Search

This paper investigates the behavior of a ductile detail of exposed base plates. This detail consists of a base plate anchored to the concrete foundation through bolts extended to a steel chair configuration. The intention is to concentrate plastic strains mainly in the extended region of the anchor bolts, forcing the other connection components to remain elastic. The scientific background of this research consists of a series of sophisticated nonlinear finite element models subjected to a cyclic load protocol in the presence of an axial compressive force. The models were validated against an experimental test reported in the literature. Forces within the connection components, stresses, strain distributions, and deformation modes were examined. A total of sixteen three-dimensional nonlinear models were created using the ABAQUS simulation platform. The models were separated into two groups: the first consists of models with dimensions similar to the specimens tested in recent experimental programs reported in the past, while the second group simulates connections representative of mid-rise industrial frames. Building on the insights gained from the simulations of the first group, a methodology is proposed to design these column base connections. This suggested methodology is validated with the second group of simulations. Results indicate that the studied configuration detail presents some advantages compared with the traditional detail presented in Design Guide 1. For instance, plastic strains are developed almost exclusively in the anchor rods, and no damage is expected at the remaining components. Another essential characteristic is the exposed stretch length, with which it is possible to achieve a target design rotation without significant strain concentrations in the anchor rods. This characteristic facilitates post-earthquake inspections and repairs, and damage is virtually eliminated in the first story.

A comprehensive methodology for the validation of simulated ground motions is presented. The suggested methodology can be geared toward any ground-motion simulation method and seismic response assessment, in a target engineering application. The methodology is founded on the comparison between conforming groups of ground-motion waveforms from recordings and simulations and their effect on a representative collection of structures that represent the engineering application. The comparison considers the statistics of earthquake scenarios at the level of the event and site parameters, the resulting waveform characteristics, and the subsequent structural responses. Regression models are developed at three levels (between structural responses and waveform characteristics, structural responses and event and site parameters, and waveform characteristics and event and site parameters). Similarities between the models from groups of recorded and simulated ground motions guide the validation process. The validation methodology is applied to CyberShake (v.15.12) simulations and for the estimation of the column drift ratio of a bridge structure. It is shown that CyberShake (v.15.12) can be used to assess the median seismic response of the used bridge. Some discrepancies between simulations and recordings are observed, which could be attributed to the basin and site-response models used for simulations. Further implementation and refinement of the suggested methodology are recommended to make broader conclusions.

There is a need for benchmarking and validating simulated ground motions in order for them to be utilized by the engineering community. Such validation may be geared towards a specific ground motion simulation method, a target engineering application, and a specific location; the validation presented herein focuses on a bridge engineering application in southern California. Catalogs of simulated ground motions representing a 200,000-year forecast are selected from the Southern California Earthquake Center CyberShake version 15.12 database for five sites in Southern California (~20,000 unscaled ground motions per site). They are used in Non-Linear Time History Analysis (NLTHA) of four Ordinary Standard Bridge structures. For each site, these data are used to obtain simulation-based Engineering Demand Parameter (EDP) hazard curves. These are compared against EDP hazard curves that are constructed using conventional methods based on empirical models, i.e., using recorded ground motions through Incremental Dynamic Analysis and integration over the Intensity Measure (IM) hazard curve. The two sets of simulation-based and conventional EDP hazard curves are compared at various return periods. To further account for the differences between simulated and recorded ground motions, direct comparisons are also made between IM hazard curves for simulated and recorded catalogs, as well as the EDP versus IM data obtained from NLTHA of the bridges. We observe that CyberShake simulates motions that yield similar EDP values compared to empirical data for shorter return periods. For longer return periods, however, EDPs from the simulation-based analysis tend to be lower than the EDPs obtained from utilizing recorded ground motions for short-period bridges, while the opposite is the case for long-period bridges. It is recommended that validation efforts go beyond IM levels and also include comparisons of the relations between IMs and EDPs. Finally, site-specific relations are proposed that correlate the ratio between the two types of EDPs (simulation-based and conventional) with the hazard level, shallow site condition, and site basin depth.

The design practice of Box-Girder Seat-Type (BGST) bridges in the Western U.S. is continuously evolving based on the results of advanced modeling and analysis techniques. This is mainly to help engineers and researchers to better understand the behavior of BGST bridges during seismic excitations. Within this backdrop, this study fills the gaps in the current knowledge of assessing the combined effect of strong motion duration and spectral shape on the response of bridges using a comprehensive set of numerical simulations and statistical analyses. Three-dimensional finite element models of two real BGST bridges are analyzed using a large set of ground motions obtained from crustal sources and subduction sources. By means of Step-wise regression – and other statistical procedures – the sensitivity of bridge response parameters to various ground motion parameters including Arias Intensity (Ia), RotD50 spectral acceleration at the bridge's first natural period (Sa(T1)), Significant Duration (D5-95), mid-frequency (f), the derivative of the mid-frequency (f’) and time at 30% of cumulated Arias Intensity (tmid) are evaluated. Results indicate that in the case of ground motions arising from shallow crustal sources, Ia and Sa(T1) are the best predictors of the bridge response, and strong motion duration (D5-95) has no statistically meaningful impact on the response of bridges. However, it is observed that the D5-95 of the ground motions ascending from the subduction sources highly affects the bridge response; utilizing D5-95 alongside Sa(T1), or Ia, can significantly increase the accuracy of bridge response estimates. Hence, it is concluded that D5-95 is not an important ground motion intensity measure for ground motion selection for bridges located in areas with crustal earthquakes. In contrast, D5-95 is important in subduction zone ground motions and must be given proper consideration in the design and analysis of BGST bridges.

This paper proposes a methodology for generating surrogate single-degree-of-freedom (SDOF) models that can be utilized to estimate the probability distribution of the roof drift ratio of multistory buildings at various ground motion intensity measures. The use of an SDOF model as a surrogate for multistory buildings can significantly alleviate the high computational cost for probabilistic seismic demand assessment considering both model uncertainty and record-to-record variability. The surrogate SDOF model generated herein explicitly accounts for model uncertainties and can be used as an alternative to the nonlinear dynamic analysis of detailed building structures. Applications for such surrogate models include regional risk and resilience analyses and comprehensive parametric studies. To showcase the proposed methodology, an SDOF surrogate model for steel special moment frame (SMF) buildings is developed using the suggested surrogate SDOF model generating methodology. The properties of the surrogate model representing a multi-degree-of-freedom (MDOF) structure are computed using a probabilistic function of the fundamental period of the structure developed using Bayesian linear regression. To validate the surrogate model for SMFs, the response statistics produced using detailed multistory SMF models are compared with those of the corresponding surrogate SDOF models. The results show that the proposed surrogate SDOF model captures the probability distribution of the roof drift ratio of SMFs up to collapse with acceptable accuracy while reducing the runtime by at least one order of magnitude.

This study presents a critical assessment of the reliability of current base plate connections in steel special moment resisting frames (SMRFs). Using a probabilistic outlook, this research evaluates the reliability of exposed column base (ECB) connections in SMRFs designed based on the current seismic design provisions; it suggests (and implements) a statistical approach to compute resistance factors for three modes of failure (concrete bearing, base plate yielding at tensile interface, and anchor bolt fracture) of ECB connections to achieve a target reliability index, β, of 4.5. Since ECB connections are limited to short buildings, therefore, this study is conducted on two-story and four-story SMRFs which are analyzed using a suite of 120 ground motions originating from strike-slip and reverse faults. ECB connections for the two-story building are designed to simulate pinned connection, while the bases of the four-story building represent moment connections. Detailed methodology for calculating the β of ECB connections is presented considering the three limit states in a moment–axial load interaction curve. Results indicate that the implementation of current seismic provisions results in β ~ 3.3 for non-moment resisting ECB connections for all tried combinations of resistance factors. For moment resisting ECB connections, however, only the designs based on a resistance factor for concrete bearing failure mode less than the current 0.65 result in an acceptable reliability factor of β > 4.5.

This paper aims to transfer the current performance-based seismic assessment (PBSA) methodology to engineering practice by providing a preliminary design procedure denoted herein as the preliminary performance-based seismic design (PPBSD). PPBSD aims at guiding a conceptual design given the tolerable expected loss and target hazard level. The suggested preliminary design procedure implicitly incorporates the main sources of variability in the seismic performance assessment of building structures including the variability associated with seismic excitation. PPBSD can help stakeholders make informed decisions on how to handle potential seismic risk at the preliminary design level with minimal computational effort. The process for development of design tools required for implementation of PPBSD are described and such development is illustrated for a 4-story reinforced concrete special moment-resisting frame (RC-SMRF) building located in Los Angeles for a 475-year ground motion return period. A design example is offered to demonstrate how PPBSD can be implemented in practice.

We describe the formulation and application of an integrated general regional seismic loss assessment (RSLA) method for buildings in seismic regions. An efficient method for RSLA is valuable for engineers involved in city planning, risk management, and insurance dealings. In contrast to previously reported methods, the framework presented herein is hazard-based and utilizes a regional rapid seismic hazard deaggregation tool that allows regional assessment to be conducted more efficiently. The proposed technique is implemented as an example to assess general regional seismic loss in Los Angeles County for a ground motion hazard with 10% probability of exceedance in 50 years.

Methods for the seismic design of base connections in steel moment frames are well-developed and routinely utilized by practicing engineers. However, design loads for these connections are not verified by rigorous analysis. This knowledge gap is addressed through nonlinear time history simulations using design-level seismic excitation that interrogate demands in column base connections in 2-, 4-, 8-, and 12-story steel moment frames, featuring base connections that reflect current U.S. practice. The results indicate that: (1) for exposed base plate connections, lower bound (rather than peak) estimates of axial compression are suitable for design because higher axial forces increase connection strength by delaying base plate uplift; (2) even when designed as pinned (as in low-rise frames), base connections carry significant moment, which can be estimated only through accurate representation of base flexibility; and (3) the failure of embedded base connections is controlled by moment, which may be estimated either through overstrength or capacity-based calculations.

This paper presents and validates an analytical formulation, denoted as Performance Prediction Equations (PPEs), that relates the seismic response engineering demand parameter (EDP) of buildings to earthquake parameters such as magnitude, epicentral distance, and type of faulting. PPEs are conceptually novel and can be readily included in any hazard calculation program to directly estimate EDP hazard curves. The PPEs presented herein are based on the linear-ization of response spectrum analysis (RSA) formulation for estimation of the seismic response of multi-degree-of-freedom (MDOF) models for planar structural systems. Equations for mean and variance are provided for floor displacement, interstory drift ratio, and normalized base shear. The input parameters needed to apply the proposed PPEs are the modal properties of the structural system and the selection of an existing ground motion model (GMM). The proposed PPEs are validated against simulated results using a set of planar building models and the Campbell-Bozorgnia 2014 GMM. The comparison confirms that the proposed PPEs provide an accurate estimate of the statistics of the said EDPs.

This paper proposes a probabilistic metamodeling framework for predicting the joint probability distribution of the interstory drift ratio (IDR) profile of multistory buildings. IDR is an engineering demand parameter that correlates well with structural and nonstructural damages. However, the use of IDR for regional loss estimation has not been practiced due to the high computational cost of such estimations. The metamodel proposed herein efficiently estimates the IDR profile while accounting for the cross-correlations. This facilitates the use of component-level damage and consequence models for regional loss estimations and significantly refines the resulting risk estimates compared with the current state of practice in regional risk analysis, which relies on a single-degree-of-freedom representation of a building. The proposed framework employs multivariate Bayesian regression calibrated using a large data set produced using detailed nonlinear time-history analyses. To showcase the methodology, metamodels are developed for predicting the IDR profile of special steel moment frame buildings of various heights ranging from three to 12 stories. The resulting models are validated against the output of detailed multidegree-of-freedom models at different levels of ground motion intensity. Finally, the proposed metamodel is employed for estimating the seismic loss of a nine-story building, and the results are compared with the loss using two extremes of modeling in terms of refinement and computational effort: the FEMA P-58 approach and the Hazus approach. The results confirm that the proposed metamodel significantly refines the loss estimates compared with those of Hazus with a computational effort that is four orders of magnitude smaller than that required by the FEMA P-58 approach. [ABSTRACT FROM AUTHOR]

This paper attempts to provide insight into the sensitivity of the collapse capacity of moment-resisting frame and shear wall structures to variation in basic structural parameters, and the choice of an appropriate ground motion intensity measure, based on probabilistic estimation of the collapse capacity of a structural system. The effects of fundamental period and base shear strength and of deformation and deterioration properties of structural components on the collapse capacity of frame and wall structures are quantified. It is shown that the collapse potential of moment-resisting frames is highly sensitive to the ratio of column to beam strength; increasing this parameter from 1.2 to 2.4 will increase the median of collapse capacity by up to 90%. Using a scalar ground motion intensity measure for estimating the collapse capacity can lead to underestimation of median collapse capacity by up to 50%, compared to using a vector-valued intensity measure. The provided information can be used to assist in the selection of a suitable structural system and associated parameters in design for collapse safety. Closed-form solutions are formulated using a database of collapse fragility curves developed for the sensitivity study. Application of these closed-form solutions for design decision making is illustrated through a comprehensive example. DOI: 10.1061/(ASCE)ST.1943-541X.0000196 CE Database subject headings: Structural failures; Building systems; Earthquakes; Frames; Shear walls; Probability; Sensitivity analysis. Author keywords: Structural collapse; Moment-resisting frames; Shear walls; Probabilistic methods; Sensitivity; Performance-based design.

A hysteretic model formulation is presented for simulation of the cyclic moment-rotation response of exposed column–base (ECB) connections, which are typically used to connect a steel column to a concrete footing in steel moment-resisting frames. The research is motivated by the potential for using these connections as dissipative elements within seismic design. The physical processes responsible for various forms of hysteretic response are outlined; these include: (1) seating and gapping between the base plate and footing leading to pinched hysteresis, and (2) a recentering effect of axial compressive load resulting in flag-shaped hysteresis. The proposed ECB model utilizes: (1) a trilinear backbone curve; (2) hysteretic rules for pinching, unloading, recentering, and reloading; and (3) modes of deterioration for four quantities, including strength and stiffness. The model has 16 parameters, of which 4 are classified as core parameters (meaning they can be determined through physics-based models), whereas 12 are classified as ancillary, such that they require empirical calibration. The model is fit to a series of experiments, and it is determined that it is able to simulate the key aspects of hysteretic response. Recommendations for calibration of model parameters are presented, and limitations of the model are outlined. [ABSTRACT FROM AUTHOR]