Narrow-band Ground Research Articles

Vector-Valued Intensity Measures Based on Spectral Shape to Predict Seismic Fragility Surfaces in Reinforced Concrete Buildings

Although some studies have been conducted to compute fragility surfaces of buildings using vector-valued seismic intensity measures (IMs), in all the cases, the first component of the vector usually is the spectral acceleration at first mode of vibration of the structure Sa(T1). In this study, fragility surfaces of three reinforced concrete buildings (RCB) subjected to narrow-band ground motions obtained from soft soil of Mexico City are computed considering vector-valued IMs based not only on Sa(T1), but also the velocity V(T1), pseudo-velocity Sv(T1), and normalized input energy by the mass EI/m(T1) as the first component. As second component of the vector-valued IMs, the Peak Ground Acceleration (PGA), Peak Ground Velocity (PGV), effective duration (tD), earthquake damage potential (ID) and four Np spectral shape-based parameters obtained through spectral acceleration (NpSa), velocity (NpV), pseudo-velocity (NpSv), and input energy (NpEI) have been analyzed. In order to obtain fragility surfaces, Multinomial Logistic Regression (MLR) was applied. It is observed that those vector-valued IMs based on the spectral shape proxies were more efficient to predict the probability of failure of RCB. For this reason, it is important to consider spectral shape vector-valued IMs in order to reduce uncertainty in the structural response of buildings under earthquakes. Thus, the use of two parameters instead of a single intensity measure improves the efficiency. Moreover, the fragility surfaces can be used for the seismic risk evaluation of buildings.

Enhanced Seismic Structural Reliability on Reinforced Concrete Buildings by Using Buckling Restrained Braces

The control of vibrations and damage in traditional reinforced concrete (RC) buildings under earthquakes is a difficult task. It requires the use of innovative devices to enhance the seismic behavior of concrete buildings. In this paper, we design RC buildings with buckling restrained braces (BRBs) to achieve this objective. For this aim, three traditional RC framed structures with 3, 6, and 9 story levels are designed by using the well‐known technique nondominated sorting genetic algorithm (NSGA‐II) in order to reduce the cost and maximize the seismic performance. Then, equivalent RC buildings are designed but including buckling restrained braces. Both structural systems are subjected to several narrow‐band ground motions recorded at soft soil sites of Mexico City scaled at different levels of intensities in terms of the spectral acceleration at first mode of vibration of the structure Sa(T1). Then, incremental dynamic analysis, seismic fragility, and structural reliability in terms of the maximum interstory drift are computed for all the buildings. For the three selected structures and the equivalent models with BRBs, it is concluded that the annual rate of exceedance is considerably reduced when BRBs are incorporated. For this reason, the structural reliability of the RC buildings with BRBs has a better behavior in comparison with the traditional reinforced concrete buildings. The use of BRBs is a good option to improve strength and seismic behavior and hence the structural reliability of RC buildings subjected to strong earthquake ground motions.

A simplified procedure to estimate peak drift demands for mid-rise steel and R/C frames under narrow-band motions in terms of the spectral-shape-based intensity measure INp

In this study, a simplified procedure to predict the maximum inter-story drift demands of mid-rise framed buildings in terms of a spectral-shape-based ground motion intensity measure is proposed. For this aim, the maximum inter-story drift of steel and reinforced concrete (R/C) frames with 4, 6, 8 and 10 stories subjected to several narrow-band ground motions is estimated as a function of the spectral acceleration at first mode of vibration of the structure Sa(T1), which is commonly used in earthquake engineering and seismology, and with the recently developed parameter related to the structural response known as INp. It is observed that the spectral-shape-based intensity measure INp is the parameter best related with the structural response of the selected frames under narrow-band motions. Therefore, a new approach is used to propose equations to compute maximum inter-story drift demands and seismic uncertainty of mid-rise frames as a function of INp. The equations can be used for the rapid seismic assessment and to obtain structural fragility curves of the steel and R/C frame models. The fragility curves obtained with the simplified approach are compared to those obtained with traditional procedure to compute structural fragility. It is observed that the proposed equations provide good accuracy to calculate fragility curves of steel and R/C structures under narrow-band earthquake ground motions in terms of the spectral-shape-based intensity measure INp.

Maximum inter-story drift demands of steel frames in terms of the intensity measure INp

In the present work, a new equation to predict the maximum inter-story drift demands of mid-rise steel framed buildings is proposed in terms of a new ground motion intensity measures based on the spectral shape. For this aim, the maxim\um inter-story drift of steel frames with 4, 6, 8 and 10 stories subjected to several narrow-band ground motions is estimated as a function of the spectral acceleration at first mode of vibration SaT1, which is commonly used in earthquake engineering and seismology, and with a new parameter related to the structural response known as INp. It is observed that the spectral-shape-based intensity measure INp is the parameter best related with the structural response of the selected steel frames under narrow-band motions. For this reason, an equation to compute the maximum inter-story drift demand of mid-rise steel frames as a function of INp is proposed. The equation is useful for the rapid seismic assessment.

Prediction of hysteretic energy demands in steel frames using vector-valued IMs

It is well known the importance of considering hysteretic energy demands for the seismic assessment and design of structures. In such a way that it is necessary to establish new parameters of the earthquake ground motion potential able to predict energy demands in structures. In this paper, several alternative vector-valued ground motion intensity measures (IMs) are used to estimate hysteretic energy demands in steel framed buildings under long duration narrow-band ground motions. The vectors are based on the spectral acceleration at first mode of the structure as first component. As the second component, IMs related to peak, integral and spectral shape parameters are selected. The aim of the study is to provide new parameters or vector-valued ground motion intensities with the capacity of predicting energy demands in structures. It is concluded that spectral-shape-based vector-valued IMs have the best relation with hysteretic energy demands in steel frames subjected to narrow-band earthquake ground motions.

Demand Diagrams Based on Inelastic Spectra: A Proposal for the Implementation of the Capacity Spectrum Method in the Romanian Seismic Design Code

During recent years, an increasing number of studies advocate the application of the capacity spectrum method based demand diagrams computed directly from inelastic response spectra, as an alternative to the basic formulation of the method, which uses elastic spectra computed for single degree-of-freedom oscillators with modified characteristics to account for inelastic behavior. When demand diagrams are based on the code-specified elastic spectrum, inelastic spectra are typically determined by dividing the elastic spectrum by strength reduction factors. The paper presents the application of the method, based on strength reduction factors computed with a unified analytical expression, deduced by the author in previous studies. The expression and is valid both for broadband and narrow band ground motions. Taking as a starting point the newly-enforced elastic design spectra in the 2013 edition of the Romanian seismic design code, the method is exemplified for the capital city, Bucharest. It is shown that, for the analyzed case, the use of the proposed strength reduction factors leads to results that are very close to those obtained by the “classical” nonlinear static procedure used in the code. This suggests a good potential for the implementation of the capacity spectrum method in the Romanian code, based on these factors.

On the Use of Vector-Valued Intensity Measure to Predict Peak and Cumulative Demands of Steel Frames under Narrow-Band Motions

In this study, various alternative vector-valued ground motion intensity measures (IMs) are used to estimate maximum inter-story drifts and hysteretic energy demands of steel framed buildings under long duration narrow-band ground motions. The vectors are based on the spectral acceleration at first mode of the structure as first parameterSa (T1). As the second parameter of the vector,IMsrelated to peak, integral and spectral shape parameters are selected. It is concluded that spectral-shape-based vector-valuedIMshave the best relation with maximum inter-story drift and energy demands in steel frames subjected to narrow-band earthquake ground motions.

Comparing vector-valued intensity measures for fragility analysis of steel frames in the case of narrow-band ground motions

Seismic fragility of steel frames subjected to narrow-band motions from soft-soils of Mexico City (Mexico) is evaluated by means of a set of vector-valued ground motion intensity measures (IMs) comprised of two parameters. All the vectors considered have, as the first component, spectral acceleration at the first mode of the structure. As the second component, compared IMs are chosen among peak and integral parameters, the former represent the spectral shape in a range of periods, while the latter refer to cumulative damage potential of earthquakes. The maximum inter-story drift and an energy-based damage index for steel frames are employed as engineering demand parameters for structural performance assessment. As a result of the comparison, it is observed that spectral-shape-based vector-valued IMs have the best explicative power with respect to seismic fragility estimation. Analyses, even if limited to the peculiar ground motions considered, suggest that a recently proposed parameter (Np) is especially promising as a candidate for the next generation of IMs when combined with spectral acceleration. This appears independent of the type of seismic response measure considered.

Nonlinear Responses of Sloshing in Square Tanks Subjected to Horizontal Random Ground Excitation

Nonlinear responses of the predominant two sloshing modes in a square tank have been investigated when the tank is subjected to horizontal, narrow-band random ground excitation. Galerkin’s method is applied to derive the modal equations of motion for nonlinear sloshing. Then the Monte Carlo simulation is used to calculate the mean square responses of these two modes. These to modes are nonlinearly coupled with each other, known as ‘autoparametric interaction’. The responses differ significantly from those of the corresponding linear model, depending on the characteristics of the narrow-band ground excitation such as the bandwidth, center frequency and the intensity. In addition, it is found that the direction of the excitation is a significant factor in predicting the mean square responses.

Probabilities of Exceeding Different Limit States for Buildings Subjected to Narrow-Band Ground Motions

Non-uniform values of recurrence intervals to seismic failure of structures are shown to occur for different limit states. The median capacity and the capacity and demand factors specified by currently accepted criteria are calculated for a set of 5-, 10-, 15-, and 24-story buildings, built on very soft soil sites and exposed to narrow-band ground motions. Using these values and the demand hazard curves for each structure, the recurrence intervals to failure are obtained for serviceability, life-safety and near-collapse limit states. Based on the results, it is concluded that for the design of structures exposed to the action of narrow-band motions, it may be appropriate to use capacity and demand factors that depend on their fundamental periods of vibration.

Seismic response of buildings with energy dissipating systems built in soft soils

A parametric study of buildings retrofitted with metallic yielding energy dissipation devices built on soft soil sites is presented. Buildings with 6, 15 and 30 stories are analyzed as representative of low-, mid- and high-rise buildings. Emphasis is placed on parameters controlling the strength and stiffness of the structure and energy dissipation devices. Response history analyses are performed using narrow band ground motions representative of typical motions in very soft soil sites. It is shown that the seismic response is strongly influenced by the ratio of the inter-story stiffness in the frame to the inter-story stiffness of the energy dissipation device support framing system and by the ratio of fundamental period of the structure to predominant period of the ground motion.

Layered antiferromagnetism with high Neel temperature in the intermetallic compound Mn2Au

On the basis of earlier experimental studies the intermetallic compound Mn2Au has been characterized as a nonmagnetically ordered material. Here we report the results of first-principles calculations based on local spin-density approximation that describes Mn2Au to have a narrow band ground state with rigid local moments on the Mn sites. Calculations of the interatomic exchange constants based on the magnetic force theorem and a Monte Carlo modeling of the resulting Heisenberg-like Hamiltonian predict a high Neel temperature of ∼1600 K. This temperature is considerably higher than for the other known high-temperature antiferromagnetic L10-type Mn based binary alloys used in magnetic storage applications.

Nonlinear Sliding Mode Control of Seismic Response of Building Frames

A nonlinear continuous sliding mode control strategy is presented to control the response of building frames to random ground motion. The control mechanism uses an active tuned mass damper placed at the top of the building frame. The control law is developed by considering uncertainties present in the control force input and in the excitations produced by the ground motion. The control forces are obtained for two cases: (1) for the state vector containing the structural displacement and velocity; and (2) for the state vector containing the parameters governing the predominant frequencies of excitations in addition to structural displacement and velocity. Using the proposed control strategy, responses of a 20-story building frame are controlled for a narrow band ground excitation represented by its power spectral density function. The responses, thus obtained, are compared with those obtained by the conventional linear control strategies. It is shown that the nonlinear sliding mode control performs better than the conventional linear control strategies for the reduction of displacement response. Further, controlled responses are found to be highly sensitive to the variation of the sliding margin and weighing matrices used to obtain the sliding surface.

A Filtered Backpropagation Algorithm for GPR

The inverse problem of determining the complex index of refraction profile of the subsurface from multistatic, narrow band ground penetrating radar (GPR) data is addressed within the context of diffraction tomography. The problem is formulated within the Born approximation which yields a coupled set of integral equations relating the sought after index of refraction profile of the subsurface to the measured scattered field data. This set of coupled equations is solved using the Tikhonov‐Phillips regularized pseudoinverse and the solution so‐obtained is found to be in the form of the filtered backpropagation algorithm of diffraction tomography. Both the full view (infinite number of incident plane waves) and limited view (finite number of incident plane waves) problems are addressed as well as the important cases where the background medium is lossy and evanescent plane waves of the scattered field are measured. The paper includes computer simulated examples illustrating the use of the finite view algorithm.

The tone sense multiaccess protocols with partial collision detections (TSMA/PCD) for packet satellite communications

The tone sense multiaccess with partial collision detection (TSMA/PCD) protocol is particularly suitable for a packet satellite system serving an area with a dense population of earth stations. By incorporating a narrowband ground radio channel for broadcasting busy ones, the earth stations are able to avoid packet collisions by sensing for the absence of busy tones before transmitting packets. Partial collision detection capability can also be achieved. Single-tone TSMA/PCD gives 97% of the carrier-sense multiaccess with collision detection (CSMA/CD) throughput when N=10 tones are used, while for multitone and slot-by-slot announcement TSMA/PCD protocols only N=8 and N=2, respectively, are sufficient to drive the system to the CSMA/CD performance.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>