Amit Kanvinde

Rotational Stiffness of Exposed Column Base Connections: Experiments and Analytical Models

AbstractThe rotational flexibility of column base connections is often ignored in structural simulation of moment frames in which the bases are assumed to be either fixed or pinned. Although structural response is sensitive to base flexibility, methods to properly characterize base flexibility are not readily available. Motivated by these issues, a new approach to characterize the rotational flexibility of exposed column base connections is presented in this paper. The method leverages existing design procedures for base connection design, such that the rotational stiffness may be calculated with modest effort once the design has been completed. The method is validated through comparison with nine experiments, which interrogate a range of parameters including base-plate size and thickness, level of axial load, anchor-rod strength, and concrete strength. The comparison between the test and the predicted values indicates that, on average, the method predicts the stiffness with accuracy, such that the averag...

Calibration of the SMCS Criterion for Ductile Fracture in Steels: Specimen Size Dependence and Parameter Assessment

The stress modified critical strain (SMCS) criterion provides a local index for the initiation of ductile fracture in metals as a function of plastic strain and stress triaxiality. Previous research has confirmed the SMCS criterion to be an accurate index for fracture initiation in mild steels and demonstrated its application to civil/structural engineering. To facilitate practical implementation of the SMCS criterion, two key aspects of its calibration for steel materials are examined. The first pertains to the sensitivity of the measured SMCS material toughness parameter to the size of the test coupon. New results from 23 tests of cylindrically notched tension (CNT) specimens of various sizes and notch geometries indicate that the toughness parameter is relatively insensitive to calibration specimen size. This finding validates the use of miniature bar specimens to calibrate the SMCS model for thin plate steels and in-service structures, where extraction of larger coupons is impossible. The second aspec...

Analytical models for the seismic performance of gypsum drywall partitions

Gypsum drywall partitions may contribute significantly to the lateral strength and stiffness of woodframe structures, whether or not the walls are explicitly designed for that purpose. This paper proposes analytical models to determine the lateral shear strength and initial elastic stiffness of wood-framed gypsum wall panels, taking into account the effects of wall geometry, door and window openings, connector type and spacing, and wall boundary conditions. The strength and stiffness models are incorporated in a multilinear curve to describe the monotonic lateral shear versus deformation response of the walls. Additional parameters to calibrate the response of a peak-oriented hysteretic cyclic model are also proposed, thus making the models suitable for nonlinear time-history simulations of woodframe buildings. The models are developed and validated using published data from 16 shear tests of full-scale gypsum wall panels.

Effect of Column-Base Flexibility on the Seismic Response and Safety of Steel Moment-Resisting Frames

The effect of column-base flexibility on the response of steel moment frames is assessed through parametric simulation. The response of four frames (2-, 4-, 8-, and 12-story), designed as per current codes, is investigated through static push-over simulations and sophisticated nonlinear response-history simulations, including collapse simulation. For each frame, a range of base fixities is interrogated, including realistic values that are calculated from the designed connections. The results indicate that a reduction in base fixity alters the force distribution and the plastic mechanism, significantly reducing ductility capacity and strength, as well as collapse resilience, while increasing member forces. For the 4-, 8-, and 12-story frames, this trend suggests that the expected response of such frames is worse than is implied by simulations and design approaches that assume a fixed-base condition. However, the trend is beneficial for the 2-story frame, which is analyzed and designed assuming a pinned base.

Probabilistic Formulation of the Cyclic Void Growth Model to Predict Ultralow Cycle Fatigue in Structural Steel

The previously developed cyclic void growth model (CVGM) has been demonstrated to accurately simulate ductile fracture initiation under monotonic and ultralow cycle fatigue loading for a variety of steel materials and geometric configurations. Prediction of ductile fracture initiation involves significant uncertainty, particularly where there is high variability in the material (e.g., welded connections) subjected to irregular cyclic loading. The reliability of the model predictions is improved through a probabilistic formulation based on maximum likelihood parameter estimation. The probabilistic formulation, which incorporates information from both the failure and nonfailure loading cycles, has the following features: (1) the calibration of model parameters provides the maximum likelihood of agreement for a given set of cyclic fracture observations, and (2) fracture predictions are provided in a probabilistic sense by generating a distribution of the expected instant of fracture. The benefit of the approach is twofold. First, it eliminates an inconsistency that is inherent in the deterministic calibration procedure, as proposed in the original development of the CVGM. Second, the degree of certainty of fracture predictions is quantified. In combination, these features significantly enhance the robustness of the framework within which the model is implemented. Although this paper applies this approach in the specific context of the CVGM, the method can be generalized to other models that share similar characteristics.

Seismic Performance of Embedded Column Base Connections Subjected to Axial and Lateral Loads

AbstractEmbedded column base (ECB) connections are used in mid- to high-rise steel moment frames to transfer base moments and forces into the footing. These connections, which feature a steel colum...

Forensic Analysis of Link Fractures in Eccentrically Braced Frames during the February 2011 Christchurch Earthquake: Testing and Simulation

The earthquake on February 22, 2011, in Christchurch, New Zealand, resulted in the first documented field fractures of links in eccentrically braced frames (EBFs). A comprehensive forensic analysis of these fractures, which occurred in the parking garage of the Christchurch Hospital, is presented. The analysis is based on mechanical and spectrochemical testing and three-dimensional (3D) scanning of procured physical samples of the fractured links. The analysis features nonlinear time history simulations to characterize deformation demands and continuum finite-element simulations to determine the capacities based on a sophisticated fracture mechanics model. The exercise represents a multiscale end-to-end simulation of the system and provides insight regarding the observed fractures. The analysis reveals the inherent challenges in determining the proximate cause of the fractures because the fractures occurred because of a confluence of several interacting factors, primarily the intensity of shaking (several times the intensity that was expected during a design-level event) and the frame geometry, which severely amplified the imposed demands. In addition, the fractured links also suffered from an erection (fit-up) error, in which the link stiffener was not located (as specified) directly above the brace flange, producing a severe strain concentration. This flaw significantly reduced the deformation capacity; however, the simulation of hypothetical scenarios indicates that, even with this flaw, the links would (with high likelihood) have survived a design-level event. Strategies for mitigation include stricter tolerances for stiffener location, consideration of frame geometry to reduce amplification of rotation, and enhancement of the seismic hazard used for design. The limitations of the study are outlined.

Predicting Fracture in Civil Engineering Steel Structures: State of the Art

AbstractFracture is an extreme limit state in steel structures, often precipitating structural failure or serious loss of function. Methods to predict fracture in civil structures include tradition...

Rotational Stiffness of Deeply Embedded Column–Base Connections

AbstractDeeply embedded column–base (DECB) connections comprise a steel column embedded into a concrete footing, and are common in mid-rise to high-rise steel moment resisting frames. These are oft...

Calibration of Continuum Cyclic Constitutive Models for Structural Steel Using Particle Swarm Optimization

AbstractThis paper presents a practical and validated method to automate the calibration of cyclic constitutive model parameters for structural steel. Due to the difficulty in developing homogenous...