Terms Of Acceleration Research Articles

Development of adjustable variable stiffness restrainer for bridge subjected to seismic excitation.

This paper presents a numerical and experimental assessment of a developed adjustable variable stiffness restrainer (AVSR) utilized for short span bridges. This restrainer has the ability to demonstrate multi stiffness capacity in different stages of bridge's superstructure movement to mitigate the severe damage of bridge due to an earthquake. The multi-level stiffness behavior of developed AVSR is achieved by using multiple mechanical springs with different lengths and placed in parallel in proposed design. A small prototype of developed AVSR has been fabricated and tested under incremental and cyclic loading in order to assess the restrainer performance and the behavior has been validated using finite element analysis. Thereafter, the constitutive model of AVSR was derived for the proposed restrainer in order to implement it in numerical simulations. Furthermore, a parametric study has been conducted numerically to evaluate the effectiveness of different parameters on the restrainer capacity. Moreover, the efficiency of AVSR application in a single degree of freedom system has been assessed by performing seismic analysis on a frame equipped with AVSR subjected to different seismic excitations using Newmark's method. The experimental and finite element results proved the efficiency of developed variable stiffness device to exhibit adjustable action against imposed loads in three designed stages. Furthermore, the parametric study results revealed that increasing the section area of the spring wire leads to increase the restrainer capacity. In contrast, the restrainer resistance is declined by an increase in the mean spring diameter and number of coils for each spring of AVSR. The time history analysis results also indicated that the frame response in terms of displacement, velocity and acceleration is improved by implementing the AVSR in the considered system.

Personalized Collision Avoidance Control for Intelligent Vehicles Based on Driving Characteristics

Collision avoidance has been widely researched in the field of intelligent vehicles (IV). However, the majority of research neglects the individual driver differences. This paper introduced a novel personalized collision avoidance control (PCAC) strategy for IV based on driving characteristics (DC), which can better satisfy various scenarios and improve drivers’ acceptance. First, the driver’s DC is initially classified into four types using K-means clustering, followed by the application of the analytic hierarchy process (AHP) method to construct the DC identification model for the PCAC design. Then, a novel PCAC is integrated with a preview-follower control (PFC) module, an active rear steering (ARS) module, and a forward collision control (FCC) module to ensure individual requirements and driving stability. Moreover, simulations verified the validity of the developed PCAC in terms of path tracking, lateral acceleration, and yaw rate. The research results indicate that DC can be identified effectively through APH, and PCAC based on DC can facilitate the development of intelligent driving vehicles with superior human acceptance performance.

A defect-correction algorithm for quadratic matrix equations, with applications to quasi-Toeplitz matrices

A defect correction formula for quadratic matrix equations of the kind is presented. This formula, expressed by means of an invariant subspace of a suitable pencil, allows us to introduce a modification of the Structure-preserving Doubling Algorithm (SDA), that enables refining an initial approximation to the sought solution. This modification provides substantial advantages, in terms of convergence acceleration, in the solution of equations coming from stochastic models, by choosing a stochastic matrix as the initial approximation. An application to solving random walks in the quarter plane is shown, where the coefficients are quasi-Toeplitz matrices of infinite size.

Broadband Strong Ground Motion Modeling Using Planar Dynamic Rupture With Fractal Parameters

AbstractDynamic rupture modeling represents a promising physics‐based approach to strong ground motion simulations. However, its application in a broad frequency range (0–10 Hz), interesting for engineering studies, is challenging. The main reason is that widely used and relatively simple planar fault models with smooth distributions of initial stress and frictional parameters, or even self‐similar initial stress, result in ground motions depleted in high‐frequency content. Here we propose an efficient approach for the linear slip‐weakening friction model on a planar fault based on the Ide and Aochi (2005, https://doi.org/10.1029/2004jb003591) multiscale model with a small‐scale random fractal distribution of the slip‐weakening distance Dc. We propose a way to combine these variations with a large‐scale deterministic dynamic model. We illustrate the approach on an elliptical model and a smooth model of the 2016 Mw 6.2 Amatrice, Italy, earthquake from low‐frequency dynamic rupture inversion. To intensify the incoherence of the rupture propagation, we also include a variation of the strength and initial stress, both correlated with Dc. These additions result in sustained high‐frequency radiation during the whole rupture propagation and omega‐square source time functions. The new model of the Amatrice earthquake generates synthetics that agree with the local ground motion model up to 5 Hz in terms of spectral accelerations while preserving the average and integral dynamic rupture parameters (e.g., stress drop, fracture and radiated energy). The fractal dynamic model can be easily implemented in any dynamic rupture propagation code and is thus readily applicable in broadband physics‐based ground motion predictions for earthquake scenarios in seismic hazard assessment.

Shaking table tests and numerical studies on the seismic behaviors of FPB in railway continuous beam bridges

Friction pendulum bearings (FPBs) were extensively used in recent years in railway beam bridges located in the earthquake-prone regions of China. While the efficiency of FPB and reliability of constitutive models have been demonstrated by many experimental studies, the seismic behaviors of FPB in continuous beam bridges in real earthquakes have rarely been investigated. Therefore, a 1/10-scale three-span railway beam bridge isolated by FPB was tested utilizing a 10.1 m × 8.1 m shaking table with six DOFs. The seismic responses of the bridge were analyzed in terms of phenomena, displacement, acceleration, and strain. Different sliding behaviors of the FPB at each pier cap appear because of the interaction with different damaged mid piers. Obvious uplift of the girder ends occurs due to the spherical surface of the FPB, which is evidenced by the measured vertical displacement of the bearings. Numerical simulation based on OpenSees indicates that the FPB element with the suggested friction coefficient model can well reproduce the peak and residual displacements of the FPB. A comparison of calculated and measured acceleration responses reveals that both damaged piers and FPB can filter out the higher frequencies of the input motion. The seismic responses of bridges isolated by the FPB and flat sliding bearing (FSB) were presented based on the validated model under PGAs from 0.2 g to 1.5 g. The FPB dissipates more earthquake energy with small peak and residual displacements, embodying better performance than that of the FSB due to the restoring capacity.

FEM investigation of full-scale tests on DSSI, including gravel-rubber mixtures as geotechnical seismic isolation

Soil-rubber mixtures underneath foundations are an eco-sustainable and low-cost Geotechnical Seismic Isolation (GSI) solution. Using rubber grains from end-of-life tyres in the mixtures can provide a modern recycling system that reduces the stockpile of scrap tyres worldwide. In recent years, gravel-rubber mixtures (GRMs) properties have been investigated, finding good behaviour in static and dynamic conditions. Laboratory tests on GRMs advantages as GSI in the form of a layer underlying the foundation of a structure are available in the literature. Nevertheless, only one experimental campaign on a full-scale prototype structure resting on GRMs with 0% and 30% rubber content per weight was performed (SOFIA-SERA Research Programme).This paper presents 3D advanced nonlinear FEM analyses to reproduce and furnish new results about the above-mentioned full-scale test. The calibration of the parameters of the different constitutive models used for modelling the foundation soil and the GRMs was supported by an extensive geotechnical characterization. The developed FEM model was validated by the results from the above-mentioned full-scale test in terms of accelerations and velocities. Then, the dynamic behaviour of the GRMs was studied, also considering the shear strains versus the shear stresses and the horizontal and vertical displacements, quantities not investigated experimentally. Finally, the GRMs static behaviour was also investigated, considering the GRMs axial strains and the GRMs and foundation vertical displacements.The paper highlights the fundamental role of both experimental tests and numerical modelling, as invaluable tools for coupled analyses of the seismic behaviour of soil-structure systems, including innovative materials, such as GRMs, and the main advantages of using GRMs as GSI.

Development of low-cost base isolation technique using multi-criteria optimization and its application to masonry building

Seismic isolation devices made of rubber have been adopted globally to protect buildings and bridges from seismic events. Recent research has focused on many low-cost rubber-based isolation devices for seismic hazard mitigation. While some techniques have demonstrated promising results, their practical application to buildings in seismic-prone rural parts of developing countries remains challenging. The present study proposes a low-cost base isolation technique that is viable and feasible for masonry buildings. The primary objective of the present study is to investigate the use of unreinforced rubber as a seismic base isolator. Initially, the rubber samples with different hardness were procured and tested for Hyperelastic and viscoelastic coefficients. These properties were used to model the rubber isolator in ABAQUS. A parametric study was conducted, and optimized rubber properties were obtained using the weighted sum approach. The optimized rubber is numerically validated with the experiment. The rubber isolator was designed and then modeled to an experimentally validated one-story masonry building. The behavior of building with and without proposed isolators is evaluated by subjecting them to seven spectrum-matched ground motions. A significant reduction in seismic response in terms of roof acceleration is observed. This study indicates the scope for using unreinforced rubber as an isolator for masonry buildings.

Dynamics of Two-dimensional Type III Electron Beams in Randomly Inhomogeneous Solar Wind Plasmas

The dynamics of a type III electron beam generating Langmuir wave turbulence and subsequent electromagnetic emissions is studied owing to two-dimensional Particle-In-Cell simulations performed in both homogeneous and randomly inhomogeneous solar wind plasmas. Important differences in the beam dynamics are highlighted between both cases, due to Langmuir waves’ transformations on the density fluctuations. This paper studies the dynamics of a weak beam interacting with Langmuir wave turbulence scattered by initially applied plasma density fluctuations, in terms of particle acceleration, non-Gaussian suprathermal electron tails, broadening and relaxation of velocity distributions, beam density localization, and electron diffusion or trapping in a turbulent plasma. Density fluctuations are the cause of beam acceleration during its relaxation stage; after Langmuir wave saturation, it gains up to half the energy lost during deceleration while wave turbulence is damping, exhibiting asymptotically a suprathermal tail of electrons carrying around 30% of its initial kinetic energy. Some important features observed for one-dimensional beams exciting Langmuir wave turbulence in randomly inhomogeneous plasmas can be recovered.

Earthquake Analysis of an Old RC Minaret Retrofitting with Shape Memory Alloy

Existing seismic vulnerability has become a topical of actuality, concerning both new and old buildings. Several techniques have been used to allow structures to better resist seismic events. In recent years, these have been so-called intelligent materials such as shape memory alloys (SMAs) due to their superelasticity and their ease in returning to their initial state after deformation, which can reach 10%. In the present article, nickel–titanium SMA is considered in a minaret of an old church transformed into a mosque to control the seismic response in terms of displacements, stresses and accelerations. The assessment of the seismic behavior was performed based on a modal and then transient analysis with Ansys software. The main objective was to determine the effectiveness of the addition of shape memory alloys by varying their number.

Enhancing Seismic Resilience of Existing Reinforced Concrete Building Using Non-Linear Viscous Dampers: A Comparative Study

After the catastrophic destruction of the October 2005 Kashmir earthquake, the first building code of Pakistan was developed in 2007. The sole purpose of the building code of Pakistan (BCP) was to incorporate advancements in earthquake-resistant design to fortify structures and ensure the safety of citizens against future seismic events. After 2007, the BCP was not revised till 2021 to include the changes over time. However, the recently updated version of BCP 2021 highlights that the seismicity of many regions in Pakistan is high, which is not truly reflected in the BCP 2007. Therefore, the advancements in earthquake-resistant design due to the growing concerns about the potential risks of seismicity in the region have been incorporated into the updated version of the BCP. However, there are concerns among researchers that many structures designed on the 2007 code may need seismic fortification. Therefore, the current study focuses on the seismic fortification of existing systems that were developed using previous codes. Non-linear viscous fluid dampers are used to improve the seismic resilience of existing structures. This study compares the seismic performance of an existing reinforced concrete building with and without non-linear viscous dampers and subjected to a non-linear dynamic analysis. The performance of the building is evaluated in terms of story displacement, story drift, story acceleration, and energy dissipation mechanisms. Adding the non-linear fluid viscous dampers in the structure caused a decrease in the inter-story drift by around 31.16% and the roof displacement was reduced by around 36.58%. In addition to that, in a controlled structure, more than 70% of energy was dissipated by the fluid viscous dampers. These results indicate that adding the non-linear fluid viscous dampers to the existing structure significantly improved the vibration performance of the system against undesirous vibrations. The outcomes of this study also provide a very detailed insight into the usage of non-linear viscous dampers for improving the seismic performance of existing buildings and can be used to develop effective strategies to mitigate the impact of seismic events on already built structures.

Dynamic stability analysis of metro tunnel in layered weathered sandstone

In this study a coupled-eulerian–lagrangian (CEL) approach has been applied to assess the internal blast loading conditions, for the tunnel built in three layers of sandstone rock. The three weathered stages of sandstone have been considered in this study i.e., slightly, medium and also highly weathered sandstone in three different layers. The sandstone rock weathering increases as we move towards the ground surface from deep subsurface. The elastoplastic finite element model with varied overburden depth with dimensions of 60 m (L × B × H) each to incorporate different parametric cases. An explosive is assumed at the center of the tunnel, with the capacity of hundred kg of trinitrotoluene (TNT), the explosive is assumed to be hanging in the air inside the tunnel opening with equal distance from all sides. The TNT sphere and air that is inside the rock tunnel has been modelled through CEL technique to simulate actual in-situ condition. Through Mohr-Coulomb, Concrete Damage Plasticity, and Johnson-Cook constructive material model the elastoplastic behaviour of different materials like rock, steel bars and concrete has been simulated respectively. The cage of steel bars has been embedded through interaction constraint in concrete liner to produce reinforced concrete liner. In the initial stages of simulation with 5 m of overburden depth the tunnel has been placed in the upper layer of sandstone. For further analysis, the position of the tunnel has been changed for overburden depth of fifteen, twenty-five and thirty-five meter. In accordance with the result of the simulation, for rock in terms of acceleration, velocity, and dis-placement, the depth of the overburden and the displacement of the crown are inversely related. No damage in terms of compression is seen in this simulation, but in all instances, a small damage owing to tensile failure has been noted.

The efficacy and accuracy of 3D-guided orthodontic piezocision: a randomized controlled trial

BackgroundNo randomized controlled trial (RCT) has studied the accuracy of surgical guides used in terms of orthodontic treatment acceleration. Therefore the aim of this trial was to assess computer-guided piezocision-based orthodontic.Materials and methodsThirty-two patients with severely crowded upper anterior teeth were enrolled and randomly allocated to either the experimental group (ExpG) or the control one. Subjects of the ExpG received three-dimensional (3D) guided piezoelectric corticotomies on the buccal alveolar bone of the anterior region. Five piezocision cuts were properly performed between each anterior teeth and the adjacent in virtual models. Surgical guides were designed and 3D-printed with preplanned slots that guide gingival and then piezoelectric incisions. The patients underwent Cone-Beam Computed Tomography CBCT before and immediately after surgery. Thus, Predesigned piezocisions were compared to the actual ones in attempt to measure three dimensional deviations of the applied peizocisions.ResultsNinety-six severe maxillary dental crowding were assigned for eligibility, 40 of them met the inclusion criteria. Thirty-two participants were randomly allocated to the trial`s groups. No patient was lost to follow-up neither from the control nor the experimental group. Overall alignment time (OAT) was reduced by 53% in the experimental group compared to the control group. The mean of the 3D deviation of the surgical guide was 0.23 mm (standard deviation 0.19 mm).ConclusionThe values of the surgical guide deviation was nearly null, which confirms that this innovative technique is clinically applicable. Furthermore, this technique was impressively effective in accelerating orthodontic tooth movement.Trial registrationThis trial was registered at The ISRCTN registry (ID: ISRCTN65498676 Registration date: 07/04/2021).

Optimum design and performance of a base-isolated structure with tuned mass negative stiffness inerter damper

In order to increase the efficiency of the structures to resist seismic excitation, combinations of inerter, negative stiffness, and tuned mass damper are used. In the present work, the optimum tuning frequency ratio and damping of the tuned mass negative stiffness damper-inerter (TMNSDI) for the base-isolated structure were determined by employing the numerical searching technique under filtered white-noise earthquake excitation and stationary white noise. The energy dissipation index, the absolute acceleration, and the relative displacement of the isolated structure were considered as the optimum parameters, obtained by their maximization. Evaluations of base-isolated structures with and without TMNSDI under non-stationary seismic excitations were investigated. The efficiency of the optimally designed TMNSDI for isolated flexible structures in controlling seismic responses (pulse-type, and real earthquakes) were evaluated in terms of acceleration and displacement. A dynamic system was used for deriving the tuning frequency and tuned mass negative stiffness damper inerter (TMNSDI) for white noise excitation by using explicit formulae of the curve fitting method. The proposed empirical expressions, for design of base-isolated structures with supplementary TMNSDI, showed lesser error. Fragility curve results and story drift ratio indicate reduction in seismic response by 40% and 70% in base-isolated structure using TMNSDI.

Effects of the quantity and arrangement of reactive jet obstacles on flame acceleration and transition to detonation: A numerical study

Solid obstacles induce the detonation in premixed gases while causing significant thrust losses. Jet obstacles can alleviate this deficiency, but there have been few studies on multi-group reactive jet obstacles. In this study, a detailed numerical simulation is performed to investigate the selection of the quantity of reactive jet obstacles and examine the effect of the arrangement of different jet obstacles on flame acceleration and the deflagration to detonation transition (DDT) processes. The results show that there is an optimal quantity of jet obstacles that can rapidly trigger detonation while reducing thrust loss and combustion chamber weight. In terms of flame acceleration, the mechanisms of flame acceleration by reactive jet obstacles with various arrangement types are complex. Specifically, the initial flame acceleration effect of reactive jet obstacles with different arrangement types is similar. However, the staggered arrangement has a longer-lasting vortex structure, stronger leading compression wave and flow field intensity, which provides favorable conditions for the ensuing flame acceleration. When the flame interacts with the jet, the larger virtual blockage ratio and stronger combustion heat release effect further amplify the flame acceleration discrepancy. In DDT, compared with solid obstacles, reactive jet obstacles have more advantages. Different types of jet obstacles arrangement differ in their ability and method in triggering DDT, with the staggered arrangement having the best DDT effect, and detonation initiation modes are divided into two categories: i) the detonation induced by the coupling of the flame surface and the high-pressure region; ii) the reflected wave impinges on the flame surface after the compression waves in front of the flame collide with the wall, resulting in a detonation. Although the quantity and arrangement of jet obstacles vary in this study, all the cases where detonation occurs are consistent with the flame acceleration model of Liberman and similar SWACER mechanism.

Covariant Wick rotation: action, entropy, and holonomies

Given an arbitrary Lorentzian metric g_{ab} and a nowhere vanishing, timelike vector field u^a, one can construct a class of metrics {widehat{g}}_{ab} which have Euclidean signature in a specific domain, with a transition to Lorentzian regime occurring on some hypersurface Sigma orthogonal to u^a. Geometry associated with {{{widehat{g}}}}_{ab} has been shown to yield some remarkable insights for classical and quantum gravity. In this work, we focus on studying the implications of this geometry for thermal effects in curved spacetimes and compare and contrast the results with those obtained through conventional Euclidean methods. We show that the expression for entropy computed using {widehat{g}}_{ab} for simple field theories and Lanczos–Lovelock actions differ from Wald entropy by additional terms depending on extrinsic curvature. We also compute the holonomy associated with loops lying partially or wholly in the Euclidean regime in terms of extrinsic curvature and acceleration and compare it with the well-known expression for temperature.

A fragility curve based on the transformation of a random variable

ABSTRACT There are two failure criteria in seismic probabilistic risk assessment of a nuclear power plant to define component failure caused by an earthquake. The first is in terms of peak ground acceleration and peak ground acceleration capacity. The second is in terms of seismic response and component capacity. These criteria are closely related, but they are not equivalent. One can derive a different fragility curve from the second criterion. First, this study analytically shows the relation of these failure criteria, pointing out that a fragility curve may not represent the uncertainty of a failure probability derived from the second criterion. In addition, we propose a probability density function of a failure probability based on the second criterion using the transformation of a random variable method applied to probability density functions of seismic response and component capacity. Then, we derive a new fragility curve based on the derived probability density function. Also, we show that the derived probability density function links to a likelihood function based on a mean fragility curve used in the literature. Finally, we discuss the non-identifiability of uncertainties of response and capacity, and we propose a Bayesian model utilizing a local response to overcome the non-identifiability.

Understanding the TeV γ-ray emission surrounding the young massive star cluster Westerlund 1

Context. Young massive star clusters (YMCs) have increasingly become the focus of discussions on the origin of galactic cosmic rays (CRs). The proposition that CRs are accelerated inside superbubbles (SBs) blown by the strong winds of these clusters avoids issues faced by the standard paradigm of acceleration at supernova remnant shocks. Aims. We provide an interpretation of the latest TeV γ-ray observations of the region around the YMC Westerlund 1 taken with the High Energy Stereoscopic System (H.E.S.S.) in terms of diffusive shock acceleration at the cluster wind termination shock, taking the spectrum and morphology of the emission into account. As Westerlund 1 is a prototypical example of a YMC, this study is relevant to the general question about the role of YMCs for the Galactic CR population. Methods. We generated model γ-ray spectra, characterised particle propagation inside the SB based on the advection, diffusion, and cooling timescales, and constrained key parameters of the system. We considered hadronic emission from proton-proton interaction and subsequent pion decay and leptonic emission from inverse Compton scattering on all relevant photon fields, including the cosmic microwave background, diffuse and dust-scattered starlight, and the photon field of Westerlund 1 itself. The effect of the magnetic field on cooling and propagation is discussed. Klein-Nishina effects are found to be important in determining the spectral evolution of the electron population. Results. A leptonic origin of the bulk of the observed γ-rays is preferable. The model is energetically plausible, consistent with the presence of a strong shock, and allows for the observed energy-independent morphology. The hadronic model faces two main issues: confinement of particles to the emission region, and an unrealistic energy requirement.

Influence of the anchorage shear hysteresis on the seismic response of nonstructural components in RC buildings

This article presents a numerical study on the influence of the anchorage shear hysteresis on the seismic response of nonstructural components (NSC) connected to multi-storey reinforced concrete (RC) buildings, and of the anchorage itself. To cover a variety of different types of shear hysteresis shapes, this contribution considered the experimental results obtained for five types of post-installed anchors. The results were used for calibrating the hysteresis model of the anchorage connecting an ideal NSC with rigid fixture and a 12-storey RC building host-structure. Using a suit of 40 earthquake records and assuming a single NSC at each storey level anchored by a single fastener, a series of non-linear dynamic analyses of the structure-fastener-nonstructural system was carried out. The results showed significant differences in terms of maximum acceleration and force of the NSC and anchorage, respectively, depending on the type of anchor. These seismic demands were sometimes larger than those required by the reviewed code provisions for rigid NSC, but also for the most restrictive code-case for flexible NSC. The results presented different amounts of scatter, mostly related to the size of the annular gap and of the loading stiffness of the anchorage. It is shown that the maximum force achieved by the anchorage is directly related to the peak relative velocity of the NSC within the gap region. It was concluded that the shape of the shear hysteresis of the anchorage highly influences the response of the NSC and the anchor itself and should not be neglected in practice.

Effect of the CHS on Seismic Responses of the Single-Layer Spherical Reticulated Shell under Vertical Seismic Motions

Single-layer spherical reticulated shells are typical roof structures for the gymnasiums. The center-hung scoreboard (CHS) is large weight display device which is usually suspended on the roof of the gymnasium. The effect of the CHS on the dynamic characteristics and seismic responses of the single-layer spherical reticulated shell (SPRS) is not fully clear. In this paper, the effect of the CHS on the SPRS under vertical seismic action is investigated. Two kinds of FE models are built with Abaqus software, including the flexibly suspended model and the simplified model. In a simplified model, the CHS is simplified as four fixed masses on the four CHS suspension nodes. The dynamic explicit method is used for the seismic responses, and the Lanczos method is used for the dynamic characteristics. The influence of the CHS weight and the sling length of on dynamic characteristics and seismic responses are analysed. It turns out that in the flexibly suspended model, the first three vibration modes are free swing of the CHS, and the CHS weight and the sling length have a significant impact on the fourth and subsequent modes. The length of the sling has a large impact on some low-order frequencies, but has little impact on the high-order frequencies. Compared with the simplified model, the axial forces of some structural members and some nodal acceleration in the flexibly suspended model under vertical seismic motions would increase by as high as 523% and 564%, respectively. It turns out that the seismic responses of the SPRS would be underestimated if a simplified model is used for analysis. The region in the central of the SPRS, the hoop members of the SPRS, and the support platform are the most affected regions in terms of both axial force and nodal acceleration.

The behaviour of a retaining wall under seismic excitation

In this research, the different methods to calculate the earth pressures on a retaining wall were exposed, for different loading conditions, namely static and dynamic. The dynamic response of a retaining wall under seismic loading was also studied. The simulation is done using the finite element code PLAXIS. The response is expressed in terms of acceleration and displacement. When using numerical models to study this type of problem, two different entities can be identified. The first one corresponds to the materials and the consideration of their behavior. The second part corresponds to the type of loading imposed, i.e. the input signal.