern over the reliability of static procedures to predict inelastic seismic demands, improved procedures overcoming these drawbacks are still forthcoming. In this paper, the potential limitations of static procedures, such as those recommended in FEMA 273, are highlighted through an evaluation of the response of instrumented buildings that experienced strong ground shaking in the 1994 Northridge earthquake. A new enhanced adaptive “modal” site-specific spectra-based pushover analysis is proposed, which accounts for the effect of higher modes and overcomes the shortcomings of the FEMA procedure. Features of the proposed procedure include its similarity to traditional response spectrum-based analysis and the explicit consideration of ground motion characteristics during the analysis. It is demonstrated that the proposed procedure is able to reasonably capture important response attributes, such as interstory drift and failure mechanisms, even for structures with discontinuities in strength and/or stiffness that only a detailed nonlinear dynamic analysis could predict.

Experimental Study on Progressive Collapse-Resistant Behavior of Reinforced Concrete Frame Structures

This paper demonstrates the feasibility of using a static unloading approach to simulate column loss in investigating the progressive failure of a reinforced concrete frame due to the loss of a lower story column. A four-bay and three-story one-third scale model representing a segment of a larger planar frame structure was tested. A constant vertical load was applied to the top of the middle column by a servo-hydraulic actuator to simulate the gravity load of the upper floors and the failure of the middle column of the first story was simulated by unloading a mechanical jacking system. The frame collapse, defined in this study as the rupture of tension steel bars in the floor beams, occurred at a vertical unloading displacement of 456 mm (18 in.) that corresponds to a beam drift angle of 10.3 degrees. The mechanical behavior of the model frame is analyzed and the redistribution and transition of the load resisting mechanisms is discussed. During the progressive collapse process, the RC frame structure experience 3 distinct phases in its response: elastic, plastic and catenary phases. Findings indicate that the calculated capacity of the frame based on the plastic limit state was approximately 70% of the tested failure capacity if catenary effects are also included. The findings in this study can contribute to the future development of collapse-resistant design methods.

Effects of fling step and forward directivity on seismic response of buildings

This paper investigates the consequences of well-known characteristics of near-fault ground motions on the seismic response of steel moment frames. Additionally, idealized pulses are utilized in a separate study to gain further insight into the effects of high-amplitude pulses on structural demands. Simple input pulses were also synthesized to simulate artificial fling-step effects in ground motions originally having forward directivity. Findings from the study reveal that median maximum demands and the dispersion in the peak values were higher for near-fault records than far-fault motions. The arrival of the velocity pulse in a near-fault record causes the structure to dissipate considerable input energy in relatively few plastic cycles, whereas cumulative effects from increased cyclic demands are more pronounced in far-fault records. For pulse-type input, the maximum demand is a function of the ratio of the pulse period to the fundamental period of the structure. Records with fling effects were found to excite systems primarily in their fundamental mode while waveforms with forward directivity in the absence of fling caused higher modes to be activated. It is concluded that the acceleration and velocity spectra, when examined collectively, can be utilized to reasonably assess the damage potential of near-fault records.

Parameter identification for degrading and pinched hysteretic structural concrete systems

Hysteretic models are frequently used to predict the non-linear behavior of reinforced concrete structural systems. Such models are typically characterized by control parameters that have to be calibrated from observed experimental testing. A system identification methodology is presented in this paper for determining the values of control parameters in a continuously smooth hysteretic model for inelastic dynamic behavior of structural concrete systems. The technique is based on a modified Gauss-Newton approach in which a non-linear relationship between model parameters and the force-deformation or moment-curvature hysteretic loops is assumed. The methodology is applied to an extended version of the well known rate-dependent Bouc-Wen hysteretic model. Six control parameters are introduced into the model which influence the degrading characteristics and, therefore, represent the target parameters that need to be optimized. The versatility of the approach is demonstrated through sample simulations of observed behavior which are drawn from reinforced and partially prestressed concrete elements and subassemblages including beams, columns and beam-column joints. Studies of convergence of the proposed algorithm and sensitivity of the model to identified parameters is presented.

Centrifuge Modeling of Bridge Systems Designed for Rocking Foundations

In good soil conditions, spread footings for bridges are less expensive than deep foundations. Furthermore, rocking shallow foundations have some performance advantages over conventional fixed-base foundations; they can absorb some of the ductility demand that would typically be absorbed by the columns, and they have better recentering characteristics than conventional reinforced-concrete (RC) columns. Foundations designed for elastic behavior do not have these benefits of nonlinear soil-structure interaction. One potential disad- vantage of rocking systems is that they can produce significant settlement in poor soil conditions. Centrifuge model tests were performed to account for the interaction between soil, footing, column, deck and abutments systems. Bridge systems with rocking foundations on good soil conditions are shown to perform well and settlements are small. An improved method for quantification of settlements is presented. The model tests are described in some detail. One of the important factors limiting the use of rocking foundations is the perception that they might tip over; experiments show that tipping instability is unlikely if the foundations are properly sized. In one experiment, a column for a system with large fixed-base foundation collapsed while the systems with smaller rocking foundations did not collapse. DOI: 10.1061/(ASCE)GT .1943-5606.0000605.

Cumulative Seismic Damage of Circular Bridge Columns: Benchmark and Low-Cycle Fatigue Tests

An experimental study was undertaken to investigate cumulative damage in reinforced concrete circular bridge piers subjected to a series of earthquake excitations. Twelve identical quarter-scale bridge columns, designed in accordance with current American Association of State Highway and Transportation Officials specifications, were fabricated and tested to failure. This paper summarizes the results of Phase I testing that consisted of benchmark tests to establish the monotonic force-deformation envelope and the energy capacity under standard cyclic loads, and constant amplitude tests to determine the low-cycle fatigue characteristics of typical flexural bridge columns. A companion paper presents the results of variable amplitude testing that focused on the effects of load path on cumulative damage. Test observations indicate two potential failure modes: low cycle fatigue of the longitudinal reinforcing bars and confinement failure caused by rupture of the confining spirals. The former failure mode is associated with relatively large displacement amplitudes in excess of 4% lateral drift, while the latter is associated with a larger number of smaller amplitude cycles. A fatigue life expression is developed that can be used in damage-based seismic design of circular, flexural bridge columns.

Gravity-Load-Designed Reinforced Concrete Buildings--Part I: Seismic Evaluation of Existing Construction

The seismic performance of nonductile reinforced concrete frame buildings in regions of low to moderate seismicity is evaluated. Several significant aspects of nonductile detailing are modeled using rotional simplifications of expected member behavior at critical sections to facilitate a complete inelastic time history analysis of the system. The detailing configurations included in the analysis are: discontinuous positive flexural reinforcement, lack of joint shear reinforcement, and inadequate transverse reinforcement for column core confinement. Seismic evaluations of three-, six-, and nine-story buildings are carried out under low- to moderate earthquake excitations. The essential parameters of the response are presented with a view to identifying vulnerability of such buildings to a potential seismic design event

Relevance of Absolute and Relative Energy Content in Seismic Evaluation of Structures

A reassessment of input energy measures taking into consideration the characteristics of near fault ground motions is presented. The difference between absolute and relative energy input to structural systems is shown to be more significant for near-fault than far-fault records. In particular, the coherent velocity pulse contained in near-fault records resulting from a distinctive acceleration pulse rather than a succession of high frequency acceleration spikes produces sudden energy demand in the early phase of the response and is typically larger than the total energy accumulated at the end. Studies using idealized pulses indicate that input energy is a function of the shape and period of the velocity pulse. For spectral periods shorter than pulse period, greater absolute energy is input into the system rather than relative energy, while the reverse is true for spectral periods larger than the pulse period. The discrepancy between two energy definitions is initiated by the phase difference in ground velocity and system relative velocity, and it tends to be minimal as the pulse period approaches to system vibration period. The significance of these findings, based on linear SDOF simulations, is further investigated by examining the nonlinear seismic response of a group of realistic buildings subjected to near-fault recordings with and without apparent acceleration pulses. This study concludes that selection of appropriate energy measure for near-fault accelerograms should be based on the shape and period of dominant pulse in the record, and the vibration properties of the structural system.

Low-Cycle Fatigue Failure of Reinforcing Steel Bars

This article reports on a comprehensive experimental study that was carried out to examine the low-cycle fatigue behavior of ordinary reinforcing bars used in reinforced concrete (RC) construction. The objective of the study was to gain a better understanding of low-cycle fatigue failure of the longitudinal steel reinforcement in potential plastic hinge zones of RC members subjected to seismic loads and to develop a fatigue life relationship to characterize the response. In the study, a pair of aluminum strips was securely fastened between the specimen and the custom-built gripping blocks that were used to act as the force transfer media. The results of both monotonic tension tests and low-cycle fatigue tests using constant amplitude cyclic strain histories on four different bar sizes are reported in this article. Preliminary findings indicate that fatigue life is influenced by the diameter of the bar and the geometry of the rolled on deformations. The authors develop fatigue life relationships based on total strain amplitude and energy, for use in damage and failure modeling. The authors point to the need for including low-cycle fatigue behavior of the bars, in addition to monotonic properties, for applications that subject the bar to reversed cyclic loads.

Amplitude-Scaled versus Spectrum-Matched Ground Motions for Seismic Performance Assessment

The need to consider only a small number of ground motions combined with the complexities of response sensitivity to both modeling choices and ground motion variability calls for an assessment of current ground motion selection and modification methods used in seismic performance evaluation of structures. Since the largest source of uncertainty and variability arises from ground motion selection, this study examines the suitability of two ground motion modification (GMM) schemes: magnitude scaling (wherein the ground motion is uniformly scaled so that the resulting spectrum matches the amplitude of the design spectrum at the structural fundamental period) and spectrum matching. Comprehensive nonlinear time-history (NTH) simulations of two reinforced concrete moment frame buildings are carried out to evaluate the GMM approaches in the context of seismic demand prediction. Findings from the investigation indicate that spectrum matching is generally more stable than scaling both in terms of the bias as well ...