IDEALS Community: Newmark Structural Engineering Laboratory

Performance-based Engineering Framework and Ductility Capacity Models for Buckling-Restrained Braces

Tue, 29 Oct 0003 22:58:59 GMT

Title: Performance-based Engineering Framework and Ductility Capacity Models for Buckling-Restrained Braces

Authors: Andrews, Blake M.; Fahnestock, Larry A.; Song, Junho

Abstract / Summary: Buckling-restrained braces (BRBs) have recently become popular in the United States for use as primary members of seismic lateral-force-resisting systems. A BRB is a steel brace that does not buckle in compression but instead yields in both tension and compression. Concentrically-braced frames incorporating BRBs are known as buckling-restrained braced frames (BRBFs). Although design guidelines for BRB application have been developed, procedures for assessing performance and quantifying reliability are needed. This report proposes a performance-based engineering framework (PBEF) for a BRBF subjected to seismic loads. The proposed framework quantifies the risk of BRB failure due to low-cycle fatigue fracture of the BRB core. The components of the PBEF include: stochastic modeling of seismic loads; dynamic analyses of BRBFs; cumulative plastic ductility (CPD) (i.e. fatigue) models for buckling-restrained braces; structural reliability analyses; parametric studies on how BRB and BRBF properties affect performance; and fragility modeling. In addition to the report, appendix files are attached which provide detailed information on the research program. For stochastic modeling of seismic loadings, input ground acceleration records were randomly generated from power spectrum models and modulated with envelope functions (to account for non-stationarity). The generated time records were used as input excitations to single-degree-of-freedom lumped-mass system models that represented the BRBFs. The BRB hysteretic behavior was modeled using a Bouc-Wen model. Non-linear dynamic time-history analyses were performed to obtain BRB core deformation time history records. In this study, significant effort was made to develop models that predict BRB CPD capacity. The result was BRB remaining capacity (RC) models, which, given the BRB core deformation history as an input, predict the remaining CPD capacity of the brace, where values less than zero indicate failure. Given BRB demand (i.e. core deformation histories generated from the dynamic analyses) and supply (i.e. remaining capacity predicted by the RC models), reliability analyses were performed to evaluate the probability of brace failure. The analyses were conducted using the first order reliability method. In the reliability analyses, the epistemic uncertainty in the fatigue capacity predictions was accounted for explicitly, and, as a result, the probabilities of brace failure were calculated in terms of mean probability, 90% confidence level probability, and 95% confidence level probability. Using the tools described above, a parametric study was conducted to explore the effects of the seismic loading, BRB, and BRBF characteristics on the probability of brace failure. For given seismic loadings, surfaces of reliability indices were constructed in order to determine the probability of brace failure directly from BRB and BRBF properties, without the need to perform individual reliability analyses each time. Also, for a given set of BRB and BRBF properties, fragility curves were created that provide conditional probability of brace failure given ground shaking intensity parameters. Though this report describes the specific application of a PBEF to the BRB fatigue problem, the components of the PBEF may be interchanged independently, leading to great overall flexibility and the potential for application of the framework to many other problems.

Keywords: Performance-Based Design; Buckling-Restrained Brace; First-Order Reliability Method; Low-Cycle Fatigue

Structural Health Monitoring Strategies for Smart Sensor Networks

Mon, 28 Apr 2008 22:58:59 GMT

Title: Structural Health Monitoring Strategies for Smart Sensor Networks

Authors: Gao, Yong; Spencer, Jr., Billie F.

Abstract / Summary: Structural health monitoring (SHM) is an emerging field in civil engineering, offering the potential for continuous and periodic assessment of the safety and integrity of civil infrastructure. Based on knowledge of the condition of the structure, certain preventative measures can be carried out to prolong the service life of the structure and prevent catastrophic failure. However, challenges remain to apply SHM to civil engineering structures. The research detailed in this report has three complimentary efforts that seek to address some of those challenges. The first component is to experimentally verify an existing damage detection method utilizing a three-dimensional 14-bay truss structure at the Smart Structures Technology Laboratory (SSTL) of University of Illinois at Urbana- Champaign (UIUC). This flexibility-matrix-based method has drawn considerable attention recently; however, only numerical validation had been previously provided. Experimental verification allows assessment of the efficacy of the method in practice. The second part of the work is directed toward extending the flexibility-matrixbased approach to continuous online SHM employing ambient vibration (i.e., unmeasured input excitations). Continuous online SHM of civil infrastructure is highly desired, because it allows early detection of the damage in a structure and therefore offers the possibility to extend the service life of the structure. Finally, a new distributed computing SHM strategy, which is suitable for implementation on arrays of densely distributed smart sensors, is proposed for monitoring of civil infrastructure. Recent development of smart sensor technology has the potential to fundamentally change how civil infrastructure will be monitored. Damage detection algorithms which can take advantage of smart sensor technology are highly desired, but currently very limited. The new approach proposed in this research is different from the traditional ones which have relied on central data acquisition and processing, and therefore meshes well with the distributed computing environment offered by smart sensor technology. A strong basis for application of SHM to civil engineering structures using smart sensors has been provided.

Keywords: Structural Health Monitoring; Damage Detection; Distributed Computing; Flexibility Matrix; Smart Sensors

Non-contact NDT of Concrete Structures Using Air Coupled Sensors

Mon, 28 Apr 2008 22:58:59 GMT

Title: Non-contact NDT of Concrete Structures Using Air Coupled Sensors

Authors: Zhu, Jinying

Abstract / Summary: Elastic wave-based non-destructive test (NDT) methods are effective for detecting flaws in concrete structures. With the recent developments in computing hardware and software, imaging techniques have become very popular in NDT applications. However the application of elastic wave-based imaging methods for concrete structures is severely limited by the physical coupling between sensors and concrete surface, which reduces testing efficiency. In this report, the air-coupled sensing technique is proposed as a solution to improve the efficiency of elastic wave-based test methods for concrete structures. Theoretical analyses are first conducted to study the propagation of leaky Rayleigh waves in fluid-solid half spaces. Closed-form solutions of the Green’s function are derived for pressure and displacement in both the fluid and solid. This analysis provides theoretical background necessary for practical air-coupled sensing of leaky Rayleigh waves in concrete. The theory is also extended to underwater NDT applications. Two applications of air-coupled sensing are considered. One is air-coupled leaky surface wave sensing in concrete. A laboratory study and field tests demonstrate that air-coupled sensors are very effective for sensing leaky surface waves in concrete. The sensitivity and accuracy of air-coupled sensors are comparable to contact sensors. Aircoupled sensors are suitable replacement for contact sensors in SASW and MASW tests and moreover help improve test efficiency. In addition, the contact-less nature of aircoupled sensing enables the study of the effect of defects on wave attenuation. The experimental results show leaky Rayleigh waves are sensitive to the existence of cracks in concrete when waves propagate across cracks; the crack positions are clearly located in a 2-D scanning test image. The second application is air-coupled impact-echo. Two reinforced concrete slabs containing different types of defects were inspected using an air-coupled impact test testing scheme. 2-D scanning impact-echo tests were conducted over the slab containing voids and delaminations. The 2-D scanning image clearly shows the location of embedded defects, and their depths are also determined. Air-coupled impact-echo is also applied to examine the grouting condition of embedded ducts. The poorly-grouted and ungrouted sections are identified within the metal duct.

Keywords: air-coupled sensing; concrete; imaging; impact-echo; Lamb's problem

A New Node-Node to-Node Approach to Contact/Impact Problems for Two-Dimensional Elastic Solids Subject to Finite Deformation

Mon, 28 Apr 2008 22:58:59 GMT

Title: A New Node-Node to-Node Approach to Contact/Impact Problems for Two-Dimensional Elastic Solids Subject to Finite Deformation

Authors: Xu, Daqing; Hjelmstad, Keith D.

Abstract / Summary: Contact analysis is an important branch of solid mechanics. Numerical simulation using the finite element method has become the dominant approach recently because of the high nonlinearity of contact problems. In the traditional Lagrangian description for solid mechanics, the numerical nodes are attached to the material particles, making it impossible to maintain node-to-node contact due to independent deformation. Various node-to-segment or segment-to-segment treatments are proposed to discretize the contact interface. But some issues still exist. Specifically, mesh distortion or element entanglement may be present if deformation is large. A new node-to-node approach for 2D contact/impact problems subject to finite deformation is proposed in this report to offer an alternative approach to these traditional methods, wherein node-to-node contact is maintained throughout the contact process. This method is based on the Arbitrary Lagrangian-Eulerian algorithm (ALE). One or both bodies in the two-body contact problem have an ALE mesh, which is independent of the material particles and has prescribed motion set to maintain node-to-node contact. The strategy of the ALE mesh motion has two steps: (1) to move nodes in the active set to maintain node-to-node contact (2) to smooth ALE mesh to improve mesh quality using the Laplacian or angle-based smoothing algorithm. Problems of interest in this study are contact/impact problems wherein the implicit mid-point rule is used as the primary time stepping algorithm to find the solution incrementally. In order to conserve the system energy, the persistency condition is incorporated as the contact constraint. The augmented Lagrangian method is primarily used to apply contact constraints. Non-classical Coulomb friction laws are used where friction is present. Several quasi-static and impact examples are given to demonstrate the performance and validity of the new approach.

Keywords: Contact; Impact; Finite Element; Moving Mesh; Arbitrary Lagrangian Eulerian (ALE)

A Synopsis of Studies of the Monotonic and Cyclic Behavior of Concrete-Filled Steel Tube Members, Connections, and Frames

Sat, 29 Mar 2008 22:58:59 GMT

Title: A Synopsis of Studies of the Monotonic and Cyclic Behavior of Concrete-Filled Steel Tube Members, Connections, and Frames

Authors: Gourley, Brett C.; Tort, Cenk; Denavit, Mark D.; Schiller, Paul H.; Hajjar, Jerome F.

Abstract / Summary: A significant amount of research has been conducted worldwide on the behavior of concrete filled steel tubes (CFTs) in the past five decades. This report provides a summary of the behavior and experimental work of concrete filled steel tube members, connections, and frames that are reported in detail in the literature. These published studies have been summarized with an emphasis on experimental setup and properties, analytical methods presented, and key results from the work. A detailed summary of the behavior of CFTs under various loading conditions is presented in the first chapter. In the second chapter, summaries of selected experimental and analytical work are presented. The third chapter contains tables documenting experimental data from the summarized work. The fourth chapter presents a categorization of tests on CFT beam-columns, sorting the experimental work by material and geometric properties. The tables in chapters three and four provide a quick reference to a wide range of published experiments. Several topics (e.g. box columns, fire studies) have been excluded from the main report for brevity, as have publications that summarize CFT research more briefly, but references to these studies have been placed in a supplemental bibliography.

Keywords: Concrete-filled steel tubes; composite construction; composite beam-column; composite connection; composite frame

Nontraditional Limitations on the Shear Capacity of Prestressed Concrete Girders

Wed, 28 Nov 2007 22:58:59 GMT

Title: Nontraditional Limitations on the Shear Capacity of Prestressed Concrete Girders

Authors: Nagle, Thomas J.; Kuchma, Daniel A.

Abstract / Summary: Code based shear design provisions principally use a sectional force design procedure in which it is assumed that plane sections remain plane. However, the shear capacity of a member may be limited by other shear related phenomena that are not captured in codes of practice. These nontraditional limitations on the shear capacity of a member can result from incorrectly evaluating the shear stress that needs to be transmitted across a crack, shearcompression failure along a web-flange interface, or insufficient capacity of longitudinal tension reinforcement at the support. A series of 20 shear tests were completed on ten 52-foot long and 63-inch deep prestressed bulb-tee bridge girders cast with high-strength concrete. An extensive amount of experimental data was gathered and advanced data analysis tools were utilized to evaluate these nontraditional limitations on shear capacity. It was determined that interface shear transfer resistance in high-strength concrete is predicted well by relationships developed from tests on normal-strength concrete specimens. It was further observed that the angle of web-shear cracking was generally steeper than the angle of principal compressive stress as given by the AASHTO LRFD Bridge Design Specifications. This difference in angles creates a significant shear demand on a crack that is not accounted for in the LRFD Specifications. A method is presented for determining if a shear-compression failure along a webflange interface is a potential mode of failure for a prestressed bridge girder. This approach provides a means of calculating the shear stress that must be transmitted across the interface as a function of the geometry and loading on the member as well as a means of calculating the shear resistance along the web-flange interface. This method can be used to guard against a shear-compression failure by placing a limitation on the maximum shear capacity a member. It was also determined that the requirement for longitudinal tension reinforcement near the support included in the 4th edition of the AASHTO LRFD Bridge Design Specifications may underestimate the demand on longitudinal tension reinforcement. Alternatively an equilibrium based approach is presented for determining demand on longitudinal tension reinforcement near the support.

Keywords: shear; high-strength concrete; prestressed girders; end regions; shear friction

Seismic Loss Assessment and Mitigation for Critical Urban Infrastructure Systems

Sat, 29 Dec 2007 22:58:59 GMT

Title: Seismic Loss Assessment and Mitigation for Critical Urban Infrastructure Systems

Authors: Kim, Young-Suk; Spencer, Jr., Billie F.; Elnashai, Amr S.

Abstract / Summary: Transportation and utility networks (e.g., water delivery, power, and oil systems) are essential in the support of all economic and social activity of an industrialized region. The functional loss of this critical urban infrastructure due to internal or external perturbations, such as earthquakes, can severely impact commercial and industrial activities on regional, national, and international scales, and on rapid and effective emergency response and repair operations following the event. Therefore, understanding the influence of hazards on these infrastructure systems and allocation of limited resources for seismic retrofitting components of infrastructure systems are critical to mitigate damage and to perform effective response and recovery efforts. This study develops an approach for estimating seismic performance of complex critical urban infrastructures and optimizing seismic retrofit of infrastructure systems based on system-level performance under the constraint of finite resources. First, postearthquake system performance of interdependent or independent infrastructure systems is assessed based on a state-of-the-art network analysis model. Subsequently, a novel optimization algorithm is developed to determine the best retrofit strategy to maximize system performance with a limited budget. The developed methodology is applied to substantial infrastructure systems for seismic loss estimation and mitigation study. The resulting output of the methodology can provide useful insight to assist in prioritizing components of infrastructure systems for seismic retrofit to enhance their post-earthquake functionality.

Keywords: seismic risk; infrastructure; interdependent network; retrofit strategy; optimization

Probabilistic Seismic Assessment of Structure, Foundation and Soil Interacting Systems

Wed, 28 Nov 2007 22:58:59 GMT

Title: Probabilistic Seismic Assessment of Structure, Foundation and Soil Interacting Systems

Authors: Kwon, Oh-Sung; Elnashai, Amr S.

Abstract / Summary: This report presents research on the probabilistic seismic performance evaluation of a structural-geotechnical interacting system. The system comprises a bridge, its foundation, and the supporting soil. The investigation includes a study on probabilistic performance evaluation methodologies, development of a multiplatform and hybrid simulation framework, verifications of numerical models of structural and geotechnical systems in comparison with measured data, and the derivation of fragility curves of a bridge in Central and Eastern United States. Seismic performance evaluation procedures are studied using a benchmark threestory, reinforced concrete (RC) building structure. Three probabilistic performance evaluation methods are applied: the Monte Carlo simulation, response surface, and SACFEMA methods. The analysis of benchmark structure shows that the effect of random variability in structural materials is small compared to the effect of input ground motion. When Peak Ground Acceleration (PGA) is used as an intensity measure, the derived vulnerability curves highly depend on ground motion sets. Three different simulation methods results in similar vulnerability curves. The computational cost are the most expensive when the Monte Carlo simulation is adopted. Methodologies for soil-structure-interaction analysis are introduced, including the newly developed multiplatform, multiresolution hybrid simulation framework. These methodologies and numerical models of soil-structure-interaction systems are verified through comparison with field measurements and experimental results. The soil-structure interacting system is verified through analyses of a heavily instrumented bridge which recorded several sets of ground motions. The verification study of soil-structure interacting system shows that detailed and meticulously developed analytical models are capable of replicating measurements of the response of complex bridge systems subjected to strong ground motion. Seismic vulnerability curves of a reference bridge in the Central and Eastern United States (CEUS) are derived employing the aforementioned methods with and without soilstructure interaction. A typical highway over-crossing bridge representing one of the most common bridge types in the CEUS is selected. Four different approaches of Soil- Structure Interaction (SSI) are tried: (a) Abutments and foundations are assumed to be fixed, (b) Conventional lumped spring approaches are adopted to model abutments and foundations, (c) Lumped springs for abutments and foundations are estimated from Finite Element (FE) analysis of geotechnical system, and (d) Multiplatform simulation is conducted. All four of the methods shows that abutment bearings in transverse direction are most vulnerable components. Failure probability of the bridge system is highly dependent on the failure probability of abutment bearings. Considering that simplified methods for SSI analysis include larger assumptions than fully coupled methods and that the multiplatform simulation is verified with measured responses from instrumented bridge, the use of multiplatform simulation is suggested if computational power and resources for FE modeling are affordable.

Keywords: seismic performance evaluation; soil structure interaction; fragility curve; hybrid simulation; multi-platform simulation

Shear Behavior and Capacity of Large-Scale Prestressed High-Strength Concrete Bulb-Tee Girders

Mon, 29 Oct 2007 22:58:59 GMT

Title: Shear Behavior and Capacity of Large-Scale Prestressed High-Strength Concrete Bulb-Tee Girders

Authors: Sun, Shaoyun; Kuchma, Daniel A.

Abstract / Summary: The current shear design provisions of the AASHTO LRFD Bridge Design Specifications limit the concrete compressive strength to 10 ksi due to a lack of experimental evidence for their extension to high strength concrete. To overcome this limitation, the National Academy of Sciences funded National Cooperative Highway Research Program (NCHRP) Project 12-56 “Application of the LRFD Bridge Design Specifications to High-Strength Structural Concrete: Shear Provisions”. This report presents an analysis of project 12-56 experimental test data from which an in-depth understanding of the shear response of prestressed girder was obtained and new models were developed. The experimental work comprised a total of 20 tests on ten 52-foot long and 6-foot deep bulb-tee girders. All girders were designed to satisfy the requirements of the LRFD Bridge Design Specifications and then subjected to a uniformly distributed load until failure occurred in shear. The primary test variables were concrete compressive strength (ranging from 10 to 18 ksi), the maximum shear design stress (0.7 to 2.5 ksi), strand anchorage details (straight, unbonded, and draped), and end reinforcement detailing (bar size, spacing, and level of confinement). A large number of both traditional and advanced instrumentation systems were used to measure response. A new data visualization tool was developed to provide a detailed analysis of the dense experimental test data. It was concluded that the AASHTO LRFD Sectional Design Method, as well as the AASHTO Standard Specifications and the Canadian Standard Association A23.3-04 Design Method, could be extended to up to 18 ksi concrete. It is also recommended that the maximum shear design stress be reduced from 0.25 fc’ to 0.18fc’. Both the angle and the strength of diagonal cracking could be accurately predicted using Mohr’s circle of stress. The web shear behavior could be characterized as a tri-linear relationship separated by web cracking, stirrup yielding, and failure and the inelastic tangent stiffness before stirrup yielding could be modeled as a polynomial function of shear reinforcement ratio. Based on the development of 350 crackbased free-body diagrams, the components of the concrete contribution to resistance (two flanges and web) over the loading history was characterized as a function of the geometric and material properties of the girders. A general expression, which adopted the calculated crack angle for the computation of shear reinforcement contribution and provided clear physical explanation for every part of concrete contribution, was suggested for the future shear design practice. From the measured test results, an analytical model, Crack Displacement Field Theory (CDFT), was developed for predicting the shear response of prestressed/reinforced concrete members. Compared to other existing models, it could capture the discrete displacement due to crack opening and crack slip along crack surface and can take account the variation of stresses in reinforcement due to bond. Based on this model, expressions were derived for shear stiffness and shear resistance at stirrup yielding, and the derived equations produced good agreement with test results.

Keywords: shear behavior; Shear behavior; Shear capacity; shear cracking; high-strength concrete; pretressed bulb-tee girders; experimental data visualization; crack displacement field theory

Model-based Strategies for Real-time Hybrid Testing

Wed, 28 Nov 2007 22:58:59 GMT

Title: Model-based Strategies for Real-time Hybrid Testing

Authors: Carrion, Juan E.; Spencer, Jr., Billie F.

Abstract / Summary: Experimental testing is an essential tool for understanding how structures respond to extreme events, thus allowing the design and construction of safer structures. Methods currently used to determine the behavior of structural systems subjected to dynamic loading are quasi-static, shaking-table, and hybrid (or pseudodynamic) testing. In hybrid testing, the dynamic response of the structure is calculated numerically on a computer, and then the restoring forces from the structure are obtained by applying the calculated displacements to a test specimen. The combination of physical testing with numerical simulation provided by hybrid testing facilitates accurate and efficient testing of large and complex structural systems. Because conventional hybrid testing is executed at slow speeds, the method is not applicable for structures with rate-dependent components (for example, devices associated with vibration control). To allow testing of such structures, researchers have proposed a variation of the method called real-time hybrid testing in which the experiment is executed in real time. Real-time hybrid testing is challenging because it requires guaranteed execution of each testing cycle within a fixed, small increment of time (typically less than 10 msec). Furthermore, unless appropriate compensation for time delays (from communication and computing time) and actuator dynamics is implemented, stability problems are likely to occur during the experiment. Traditionally, researchers have lumped the effects of time delays and actuator dynamics together and treated them as a constant time delay; techniques were then developed to compensate for this total time delay. However, these techniques only perform well when the delay is small compared to the fundamental period of the structure. The focus of this report is to develop an approach for real-time hybrid testing that uses model-based methods to compensate for time delays and actuator dynamics and combines fast hardware and software (for high-speed computations and communication) with high performance hydraulic components. The studies presented in this report extend the capabilities of real-time hybrid testing by facilitating accurate testing of structural systems with larger natural frequencies (e.g., stiff structures or multi-degree-of-freedom systems) and handling larger delays/lags which are typically associated with actuators with high force capacity. Furthermore, these studies demonstrate that real-time hybrid testing is an effective and practical technique to evaluate the response of structures incorporating devices for passive and semiactive structural control.

Keywords: real-time hybrid testing; pseudodynamic testing; substructuring; actuator dynamics; delay compensation; model-based compensation; MR damper; semi-active control