Ricardo A. Medina

Strength Demand Issues Relevant for the Seismic Design of Moment-Resisting Frames

This paper deals with the evaluation of strength demands relevant for the seismic design of columns that are part of moment-resisting frames. Regular frames with fundamental periods from 0.3 sec. to 3.6 sec. and number of stories from 3 to 18 are investigated. An evaluation of the relationships between strength demands (e.g., story shear forces, story overturning moments, and moments in columns), ground motion intensity, fundamental period, and number of stories is the focus of this paper. The results from this study demonstrate that the magnitude and distribution over the height of maximum axial and shear forces in columns exposed to severe earthquakes often are not adequately estimated by current seismic design and analysis procedures (e.g., the nonlinear static pushover). Moreover, the potential of plastic hinging in columns is high for regular frames designed according to the strong-column/weak-beam requirements of current code provisions, and more stringent strong-column/weak-beam criteria appear to be called for. The presented results are intended to provide guidance for improvement of seismic design provisions to avoid brittle failure modes in columns of moment-resisting frames.

Thermal Loading and Material Property Characterization of a Functionally Graded Plate With a Hole Using an Inverse Problem Methodology

ABSTRACT An existing axisymmetric analytical solution examines the radial and tangential thermal stresses and strains, and the radial displacements around a circular hole in a functionally graded material (FGM) plate. This solution is the point of departure to apply an inverse problem methodology to pose two inverse problems from measurements of the displacement and/or stress field: First, for known material distribution gradation function and material behavior the aim is to evaluate the thermal load. Second, for known thermal load and material behavior the objective is to define the gradation function coefficients relevant to determination of the properties. The inverse problem methodology explored in this paper for a FGM infinite plate with a hole provides a robust approach to determining the material gradation function and/or thermal loading conditions. It also demonstrates that careful attention needs to be paid to the analytical formulation of the behavior of FGMs so that the experimental necessity for evaluating and characterizing actual mechanical property variation inherent to these manufactured FGM structures may become a reality.

Proposed Method for Probabilistic Estimation of Peak Component Acceleration Demands

A probabilistic method is proposed to quantify peak component acceleration (PCA) demands for nonstructural components attached to elastic and inelastic structures. Incremental dynamic analyses and site-specific ground-motion hazard information are used to estimate PCA hazard curves and component uniform hazard spectra (CUHS) based on various structural and nonstructural parameters. For a given structural system, the primary parameters of interest are the location of the component within the structure, the ratio of the period of the component to the modal periods of the primary structure, and the component damping ratio. Representative results for shear wall structures illustrate the value of applying the method to acceleration-sensitive components, as the quantification of CUHS facilitates the implementation of performance-based design and evaluation approaches. The variability in the component responses presented highlights the need for a robust probabilistic seismic demand estimation methodology for nonstructural components in which the major sources of variability are incorporated.

Assessment of Numerical and Experimental Errors in Hybrid Simulation of Framed Structural Systems through Collapse

Hybrid simulation can provide significant advantages for large-scale experimental investigations of the seismic response of structures through collapse, particularly when considering cost and safety of conventional shake table tests. Hybrid simulation, however, has its own challenges and special attention must be paid to mitigate potential numerical and experimental errors that can propagate throughout the simulation. Several case studies are presented here to gain insight into the factors influencing the accuracy and stability of hybrid simulation from the linear-elastic response range through collapse. The hybrid simulations were conducted on a four-story two-bay moment frame with various substructuring configurations. Importantly, the structural system examined here was previously tested on a shake table with the same loading sequence, allowing for direct evaluation of the hybrid simulation results. The sources of error examined include: (1) computational stability in numerical substructure; (2) setup and installation of the physical specimen representing the experimental substructure; and (3) the accuracy of the selected substructuring technique that handles the boundary conditions and continuous exchange of data between the subassemblies. Recommendations are made regarding the effective mitigation of the various sources of errors. It is shown that by controlling errors, hybrid simulation can provide reliable results for collapse simulation by comparison to shake table testing.

Seismic Acceleration Demands on Nonstructural Components Attached to Elastic and Inelastic Structures

This paper addresses the quantification of peak component acceleration demands for acceleration-sensitive nonstructural components attached to or suspended from inelastic structural wall and moment-resisting frame structures. Under certain conditions, for a given ground motion hazard level, inelasticity of the primary structures increases the peak component acceleration demands when compared to the accelerations experienced by the component if the primary structure behaves in the linear elastic range. These conditions include nonstructural components located near the bottom of the structure with periods of vibration that are between the higher modal periods of the supporting structure. This amplification of peak component acceleration demands increases with a decrease in the damping ratio of the component and is more pronounced for structural systems that exhibit a concentration of inelastic behavior near the bottom of the structure. A short discussion on the relevance of these results in the context of current seismic strength demand estimation for nonstructural components is included.

Decision Tree for Postflooding Roadway Operations

When a pavement is flooded, transportation agencies are faced with the decision to leave the road open or close it to traffic. On the one hand, agencies may want to keep the road open and prioritize connectivity if the road serves a critical economic purpose for a region. On the other hand, agencies may choose to close the road to prevent additional structural damage (which may incur significant repair costs) because of high water content in (or even saturation of) the subgrade. Nondestructive testing (NDT) tools such as the falling weight deflectometer (FWD) can be applied to assist decision makers by facilitating a more reliable evaluation of the structural condition of the pavement. Reliable decision making ought to incorporate inherent uncertainties in the process, including the structural state of the pavement after flooding, as well as the reliability of FWD testing and variability in costs. This paper presents a method that uses a Bayesian decision tree approach for highway emergency operations after flooding. Uncertainties in the structural state of the pavement after flooding, NDT, and costs are addressed with Monte Carlo simulations. A case study of a flooded roadway section in North Dakota demonstrated this approach: user delay costs caused by closure of the roadway were explicitly considered. The results of the decision tree analysis provide objective recommendations about opening or closing a road after flooding, as well as whether FWD testing of the flooded road should be conducted once the water recedes.

VERTICAL ACCELERATION DEMANDS ON NONSTRUCTURAL COMPONENTS IN BUILDINGS

This paper addresses the statistical evaluation of vertical peak floor acceleration (PFAv) demands on elastic multistory buildings using recorded ground motions. Typical buildings are considered to be relatively flexible in the horizontal (lateral) direction and relatively rigid in the vertical (longitudinal) direction. The vast majority of studies conducted on the quantification of component acceleration demands have considered the flexibility of the nonstructural component (NSC) and its supporting structure primarily in the lateral direction. Studies on the evaluation of vertical component acceleration demands throughout a building are scarce and different opinions exist on the relevance of vertical accelerations in buildings. This paper focuses on the quantification of PFAv, which implies that NSCs are assumed to be rigid in the vertical direction. Only rigid NSCs located close to columns of moment-resisting frames are considered. Thus, the influence of vertical floor vibrations and their dependence on the properties of the floor system is not addressed. The results demonstrate that the amplification of vertical ground acceleration demands throughout a building depends on the vertical stiffness of the load bearing structure, and hence, the common assumption of rigid-body responses in the vertical direction is highly questionable.

Risk Assessment of Structural Integrity of Transportation Casks after Extended Storage

This study is part of an investigation into the structural reliability of a spent nuclear fuel (SNF) cask subjected to normal or accidental conditions of transport, after a dry storage period of up to 300 years. A probabilistic approach is chosen for this assessment to account for the uncertainties related to the timedependent material degradation mechanisms acting on the cask components. Preliminary investigations revealed that the fuel rod cladding is expected to control structural failure of the cask. The probability of failure is likely increased by hydride-related material degradation in the cladding and due to low cladding temperature after long-term storage. Mechanical testing revealed a tendency towards a brittle response on pinching loads at low material temperature of cladding that contains an excessive amount of hydrogen, or after the application of high cladding hoop stresses (CHSs) at high cladding temperatures. Therefore, the hydrogen content (CH) of the cladding and the value of the peak CHS during vacuum drying of the rods were identified as possible controlling parameters of hydride-induced cladding embrittlement. In the scope of the probabilistic risk assessment (PRA), statistical methods are used to predict the expected fuel rod conditions for the moment of transport. This paper presents an exemplary assessment of the probability of hydride-related cladding embrittlement due to high CHSs during vacuum drying or due to excessive hydrogen incorporation in the cladding considering in-reactor corrosion and fission gas release (FGR). The analysis reveals that between 5 and 12% of the SNF rods likely have an offset strain capacity below 2%, and therefore, could react brittle under pinching loads.

Lessons Learned from Evaluating the Response of Instrumented Buildings in the US: The Effect of Supporting Building Characteristics on Floor Response Spectra

Floor spectra of many instrumented buildings are evaluated to identify and quantify influential parameters on the horizontal seismic responses of acceleration-sensitive nonstructural components (NSCs). It is shown that many of these parameters are not explicitly incorporated into the American Society of Civil Engineers ASCE 7-16 design equations and are challenging to capture through numerical building models. Significant torsional responses are identified, even for nominally regular buildings, which can increase seismic demands on NSCs located at a floor periphery. For many instrumented buildings, especially single-story ones, floor diaphragms behave as flexible in their plane. This behavior, while mitigating torsional responses, can increase demands on NSCs located away from elements of the lateral-force resisting systems. An evaluation of floor acceleration responses of instrumented buildings with basements reveals that in many cases, even with the presence of perimeter concrete basement walls, accelerations at grade level could be significantly larger than those at lower basement levels. Consideration should be given to establishing the seismic base at the lowermost basement elevation.

A Ground Motion Record Selection Approach Based on Multiobjective Optimization

ABSTRACT The evaluation of the seismic safety and reliability of buildings and building contents within a probabilistic framework often requires response history analyses using site-specific ground motion records. The ground motion selection method proposed in this paper addresses this issue by a stochastic search procedure in which record sets are selected such that first- and second-order statistics (median and dispersion) satisfy predefined ground motion spectrum targets over a wide period range. Once a ground motion record set is selected, it can be used for seismic assessment of a broad class of buildings within the target period range at the given location.