Journal of Structural Engineering

Magneto-rheological (MR) fluid dampers have been identified as a particularly promising type of semiactive control device for hazard mitigation in civil engineering structures. Large-scale experimental testing is important to verify the performance of MR fluid dampers for seismic protection of civil structures. Real-time hybrid testing, where only the critical components of the system are physically tested while the rest of the structure is simulated, can provide a cost-effective means for large-scale testing of semiactive controlled structures. This paper describes the real-time hybrid simulation experimental setup for multiple large-scale MR fluid dampers and demonstrates the capability at the University of Colorado at Boulder shared-use Fast Hybrid Test facility to conduct real-time hybrid testing within the Network for Earthquake Engineering Simulation.

The hybrid simulation test method is a versatile technique for evaluating the seismic performance of structures by seamlessly integrating both physical and numerical simulations of substructures into a single test model. Using hybrid simulation, the seismic response of complex structural systems partitioned into multiple large-scale experimental and numerical substructures at networked geographically distributed experimental and computational facilities can be evaluated. A scalable framework with a fault-tolerant distributed controller is presented to support the implementation of advanced hybrid testing methods with distributed substructures. The control strategy is based on a multithreaded simulation coordinator for parallel communication with remote sites and an event-driven controller at each remote experimental site to implement continuous loading. The distributed controller provides faster rates of testing and improved accuracy in the simulation results. The effectiveness of the proposed framework is demonstrated by computing the earthquake response of a six-span bridge model with multiple remote experimental and numerical column substructures distributed within NEES laboratories across the United States. Further, the distributed tests were implemented using a secure network link between the testing sites that was developed for the NEES cyber infrastructure.

Deterioration mechanisms of existing structures have been identified and studied over the last decades. Uncertainties associated with mechanical loadings and environmental stressors (e.g., chloride intrusion, freeze-thaw cycles) make it difficult to accurately predict the life-cycle performance of these structures. In general, deteriorating structures are maintained by periodical interventions based on their condition states. Available resources are limited and maintenance decisions resulting from management systems concentrating on condition states of deteriorating structures are not always cost effective. Therefore, the need for reliability-based structure management is evident. Models for time-based and performance-based (condition-based or reliability-based) maintenance strategies are applied in this study to a deteriorating bridge in Colorado. Several maintenance interventions are considered and combined. Realistic data consisting of condition, reliability, and cost of maintenance actions are used. Finally, the optimum maintenance strategy considering condition, reliability, and cost is selected based on different criteria.

A system identification and model updating methodology that accounts for factors influencing the reliability of identification is proposed. An important aspect of this methodology is the generation of a population of candidate models. This paper presents an analysis of error sources that are used to define model populations. A case study illustrates the need for such an approach even when a single conservative model has been appropriate for design. Data mining techniques such as principal component analysis and $k$ -means clustering combined to interpret model predictions. These methods are useful for estimating the dependability of system identification.

A framework for simulating the material and geometric nonlinear response of frame members is developed from the equations of beam mechanics. The implementation of a beam-column finite element is reduced to the state determination procedure for a basic system that displaces and rotates with the element. An abstraction for geometric nonlinearity represents the kinematic and equilibrium transformations between the basic and global reference systems, while an abstraction for force-deformation response represents material nonlinearity for the basic system. Separate objects encapsulate material stress-strain behavior and cross-sectional integration in order to increase the modeling flexibility for computing the response of fiber-discretized cross sections. Multiple forms of distributed plasticity in beam-column elements are incorporated in the framework through objects that encapsulate one-dimensional quadrature rules. Software design patterns are utilized to create complex beam-column simulation models by composition of basic building blocks. The modeling flexibility of the software design is demonstrated through the simulation of a reinforced concrete column.

Honeycomb structures have been used in different engineering fields. In civil engineering, honeycomb fiber-reinforced polymer (FRP) structures have been used as bridge decks to rehabilitate highway bridges in the United States. In this work, a simplified finite-element modeling technique for honeycomb FRP bridge decks is presented. The motivation is the combination of the complex geometry of honeycomb FRP decks and computational limits, which may prevent modeling of these decks in detail. The results from static and modal analyses indicate that the proposed modeling technique provides a viable tool for modeling the complex geometry of honeycomb FRP bridge decks. The modeling of other bridge components (e.g., steel girders, steel guardrails, deck-to-girder connections, and pier supports) is also presented in this work.

This paper investigates the consequence of using Rayleigh proportional damping in the analysis of inelastic structural systems. The discussion is presented theoretically, as well as by example through the analysis of a simple five-story structure. It is shown that when the stiffness portion of the system damping matrix is based on the original system stiffness, artificial damping is generated when the structure yields. When the damping matrix is based on the tangent stiffness but the Rayleigh proportionality constants are based on the initial stiffness, a significant but reduced amplification of damping occurs. When the damping is based on the tangent stiffness and on updated frequencies based on this stiffness, virtually no artificial damping occurs. The paper also investigates the influence on effective damping when localized yielding occurs in areas of concentrated inelasticity. In such cases, it is possible to develop artificial viscous damping forces that are extremely high, but that are not easy to detect. Such artificial damping forces may lead to completely invalid analysis. The paper ends with recommendations for performing analysis where the artificial damping is eliminated, or at least controlled.

This paper is concerned with an application of the Ritz method for elastic, axisymmetric, buckling analysis of moderately thick, rotational orthotropic shells under uniform external pressure. In order to capture the effect of transverse shear deformation, which is significant for thick shells, the Mindlin shell theory is used. In applying the Ritz method, the displacement components of the shell are approximated by the product of one-dimensional polynomial functions, and the boundary equations are raised to the appropriate powers so as to ensure the satisfaction of geometric boundary conditions a priori. The validity of the method, convergence and accuracy of solutions are demonstrated using spherical shells, which is a special case of rotational shells, where closed-form solutions exist for some cases. A parametric study is conducted on spherical and parabolic shells, considering the effects of height-to-base-radius ratios, thickness-to-radius ratios, and different support conditions on the buckling solutions. The new solutions should be useful to researchers and engineers who are developing analytical tools and designs of shells.

This paper presents analytical approximate solutions for the geometrically nonlinear, large deformation postbuckling of a linear-elastic and hygrothermal beam with axially nonmovable pinned-pinned ends and subjected to a significant increase in swelling. The solution for the limiting case of a string is also obtained. Analytical approximations to large deformation of the beam are established by combining the Newton’s method with the method of harmonic balance. In contrast to the classical method of harmonic balance, the linearization is performed prior to proceeding with harmonic balancing, thus resulting in a set of linear algebraic equations instead of one of nonlinear algebraic equations. We are hence able to establish analytical approximate solutions. These approximate solutions show excellent agreement with the referenced solution obtained by the shooting method, and are valid for a small as well as a large angle of rotation at the end of the beam.

The quasi-plastic-hinge approach proposed by Attalla et al. in 1994 for material nonlinear analysis is formulated using the total form expressions, which is generally computationally inefficient as numerical integrations and finite-difference schemes are required to obtain the elastic-plastic flexibility coefficients. To overcome the above drawback, the quasi-plastic-hinge approach is formulated in an incremental form for beams with uniform cross sections, by which the elastic-plastic flexibility coefficients are explicitly given. The procedure is directly extended to nonuniform beams through use of different moment-curvature relations for the two ends of the beam, or indirectly through subdivision of the beam into a number of inner segments. The elastic-plastic flexibility coefficients derived for both cases are also explicitly given. For comparison, three steel frames composed of nonuniform cross sections are studied, from which the accuracy of the proposed approaches for treating frames with uniform and nonuniform cross sections is verified.

The paper presents a model for the inelastic buckling behavior of steel braces. The model consists of a force-based frame element with distributed inelasticity and fiber discretization of the cross section. With this approach, the response of the element can be derived by integration of the uniaxial stress-strain relation of the fibers and can account for kinematic and isotropic hardening as well as the Bauschinger effect of the material. The interaction between axial force and bending moment is thus accounted for. Even though the element only accounts for small deformations in the basic system, large displacement geometry is included in the nonlinear transformation of the force-deformation relation of the basic element following the concept of the corotational formulation. With this approach, two elements for each brace suffice to yield results that match an extensive set of experimental data of braces with different cross sections and slenderness ratios. Even though the model does not account for the effect of local buckling, this phenomenon does not seem to appreciably affect the global response of braces with compact sections.

It is a well established experimental fact that slippage of reinforcement may sometimes play an important role in the response of cyclically loaded reinforced concrete (RC) structures, especially in cases of beam-column subassemblages. In the past, analyses with 2D plane or 3D solid finite elements that assume a nonlinear bond-slip relationship to describe an arbitrary response of the interface have only been performed using elements connecting concrete nodes with discrete reinforcement nodes. This modeling exhibits restrictions in the bar topology, which can be removed only with embedded reinforcement formulations. In the present work, a 3D solid element, based on a simple smeared crack one-parameter model that describes concrete’s triaxial stress-strain behavior is extended for cases of cyclically loaded RC structures, allowing embedded reinforcement slippage. This modeling is combined with an existing bond-slip mathematical description to give stable numerical results. The proposed procedure is applied successfully in a long anchorage rebar test, as well as two cases of bond critical exterior and interior column-beam joints, and numerical results compare well with existing experimental data.

Fragility estimates are developed to assess the seismic vulnerability of reinforced concrete (RC) frame buildings, which were designed primarily for gravity loads. Five buildings of various heights (one-, two-, three-, six-, and ten-stories) are used to represent generic RC frame buildings of one- to ten-stories tall. Using a Bayesian methodology, probabilistic demand models are developed to predict the seismic structural demand using the peak interstory drift response for an imposed earthquake. Seismic structural capacity values are selected corresponding to the performance levels or damage state as specified in FEMA 356 or as computed by nonlinear pushover analyses. For each building, fragility estimates are obtained by assessing the conditional probability that the drift demand reaches or exceeds the drift capacity for a given earthquake spectral acceleration. Confidence bounds are developed to represent the epistemic uncertainty inherent in the demand models used in the fragility estimates. Bivariate fragility estimates, formulated as a function of spectral acceleration and the fundamental building period, are developed from the fragility estimates of the individual buildings. The bivariate fragilities can be used to quantify the seismic vulnerability of gravity load designed RC frame buildings one- to ten-stories tall.

A multiplatform simulation of a highway overcrossing bridge is introduced where the embankments, abutments, and pile groups are modeled in a powerful finite-element (FE) package with nonlinear soil material model while the bridge is modeled in another FE package that excels in fiber-based frame analysis. The Meloland Road Overcrossing Bridge, which is heavily instrumented, is selected as a reference analysis model due to the existence of measured data. The components as well as the integrated system are verified through comparison with previous studies and measured data. The stiffness of abutments and pile foundations are evaluated by three-dimensional finite-element analysis and compared with those from previous investigations. The analytical models of abutments and foundations are combined with the structural model using the multiplatform analysis framework, UI-SimCor. The dynamic properties of the model for multiplatform analysis are compared with those evaluated from system identification techniques using recorded ground motion. Finally, response history analyses are conducted with recorded free-field ground motion. The analysis results are in very good agreement with measured response showing the promising potential of multiplatform dynamic simulation.

An extensive parametric study is conducted with eight code-designed steel moment-resisting frames and a series of linear and nonlinear single-degree-of-freedom nonstructural components to investigate to what degree, how often, and under what conditions the nonlinear behavior of a building may amplify the seismic response of a nonstructural component attached to it. The study comprises the time-history analysis of each of the considered frames with one of the considered nonstructural components connected to it. In each case, an ensemble of 25 recorded earthquake ground motions is used, alternatively scaling them to three different intensity levels. The study also comprises a comparison between the nonstructural component responses obtained when the supporting structure is modeled alternatively as a nonlinear and a linear system. The influence of the nonstructural component location, nonlinearity, and damping ratio and the relative values of the building and component masses and natural frequencies is investigated. It is found that, in general, the nonlinear behavior of the supporting structures reduces the seismic response of the nonstructural components in comparison with the linear counterparts. In a few cases, however, the nonstructural component response is amplified by a factor that can be as large as 5.2. These cases correspond to components located on the lower building floors, with a natural period equal to the second or third natural period of the building, and subjected to a narrow-band excitation with a dominant period close to the building’s fundamental natural period.

This paper presents the experimental and analytical results of a shaking-table study on the elastic and inelastic behavior of a $2/3$ -scale three-story steel structure retrofitted by the nonlinear viscous dampers. The properties of the dampers used in the test are designed based on the displacement-based design procedure. The retrofitted frame exhibits moderate inelastic behavior under the design ground motion of the 275% El Centro earthquake. The lateral floor displacements, story drifts, floor accelerations, story shears, and damper axial forces measured from the frame tested are compared with those obtained from the displacement-based method as well as the nonlinear time-history analysis. It is shown from the study that the addition of nonlinear viscous dampers to the structure results in displacement and force reduction by about 68 to 80% under the 30% El Centro earthquake $(\mathrm{PGA}\approx 0.1\mathrm{g})$ . Higher-mode responses are significantly diminished. In addition, the displacement and acceleration responses of the structure with dampers are appropriately captured by analytical models in the elastic range. The differences between the experimental and analytical results become noticeable in the inelastic range. The displacement-based evaluation procedure tends to underestimate the responses of the damped structure in the elastic range and overestimate them in the inelastic range.