Dimitrios Vamvatsikos

APPLIED INCREMENTAL DYNAMIC ANALYSIS

We are presenting a practical and detailed example of how to perform incremental dynamic analysis (IDA), interpret the results and apply them to performance-based earthquake engineering. IDA is an emerging analysis method that offers thorough seismic demand and capacity prediction capability by using a series of nonlinear dynamic analyses under a multiply scaled suite of ground motion records. Realization of its opportunities requires several steps and the use of innovative techniques at each one of them. Using a nine-story steel moment-resisting frame with fracturing connections as a test bed, the reader is guided through each step of IDA: (1) choosing suitable ground motion intensity measures and representative damage measures, (2) using appropriate algorithms to select the record scaling, (3) employing proper interpolation and (4) summarization techniques for multiple records to estimate the probability distribution of the structural demand given the seismic intensity, and (5) defining limit-states, such as the dynamic global system instability, to calculate the corresponding capacities. Finally, (6) the results can be used to gain intuition for the structural behavior, highlighting the connection between the static pushover (SPO) and the dynamic response, or (7) they can be integrated with conventional probabilistic seismic hazard analysis (PSHA) to estimate mean annual frequencies of limit-state exceedance. Building upon this detailed example based on the nine-story structure, a complete commentary is provided, discussing the choices that are available to the user, and showing their implications for each step of the IDA.

Reliability Assessment of Urban Water Distribution Networks Under Seismic Loads

Presented herein is a methodology for the seismic assessment of the reliability of urban water distribution networks (UWDN) based on general seismic assessment standards, as per the American Lifelines Alliance (ALA) guidelines, and localized historical records of critical risk-of-failure metrics pertaining to the specific UWDN under assessment. The proposed methodology is applicable to UWDN under both normal or abnormal operating conditions (such as intermittent water supply), and the assessment of reliability incorporates data of past non-seismic damage, the vulnerabilities of the network components against seismic loading, and the topology of a UWDN. Historical data obtained using records of pipe burst incidents are processed to produce clustered ‘survival curves’, depicting the pipes’ estimated survival rate over time. The survival curves are then used to localize the generalized fragility values of the network components (primarily pipes), as assessed using the approach suggested by the ALA guidelines. The network reliability is subsequently assessed using Graph Theory (Djikstra’s shortest path algorithm), while the system reliability is calculated using Monte Carlo simulation. The methodology proposed is demonstrated on a simple small-scale network and on a real-scale district metered area (DMA). The proposed approach allows the estimation of the probability that a network fails to provide the desired level of service and allows for the prioritization of retrofit interventions and of capacity-upgrade actions pertaining to existing water pipe networks.

Application of Nonlinear Static Procedures for the Seismic Assessment of Regular RC Moment Frame Buildings

The applicability of nonlinear static procedures for estimating the seismic demands of typical regular RC moment-resisting frames is evaluated. This work, conducted within the framework of the ATC-76-6 project, shows the degree to which nonlinear static methods can characterize global and local response demands vis-à–vis those determined by nonlinear dynamic analysis for three RC moment-frame buildings. The response quantities (engineering demand parameters) considered are peak story displacements, story drifts, story shears, and floor overturning moments. The single-mode pushover methods evaluated include the N2 and the ASCE-41 coefficient methods. Multi-modal pushover methods, such as modal pushover analysis and the consecutive modal pushover method, were also evaluated. The results indicate that the relatively good performance of the single-mode methods observed for low-rise buildings rapidly deteriorates as the number of stories increases. The multi-modal techniques generally extend the range of applicability of pushover methods, but at the cost of additional computation and without ensuring the reliability of the results.

Vector and Scalar IMs in Structural Response Estimation, Part II: Building Demand Assessment

The advantages and disadvantages of using scalar and vector ground motion intensity measures (IMs) are discussed for the local, story-level seismic response assessment of three-dimensional (3-D) buildings. Candidate IMs are spectral accelerations, at a single period (Sa) or averaged over a period range (Sa avg ). Consistent scalar and vector probabilistic seismic hazard analysis results were derived for each IM, as described in the companion paper in this issue (Kohrangi et al. 2016). The response hazard curves were computed for three buildings with reinforced concrete infilled frames using the different IMs as predictors. Among the scalar IMs, Sa avg tends to be the best predictor of both floor accelerations and inter story drift ratios at practically any floor. However, there is an improvement in response estimation efficiency when employing vector IMs, specifically for 3-D buildings subjected to both horizontal components of ground motion. This improvement is shown to be most significant for a tall plan-asymmetric building.

Vector and Scalar IMs in Structural Response Estimation, Part I: Hazard Analysis

A realistic assessment of building economic losses and collapse induced by earthquakes requires the monitoring of several response measures, both story-specific and global. The prediction of such response measures benefits from using multiple ground motion intensity measures (IMs) that are, in general, correlated. To allow the inclusion of multiple IMs in the risk assessment process, it is necessary to have a practical tool that computes the vector-valued hazard of all such IMs at the building site. In this paper, vector-valued probabilistic seismic hazard analysis (VPSHA) is implemented here as a post-processor to scalar PSHA results. A group of candidate scalar and vector IMs based on spectral acceleration values, ratios of spectral acceleration values, and spectral accelerations averaged over a period range are defined and their hazard evaluated. These IMs are used as structural response predictors of three-dimensional (3-D) models of reinforced concrete buildings described in a companion paper (Kohrangi et al. 2016).

Performance-Based Seismic Design: Avant-Garde and Code-Compatible Approaches

AbstractCurrent force-based codes for the seismic design of structures use design spectra and system-specific behavior factors to satisfy one or two predefined structural limit states . In contrast, performance-based seismic design aims to design a structure to fulfill any number of target performance objectives, defined as user-prescribed levels of structural response, loss, or casualties to be exceeded at a mean annual frequency less than a given maximum. First, a review of recent advances in probabilistic performance assessment is offered. Second, the salient characteristics of methodologies that have been proposed to solve the inverse problem of design are discussed. Finally, an alternative approach is proposed that relies on a new format for visualizing seismic performance, termed yield frequency spectra (YFS). YFS offer a unique view of the entire solution space for structural performance of a surrogate single-degree-of-freedom oscillator, incorporating uncertainty and propagating it to the output r...

Surrogate Modeling for the Seismic Performance Assessment of Liquid Storage Tanks

AbstractA three-dimensional surrogate model is presented for the seismic performance assessment of cylindrical atmospheric liquid storage tanks. The proposed model consists of a concentrated fluid mass attached to a single vertical beam-column element that rests on rigid beam-spokes with edge springs. The model is suitable for rapid static and dynamic seismic performance assessment. Contrary to other simplified models for tanks, its properties are determined through a simple structural analysis that can be performed in any nonlinear analysis software, without the need for complex finite-element models. The results compare favorably to those of three-dimensional finite-element models on three tanks of varying aspect ratios. A step-by-step example of the modeling procedure is presented for a squat unanchored tank, and a sensitivity analysis is conducted in order to investigate the effect of various modeling parameters on the seismic response of the proposed tank model.

Implications of Intensity Measure Selection for Seismic Loss Assessment of 3-D Buildings

Present building-specific loss assessment state-of-art involves the convolution of seismic hazard and building seismic demands. The latter is conditioned on spectral acceleration, Sa(T1), at the buildings first mode as the ground motion intensity measure (IM) and is typically estimated by carrying out nonlinear dynamic analyses on a two-dimensional (2-D) model. By new proposals on the use of improved IMs that can introduce higher fidelity, the accuracy in loss estimation becomes an open question. In reply, we offer a uniform basis for comparing the loss estimates for a set of eight different scalar and vector IMs whose hazard can be predicted with existing GMPEs. Despite all eight being legitimate IMs, and despite the consistent use of conditional spectrum record selection, we find large differences in the estimated loss hazard. This points to the large uncertainty still lingering when connecting hazard to loss. Among the IMs considered here, the vector IMs and at least a scalar average of spectral accelerations showed a remarkable stability in their predictions for the three-dimensional (3-D) buildings, pointing to a potential for reliable applications.

PROBABILISTIC ASSESSMENT OF INNOVATIVE MITIGATING MEASURES FOR BURIED STEEL PIPELINE - FAULT CROSSING

A methodology is presented on assessing the effectiveness of flexible joints in mitigating the consequences of faulting on buried steel pipelines through a comprehensive analysis that incorporates the uncertainty of fault displacement magnitude and the response of the pipeline itself. The proposed methodology is a two-step process. In the first step the probabilistic nature of the fault displacement magnitude is evaluated by applying the Probabilistic Fault Displacement Hazard Analysis, considering also all pertinent uncertainties. The second step is the “transition” from seismological data to the pipeline structural response through the fault displacement components as the adopted vector intensity measure. To mitigate the consequences of faulting on pipelines, flexible joints between pipeline parts are proposed as innovative measure for reducing the deformation of pipeline walls. Thus, the mechanical behavior of continuous pipelines and pipelines with flexible joints is numerically assessed and strains are extracted in order to develop the corresponding strain hazard curves. The latter are a useful engineering tool for pipeline – fault crossing risk assessment and for the effectiveness evaluation of flexible joints as innovative mitigating measures against the consequences of faulting on pipelines.Copyright

Simplified Mechanical Model to Estimate the Seismic Vulnerability of Heritage Unreinforced Masonry Buildings

Traditional or historic masonry structures occur in large populations throughout the world, particularly in preserved historical city clusters. Being non-engineered and aging these structures are in urgent need of assessment and seismic repair/rehabilitation. However, traditional masonry presents important challenges to computational modeling, owing to complexity of structural system, material inhomogeneity, and contact interactions that collectively can only be addressed through detailed 3D nonlinear representation. In this article, a simple performance assessment model is developed in order to address the need for preliminary assessment tools for this class of structures. The objective is to be able to rapidly identify buildings that are at higher risk in the event of a significant earthquake, potentially justifying a second round of more detailed evaluation. The proposed model defines the characteristics of a Single Degree of Freedom representation of the building, formulating consistent 3D shape functions to approximate its fundamental mode of vibration considering both in-plane and out-plane wall bending as a result of insufficient diaphragm action. Parametric expressions for the dynamic properties are derived in terms of the important geometric, material, and system characteristics, and are used to express local demand from global estimates. Acceptance criteria are established both in terms of deformation and strength indices to guide retrofit. An application example of the proposed assessment methodology is included to demonstrate the ability of the model to reproduce the essential features of traditional masonry buildings under seismic action.