Showing papers in "Earthquake Engineering & Structural Dynamics in 2018"

Is rocking motion predictable

Abstract: Summary An argument of engineers and researchers against the use of rocking as a seismic response modification technique is that the rocking motion of a structure is chaotic and the existing models are incapable of predicting it well. This argument is supported by the documented inability of rocking models to predict the motion of a specimen excited by a single ground motion. A statistical comparison of the experimental and the numerical responses of a rigid rocking oscillator not to a specific ground motion, but to ensembles of ground motions that have the same statistical properties, is presented. It is shown that the simple analytical model proposed by Housner in 1963 is capable of predicting the statistics of seismic response of a rigid rocking oscillator.

Permanent earthquake‐induced actions in buried pipelines: Numerical modeling and experimental verification

Abstract: Buried pipelines are often constructed in seismic and other geohazards areas, where severe ground deformations may induce severe strains in the pipeline. Calculation of those strains is essential for assessing pipeline integrity and, therefore, the development of efficient models accounting for soil-pipe interaction is required. The present paper is aiming at developing efficient tools for calculating ground-induced deformation on buried pipelines, often triggered by earthquake action, in the form of fault rupture, liquefaction-induced lateral spreading, soil subsidence, or landslide. Soil-pipe interaction is investigated using advanced numerical tools, which employ solid elements for the soil, shell elements for the pipe and account for soil-pipe interaction, supported by largescale experiments. Soil-pipe interaction in axial and transverse directions is evaluated first, using results from special-purpose experiments and finite element simulations. The comparison between experimental and numerical results offers valuable information on key material parameters, necessary for accurate simulation of soil-pipe interaction. Furthermore reference is made to relevant provisions of design recommendations. Using the finite element models, calibrated from these experiments, pipeline performance at seismic-fault crossings is analyzed, emphasizing on soil-pipe interaction effects in the axial direction. The second part refers to fullscale experiments, performed on a unique testing device. These experiments are modeled with the finite element tools, to verify their efficiency in simulating soil-pipe response under landslide or strike-slip fault movement. The large-scale experimental results compare very well with the numerical predictions, verifying the capability of the finite element models for accurate prediction of pipeline response under permanent earthquake-induced ground deformations.

Modeling spatially correlated spectral accelerations at multiple periods using principal component analysis and geostatistics

Abstract: Funding information National Science Foundation, Grant/Award Number: CMMI 0952402 Summary Regional seismic risk assessments and quantification of portfolio losses often require simulation of spatially distributed ground motions at multiple intensity measures. For a given earthquake, distributed ground motions are characterized by spatial correlation and correlation between different intensity measures, known as cross-correlation. This study proposes a new spatial cross-correlation model for within-event spectral acceleration residuals that uses a combination of principal component analysis (PCA) and geostatistics. Records from 45 earthquakes are used to investigate earthquake-to-earthquake trends in application of PCA to spectral acceleration residuals. Based on the findings, PCA is used to determine coefficients that linearly transform cross-correlated residuals to independent principal components. Nested semivariogram models are then fit to empirical semivariograms to quantify the spatial correlation of principal components. The resultant PCA spatial cross-correlation model is shown to be accurate and computationally efficient. A step-by-step procedure and an example are presented to illustrate the use of the predictive model for rapid simulation of spatially cross-correlated spectral accelerations at multiple periods.

A numerical coupling scheme for nonlinear time history analysis of buildings on a regional scale considering site‐city interaction effects

Abstract: Key Laboratory of Civil Engineering Safety and Durability of China Education Ministry, Department of Civil Engineering, Tsinghua University, Beijing, P.R. China, 100084 Beijing Engineering Research Center of Steel and Concrete Composite Structures, Tsinghua University, Beijing, China 100084 Department of Civil and Environmental Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong Department of Hydraulic Engineering, Tsinghua University, Beijing, P.R. China 100084

Design and full‐scale experimental evaluation of a seismically endurant steel buckling‐restrained brace system

Abstract: Summary This paper presents the results of 12 full-scale tests on buckling-restrained brace (BRB) specimens. A simple-to-fabricate all-steel encasing joined by high-strength bolts was used as the buckling-restrainer mechanism. Steel BRBs offer significant energy dissipation capability through nondeteriorating inelastic response of an internal ductile core. However, seismic performance of BRBs is characterized by interaction between several factors. In this experimental study, the effects of core-restrainer interfacial condition, gap size, loading history, bolt spacing, and restraining capacity are evaluated. A simple hinge detail is introduced at the brace ends to reduce the flexural demand on the framing components. Tested specimens with bare steel contact surfaces exhibited satisfactory performance under the American Institute of Steel Construction qualification test protocol. The BRBs with friction-control self-adhesive polymer liners and a graphite-based dry lubricant displayed larger cumulative inelastic ductility under large-amplitude cyclic loading, exceeding current code minimum requirements. The BRB system is also examined under repeated fast-rate seismic deformation history. This system showed significant ductility capacity and remarkable endurance under dynamic loading. Furthermore, performance is qualified under long-duration loading history from subduction zone's megathrust type of earthquake. Predictable and stable performance of the proposed hinge detail was confirmed by the test results. Internally imposed normal thrust on the restrainer is measured using series of instrumented bolts. Weak- and strong-axis buckling responses of the core are examined. Higher post-yield stiffness was achieved when the latter governed, which could be advantageous to the overall seismic response of braced frames incorporating BRBs.

Improved risk‐targeted performance‐based seismic design of reinforced concrete frame structures

Abstract: Summary This paper presents a procedure for seismic design of reinforced concrete structures, in which performance objectives are formulated in terms of maximum accepted mean annual frequency (MAF) of exceedance, for multiple limit states. The procedure is explicitly probabilistic and uses Cornell's like closed-form equations for the MAFs. A gradient-based constrained optimization technique is used for obtaining values of structural design variables (members' section size and reinforcement) satisfying multiple objectives in terms of risk levels. The method is practically feasible even for real-sized structures thanks to the adoption of adaptive equivalent linear models where element-by-element stiffness reduction is performed (2 linear analyses per intensity level). General geometric and capacity design constraints are duly accounted for. The procedure is applied to a 15-storey plane frame building, and validation is conducted against results in terms of drift profiles and MAF of exceedance, obtained by multiple-stripe analysis with records selected to match conditional spectra. Results show that the method is suitable for performance-based seismic design of RC structures with explicit targets in terms of desired risk levels.

Evaluation of ASCE 7 equations for designing acceleration sensitive nonstructural components using data from instrumented buildings

Abstract: A wide variety of instrumented buildings and models of a code-based designed building are used to validate the results of previous studies that highlighted the need to revise the ASCE 7-16 Fp equation for designing acceleration-sensitive nonstructural components (NSCs) through utilizing simplified elastic and inelastic numerical models. The conducted evaluation shows that, unlike the ASCE 7 approach, the component amplification factor is strongly dependent on the ratio of NSC period to the supporting building modal periods, the ground motion intensity, and the NSC location. Results illustrate that with increasing the ground motion intensity, the in-structure and component amplification factors tend to decrease. It is shown that the recorded ground motions at the base of most instrumented buildings were significantly lower than the design earthquake (DE). Because ASCE 7 is currently meant to provide demands at a DE level, for a more reliable evaluation of the Fp equation, a representative code-based designed building is exposed to ground motions with various intensity levels. Results shows that at the DE level the ASCE 7 in-structure amplification factor, 1+ 2 (z h) , tends to significantly overestimate demands at all floor levels, whereas the ASCE 7 component amplification factor, ap, in many cases underestimate the computed values. The product of these two amplification factors, which represents the normalized peak component acceleration, in several floors exceeds the normalized Fp equation. PhD candidate, Dept. of Civil and Environmental Engineering, University of New Hampshire Durham, NH 03824 (email: ha2006@wildcats.unh.edu). Associate Professor, Dept. of Civil and Environmental Engineering, University of New Hampshire Durham, NH 03824 (email: ricardo.a.medina@unh.edu). Anajafi H, Medina R. Evaluation of ASCE 7 Equations for Designing Acceleration-Sensitive Nonstructural Components. Proceedings of the 11th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Los Angeles, CA. 2018. Eleventh U.S. National Conference on Earthquake Engineering Integrating Science, Engineering & Policy

Lifecycle cost optimization of tuned mass dampers for the seismic improvement of inelastic structures

Abstract: The seismic performance of tuned mass dampers (TMDs) on structures undergoing inelastic deformations may largely depend on the ground motion intensity. By estimating the impact of each seismic intensity on the overall cost of future seismic damages, lifecycle cost (LCC) proves a rational metric for evaluating the benefits of TMDs on inelastic structures. However, no incorporation of this metric into an optimization framework is reported yet. This paper presents a methodology for the LCC‐optimal design of TMDs on inelastic structures, which minimizes the total seismic LCC of the combined building‐TMD system. Its distinctive features are the assumption of a mass‐proportional TMD cost model, the adoption of an iterative suboptimization procedure, and the initialization of the TMD frequency and damping ratios according to a conventional linear TMD design technique. The methodology is applied to the seismic improvement of the SAC‐LA benchmark buildings, taken as representative of standard steel moment‐resisting frame office buildings in LA, California. Results show that, despite their limited performance at the highest intensity levels, LCC‐optimal TMDs considerably reduce the total LCC, to an extent that depends on both the building vulnerability and the TMD unit cost. They systematically present large mass ratios (around 10%) and frequency and damping ratios close to their respective linearly designed optima. Simulations reveal the effectiveness of the proposed design methodology and the importance of adopting a nonlinear model to correctly evaluate the cost‐effectiveness of TMDs on ordinary structures in highly seismic areas.

Simulation of orthogonal horizontal components of near-fault ground motion for specified earthquake source and site characteristics

Abstract: A method for simulating an ensemble of orthogonal horizontal near-fault ground motion components for a specified set of earthquake source and site characteristics is presented. We develop a parameterized stochastic model of the near-fault ground motion in two orthogonal horizontal directions. Near-fault ground motions may or may not contain a forward directivity pulse in at least one direction, and our final model will account for both pulselike and nonpulselike cases. In this paper we focus only on the pulselike motions. By fitting the model to a database of pulselike near-fault ground motions, we develop predictive equations for the model parameters in terms of the earthquake source and site characteristics, and we estimate the correlations between the model parameters. We use the predictive equations to generate sets of correlated model parameters and a corresponding ensemble of synthetic pairs of orthogonal horizontal near-fault ground motion components for a given design scenario specified in terms of earthquake and site characteristics. The resulting synthetic motions have the same statistical characteristics as the motions in the database, including the variability for the given set of earthquake source and site characteristics. Use of simulated motions is of particular interest in performance-based earthquake engineering due to scarcity of near-fault recordings.

Optimization-based minimum-cost seismic retrofitting of hysteretic frames with nonlinear fluid viscous dampers

Abstract: In this paper we discuss an optimization-based approach for minimum-cost seismic retrofitting of hysteretic frames with nonlinear fluid viscous dampers. The proposed approach accounts also for for moment-axial interaction in the structural elements, to consider a more realistic coupling between added dampers and retrofitted structure. The design variables of the problem are the damping coefficients of the dampers. Indirectly, the design involves also the stiffness coefficients of the supporting braces. In the optimization analysis, we minimize a realistic retrofitting cost function with constraints on inter-story drifts under a suite of ground motion records. The cost function includes costs related to the topological and mechanical properties of the dampers’ designs. The structure is modeled with a mixed finite element approach, where the hysteretic behavior is defined at the beams’ and columns’ cross-sections level. We consider damper-brace elements with a visco-elastic behavior characterized by the Maxwell model. The dampers’ viscous behavior is defined by a fractional power law. Promising results obtained for a two-story, a nine-story, and a twenty-story 2-D frames are presented and discussed.

Predictive model for site specific simulation of ground motions based on earthquake scenarios

Abstract: Summary A predictive stochastic model is developed based on regression relations that inputs a given earthquake scenario description and outputs seismic ground acceleration time histories at a site of interest. A bimodal parametric non-stationary Kanai-Tajimi (K-T) ground motion model lies at the core of the proposed predictive model. The functional forms that describe the temporal evolution of the K-T model parameters can effectively represent strong non-stationarities of the ground motion. Fully non-stationary ground motion time histories can be generated through the powerful Spectral Representation Method. A Californian subset of the available NGA-West2 database is used to develop and calibrate the predictive model. Samples of the model parameters are obtained by fitting the K-T model to the database records, and the resulting marginal distributions of the model parameters are efficiently described by standard probability models. The samples are translated to the standard normal space and linear random-effect regression models are established relating the transformed normal parameters to the commonly used earthquake scenario defining predictors: moment magnitude Mw, closest-to-site distance Rrup, and average shear-wave velocity VS30 at a site of interest. The random-effect terms in the developed regression models can effectively model the correlation among ground motions of the same earthquake event, in parallel to taking into account the location-dependent effects of each site. For validation purposes, simulated acceleration time histories based on the proposed predictive model are compared with recorded ground motions. In addition, the median and median plus/minus one standard deviation elastic response spectra of synthetic ground motions, pertaining to a variety of different earthquake scenarios, are compared to the associated response spectra computed by the NGA-West2 ground motion prediction equations and found to be in excellent agreement.

Assessment of the dynamic response of unreinforced masonry structures using a macroelement modeling approach

Abstract: Innovate Peru-Fondo para la Innovacion, Ciencia y Tecnologia, Grant/Award Numbers: PhD grant BECA-1-P-078-13 and BECA-1-P-078-13; Foundation for Science and Technology, Grant/Award Number: POCI-01-0145-FEDER-007633; FEDER / ERDF: "Fundo Europeu de Desenvolvimento Regional / European Regional Development Fund"

Design of transverse reinforcement to avoid premature buckling of main bars

Abstract: Summary This paper proposes an enhancement to the current strength and confinement-based design of transverse reinforcement in rectangular and circular reinforced concrete members to ensure that the flexural strength of reinforced concrete sections does not degrade excessively due to buckling of longitudinal bars until the desired level of plastic deformation is achieved. Antibuckling design criteria are developed based on a popular bar buckling model that uses a bar buckling parameter (combining the bar diameter, yield strength, and buckling length) to solely describe the bar buckling behavior. The value of buckling parameter that limits the buckling-induced stress loss to 15% in compression bars at the strain corresponding to the design ductility is determined. For a bar of known diameter and yield strength, the maximum allowable buckling length can then be determined, which serves as the maximum limit for the tie/stirrup/hoop spacing. Lateral stiffness required to restrain the buckling tendency of main bars at the locations of the ties/stirrups/hoops depends on the flexural rigidity of the main bars and the buckling length (equal to or multiple of tie/hoop/stirrup spacing), whereas the antibuckling stiffness (ie, resistance) provided by the ties/stirrups/hoops depends on their size, number, and arrangement. Using the above concept, design recommendations for the amount, arrangement, and spacing of rectangular and circular ties/stirrups/hoops are then established to ensure that the antibuckling stiffness of the provided transverse reinforcement is greater than the stiffness required to restrain the buckling-prone main bars. Key aspects of the developed method are verified using experimental tests from literature.