IDEALS Community: Dept. of Civil and Environmental Engineering

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

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

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

Post Processing of Cone Penetration to Assess Seismic Ground Hazards, with Specific Application to the New Madrid Seismic Zone

Title: Post Processing of Cone Penetration to Assess Seismic Ground Hazards, with Specific Application to the New Madrid Seismic Zone

Authors: Liao, Tianfei; Mayne, Paul W.

Abstract / Summary: The seismic cone penetration test (SCPTu) is the most efficient means for geotechnical site characterization and the evaluation of seismic ground hazards, as it provides up to 5 independent readings in a single sounding: cone tip stress (qT), sleeve friction (fs), penetration porewater pressure (ub), time rate of dissipation (t50), and downhole shear wave velocity (Vs). During SCPTu tests, a very large number of digital measurements are recorded. The overwhelming data provide more detailed information for engineering analysis, but also pose challenges in post-processing of “information overload”. In this thesis, software systems including ShearPro, ClusterPro, and InSituData, are developed to automate post processing of these SCPTu data. ShearPro is developed to automate the post-processing of the shear wave signals. ClusterPro uses the proposed threedimensional cluster analysis approach for soil stratification. InSituData facilitates the post processing of penetration data for seismic ground hazards analysis. A new threedimensional soil classification chart is also proposed in this thesis to help discern soil layers that may be subject to seismic ground hazards, such as loose liquefied sands and silty sands. These methods are then applied to SCPTu data collected at previously-identifed paleoliquefaction sites located in the New Madrid Seismic Zone (NMSZ). For liquefaction evaluation, the cyclic stress ratio (CSR) is computed using site response analysis by DeepSoil and a measured profile of shear waves derived from the 30-m SCPTU soundings and deep suspension loggings in AR and TN. The natural resistance of the soil to liquefaction, termed the cyclic resistance ratios (CRRs), is evaluated based on both deterministic procedures and probabilistic procedures. Based on liquefaction evaluation results at selected paleoliquefaction sites, regional CRR criteria for liquefaction are developed for the NMSZ. As even the latest major earthquakes in NMSZ occurred nearly 200 years ago, aging effects might be an important factor to consider in utilizing the liquefaction criteria to assess the seismic parameters associated with the previous earthquakes. The aging effects in the NMSZ were investigated through large scale blast-induced liquefaction tests conducted by the USGS and supplemented by the author by series of CPTs. Then a procedure to estimate seismic parameters associated with previous earthquakes is proposed. It utilizes both the liquefaction criteria based on SCPTu tests and the empirical attenuation relations developed for the corresponding regions. The approach is validated through data evaluation related to the 1989 Loma Prieta earthquakes in California and then applied to previous historic earthquakes in the NMSZ.

Keywords: MAE Center; Cone; Penetration; Hazards; NMSZ

Out-of-Plane Capacity and Rehabilitation of Partial Height Masonry Partitions

Title: Out-of-Plane Capacity and Rehabilitation of Partial Height Masonry Partitions

Authors: Goodno, Barry J.; Craig, James I.; Losiriluk, Thitikorn

Abstract / Summary: This project examined the behavior of nonstructural components present in essential facilities in Mid-America. School buildings were chosen for detailed studies because of the vulnerability of their occupants and the importance of these particular facilities to post earthquake emergency response. An inventory of nonstructural components was made for this class of essential facilities using the categorization of components developed by FEMA. In addition, the results of an inventory of schools and other essential facilities from Project SE-1 (Inventories of Essential Facilities in Mid-America) were used in this study. Partial height interior unreinforced masonry walls were found in many schools and because of their clear vulnerability to seismic forces were selected as the focus of this study. The primary objective of the present investigation was to evaluate and then suggest retrofit strategies for these components to enable them to meet the life safety and/or immediate occupancy performance levels specified in current FEMA guidelines. Additional objectives were to assess the accuracy of current evaluation methods recommended by FEMA and if necessary to develop improved analysis procedures, rehabilitation guidelines, and performance measures.

Keywords: MAE Center; Partial Height; Masonry; Capacity

New Madrid Seismic Zone Catastrophic Earthquake Response Planning

Title: New Madrid Seismic Zone Catastrophic Earthquake Response Planning

Authors: Cleveland, Lisa J.; Elnashai, Amr S.; Pineda, Omar

Abstract / Summary: The New Madrid seismic zone in the Central USA has experienced some of the strongest earthquake ground motions observed nationwide. The historic series of three earthquakes during 1811 and 1812 shook this Midwest region with magnitudes around 8. The earthquakes were extensively reported. However, limited damage occurred because of the area was sparsely populated. A recurrence of the 1811 and 1812 earthquakes would cause widespread and severely impacts affecting over 45 million residents of the states surrounding the New Madrid seismic zone. A repeat of these historical events would subject the major urban center of Memphis, Tennessee to intense ground shaking while other urban centers such as St. Louis, Missouri, would experience less intense shaking. This does not indicate that St. Louis is less vulnerable, however. Though not undertaken in this report, subsequent work will include the examination of other hazard scenario within the region of interest. These scenarios will represent seismic activity in the Wabash Valley Seismic Zone of M7.1 as well as near St. Louis, Missouri, of M6.0.

Keywords: MAE Center; FEMA; NMSZ; Response Planning

Modeling of Hysteretic Behavior of Beam-Column Connections Based on Self-Learning Simulation

Title: Modeling of Hysteretic Behavior of Beam-Column Connections Based on Self-Learning Simulation

Authors: Yun, Gun Jin; Ghaboussi, Jamshid; Elnashai, Amr S.

Abstract / Summary: Current AISC-LRFD code requires that the moment-rotation characteristics of connections be known. Moreover, it requires that these characteristics be incorporated in the analysis and member design under factored loads (AISC, 2001). Conventional modeling approaches to improve the prediction of cyclic behavior starts with a choice of a phenomenological model followed by calibration of the model parameters. However, not only is the improvement limited due to inherent limitations of this approach, but also test results indicate a large variability in load-carrying capacity under earthquake loading. In this research, a new neural network (NN) based cyclic material model is applied to inelastic hysteretic behavior of connections. In the proposed model, two energy-based internal variables are introduced to expedite the learning of hysteretic behavior of materials or structural components. The model has significant advantages over conventional models in that it can handle complex behavior due to local buckling and tearing of connecting elements. Moreover, its numerical implementation is more efficient than the conventional models since it does not need an interaction equation and a plastic potential. A new approach based on a self-learning simulation algorithm is used to characterize the hysteretic behavior of the connections from structural tests. The proposed approach is verified by applying it to both synthetic and experimental examples. For its practical application in semi-rigid connections, design variables are included as inputs to the model through a physical principle based module. The extended model also gives reasonable predictions under earthquake loads even when it is presented with new geometrical properties and loading scenario as well.

Keywords: Hysteretic; Beam-Column; Self-Learning; MAE Center

Fragility Relationships for Populations of Buildings Based on Inelastic Response

Title: Fragility Relationships for Populations of Buildings Based on Inelastic Response

Authors: Gencturk, Bora; Elnashai, Amr S.; Song, Junho

Keywords: Fragility; Buildings; Inelastic

Deterministic and Probabilistic Evaluation of Retrofit Alternatives for a Five-Story Flat-Slab RC Building

Title: Deterministic and Probabilistic Evaluation of Retrofit Alternatives for a Five-Story Flat-Slab RC Building

Authors: Bai, Jong-Wha; Hueste, Mary Beth

Abstract / Summary: The effectiveness of seismic retrofitting applied to enhance seismic performance was assessed for a five-story reinforced concrete (RC) flat-slab building structure in the central United States. In addition to this, an assessment of seismic fragility that relates the probability of exceeding a performance level to the earthquake intensity was conducted. The response of the structure was predicted using nonlinear static and dynamic analyses with synthetic ground motion records for the central U.S. region. In addition, two analytical approaches for nonlinear response analysis were compared. FEMA 356 (ASCE 2000) criteria were used to evaluate the seismic performance of the case study building. Two approaches of FEMA 356 were used for seismic evaluation: global-level and member-level using three performance levels (Immediate Occupancy, Life Safety and Collapse Prevention). In addition to these limit states, punching shear drift limits were also considered to establish an upper bound drift capacity limit for collapse prevention. Based on the seismic evaluation results, three possible retrofit techniques were applied to improve the seismic performance of the structure, including addition of shear walls, addition of RC column jackets, and confinement of the column plastic hinge zones using externally bonded steel plates. Seismic fragility relationships were developed for the existing and retrofitted structure using several performance levels. Fragility curves for the retrofitted structure were compared with those for the unretrofitted structure. For development of seismic fragility curves, FEMA global drift limits were compared with the drift limits based on the FEMA member-level criteria. In addition to this, performance levels which were based on additional quantitative limits were also considered and compared with FEMA drift limits. Finally, recommendations are made for implementing the seismic fragility analysis results into MAEviz, the damage visualization module developed by the Mid- America Earthquake Center.

Keywords: MAE Center; Retrofit Alternatives; Flat-slab; RC Building; Deterministic; Probabilistic

Comprehensive Seismic Loss Assessment for the State of Illinois

Title: Comprehensive Seismic Loss Assessment for the State of Illinois

Authors: LaFore, Susan; Elnashai, Amr S.

Abstract / Summary: A seismic loss assessment was conducted for the State of Illinois for the purposes of determining the vulnerable infrastructure elements, prioritizing mitigation efforts in the state, quantifying damage in economic terms, and aiding in the development of public awareness projects. The study was performed using FEMA’s HAZUS loss estimation software, and included several levels of analysis. First, a level 1 loss estimation was conducted using HAZUS default inventories and loss parameters but with user-supplied ground motion. Additional loss estimations were performed using site class maps to refine the ground motion, liquefaction susceptibility maps to estimate the effects of liquefaction, pipeline inventories from FEMA’s HSIP Gold Dataset, improved essential facilities inventories, bridge inventories provided by IDOT, and improved building and highway bridge fragilities.

Keywords: IEMA; Loss; MAE Center; Illinois

Assessment of Seismic Integrity of Multi-Span Curved Bridges in Mid-America

Title: Assessment of Seismic Integrity of Multi-Span Curved Bridges in Mid-America

Authors: Mwafy, A. M.; Elnashai, Amr S.

Abstract / Summary: The study presents a detailed seismic performance assessment of a complex office-designed bridge using state-of-the-art assessment tools and metrics. The impact of design assumptions on the capacity estimates and dynamic characteristics of a multi-span curved bridge are investigated. A single nine-span bridge is studied whilst the level of attention to detail is significantly higher than can be achieved in a mass parametric study of a population of bridges. The objective is achieved by in-depth investigation of the bridge representing the ‘as-designed’ (including features assumed in the design process) and that representing the ‘as-built’ (actual expected characteristics) structure. Three-dimensional detailed dynamic response simulations of the investigated bridge including soil-structure interaction effects are undertaken. The behavior of the ‘as-designed’ bridge is investigated on two different analytical platforms for elastic and inelastic analysis. A third idealization is adopted to investigate the ‘as-built’ behavior by realistically modeling bridge bearings, structural gaps and materials. A comprehensive list of local and global, action and deformation, performance indicators are selected to monitor the response to earthquake action, including bearing slippage and segment collision. The adopted methodology and results of elastic and inelastic analyses are discussed. The comparative study has indicated that the lateral capacity and dynamic characteristics of the as-designed bridge are significantly different than the as-built behavior. The potential of pushover analysis in identifying structural deficiencies, estimation of capacities and providing insight into the pertinent limit state criteria are demonstrated. The conclusions from this study are important for designers and assessors of the seismic response of complex bridges since it highlights potentially non-conservative assumptions that are frequently used in the design office.

Keywords: Integrity; MAE Center; Multi-Span; Bridges