Higher-order gradient continuum modelling of periodic lattice materials

The dynamic behaviour of periodic lattice materials is investigated using an equivalent higher-order continuum model obtained by homogenisation of the equations of motion. A gradient continuum enriched with higher-order inertia terms is developed using a combination of finite element discretisation of the unit cell and the continualisation approach. The analysis of the dispersion relations shows that the proposed model is able to capture correctly the physical phenomenon of wave dispersion in lattice structures which is overlooked by classical continuum theories

Stochastic representation of the mechanical properties of irregular masonry structures

A procedure for the stochastic characterization of the elastic moduli of plane irregular masonry structures is presented in this paper. It works in the field of the random composite materials by considering the masonry as a mixture of stones (or bricks) and mortars. Once that the elastic properties of each constituent are known (deterministically or stochastically), the definition of the overall masonry elastic properties requires the knowledge of the random field describing the irregular geometry distribution. This last one is obtained by a software, implemented ad hoc, that, starting from a colour digital photo of the masonry and using the instruments of the digital image processing techniques, gives the random features of this field in both the space and frequency domain. The definition of the stochastic properties of masonry structures may be very useful both for the application of the stochastic homogenization techniques and for the direct stochastic analysis of the structures

A new modal correction method for linear structures subjected to deterministic and random loadings

In the general framework of linear structural dynamics, modal corrections methods allow improving the accuracy of the response evaluated with a reduced number of modes. Although very often neglected by researchers and practitioners, this correction is particularly important when strains and stresses are computed. Aimed at overcoming the main limitations of existing techniques, a novel dynamic modal acceleration method (DyMAM) is presented and numerically validated. The proposed correction involves a set of additional dummy oscillators, one for each dynamic loading, and can be applied, with a modest computational effort, to discrete and continuous systems under deterministic and random inputs

Elastic wave dispersion in microstructured membranes

The effect of microstructural properties on the wave dispersion in linear elastic membranes is addressed in this paper. The periodic spring-mass lattice at the lower level of observation is continualised and a gradient-enriched membrane model is obtained to account for the characteristic microstructural length scale of the material. In the first part of this study, analytical investigations show that the proposed model is able to capture correctly the physical phenomena of wave dispersion in microstructured membrane which is overlooked by classical continuum theories. In the second part, a finite element discretisation of microstructured membrane is formulated by introducing the pertinent inertia and stiffness terms. Importantly, the proposed modifications do not increase the size of the problem with respect to the classical elasticity. Numerical simulations are included for validation purposes. The results confirm that the structural characteristics of the material can have a huge impact on the vibrational properties, particularly in the high-frequency range

Using the vibration envelope as a damage-sensitive feature in composite beam structures

A novel approach of damage detection in composite steel-concrete composite beams is suggested. Based on the idea of using the envelope's profile deflections and rotations induced by a moving load, this approach can lead to a practical cost-effective alternative to the traditional use of accelerometers and laser vibrometers.A parametric study has been undertaken, quantifying the sensitivity of the dynamic response of a realistic composite bridge to the presence of damage at different levels of partial steel-concrete interaction and velocity of the moving load.When compared to shifts in the natural frequencies, it has been verified that the proposed approach generally enjoys a higher sensitivity (so damage can be detected at an early stage), is more effective when closer to the ends of the bridge (where shear studs are more likely to be damaged), and displays an ordered set of results (which would reduce the possibility of a false damage).Further work is required to assess the effects of uncertainties and the adoption of more refined models for the moving load

Seismic response of subsystems in irregular buildings

The component-mode synthesis method is applied to investigate the seismic response of secondary subsystems multi-connected to primary structures with irregularities. The proposed approach is more accurate than the cascade approximation, often used in the design practice, as the primarysecondary dynamic interaction is considered through the modes of vibration of the two components. The results of parametric analyses on a representative case study reveal similar trends in the displacements of the two components for mass irregularities in elevation, while sti ness irregularities in plan can result in significant torsional motion in both components, with the effects in terms of absolute accelerations being in general larger than those associated with the lateral drifts. This suggests that dynamic analyses with the component-mode synthesis method are particularly indicated for the seismic assessment of acceleration-sensitive secondary subsystems

Design floor spectra for linear and nonlinear SDoF oscillators

The seismic analysis and design of secondary attachments to buildings or industrial facilities is a topic of broad engineering interest, increasingly attracting the attention of researchers and practitioners. Examples of secondary systems include suspended ceilings and non-structural walls, piping systems and antennas, storage tanks, electrical transformers and glass façades. Although not part of the load bearing structure, their significance stems from the survivability requirement in the aftermath of a seismic event and their vast contribution to the overall construction costs. Nevertheless, past earthquakes have demonstrated that current methods for the seismic analysis and design of secondary structures lack the necessary rigor and robustness, resulting in expensive and often unreliable solutions. Secondary systems can be highly sensitive to accelerations and inter-story drifts, and their seismic performance is influenced by the primary-secondary dynamic interaction. In many situations however, the mass of the secondary system may be much lower than the mass of the floor at which it is connected and therefore a cascade approach is admissible. If the secondary system can be realistically modeled as a single-degree-of-freedom (SDoF) system, then the floor response spectra could be a powerful tool for quantifying its seismic response. In this study, the performance of light secondary systems is examined in presence of uncertainties in the seismic input. A set of principal axes of ground shaking is initially identified and an ensemble of bi-directional time series is generated. The response of a set of SDoF secondary oscillators (i.e. linear, Bouc-Wen, sliding and rocking) attached to a representative primary structure is then investigated and their design spectra are established. As demonstrated with Monte Carlo simulations for the selected case study, the angle of seismic incidence causes the highest variations in the engineering demand parameters for the sliding oscillators, while the elasto-plastic oscillator with the Bouc-Wen model experiences the least variations. Furthermore, investigations at different elevations show higher variations in the sliding and linear oscillators, depending on the seismic input. As expected, the viscous damping ratio is found to significantly influence the response of secondary systems vibrating close to the fundamental frequency of the primary structure. Moreover, the peak response of sliding oscillators is shown to be a smooth function of the sliding friction coefficient, while the rocking spectra, due to the strong nonlinear dynamics of the rocking blocks, are characterized by large values of the coefficient of variations

Dynamic analysis of multi-cracked Euler-Bernoulli beams with gradient elasticity

Galerkin-type approach is presented and numerically validated for the vibration analysis of non-local slender beams with multiple cracks, in which a hybrid gradient elasticity (HGE) model accounts for the microstructural e ects. It is shown that: i) a smoother and more realistic profile of beam’s rotations is obtained at the damaged locations; ii) independently of support restraints and damage scenarios, only four boundary conditions are required, meaning that the computational e ort does not increase with the number of cracks; iii) the microstructural effects become significant when the modal wave lengths are less then about forty times the HGE length-scale parameters

Experimental study of wave propagation in heterogeneous materials

The phenomenon of wave propagation through concrete materials is affected by dispersion due to its intrinsic heterogeneous microstructure. Previous experiments have shown an increase of phase velocity at high frequencies. This behaviour cannot be analytically described by the classical elasticity theory, due to its non-dispersive nature. Instead, enhanced theories can be adopted. In this work the dynamically consistent non local model, able to take into account the microstructural effects by two additional length scale parameters, is retrieved. The main subject of this contribution is the experimental identification of the dispersive behaviour of cementitious materials and the validation of the gradient continuum to predict the dispersion of the wave born out of the heterogeneity of the material. The proposed work extends the applicability of non-local theories from a purely heoretical/analytical domain to the laboratory territory

A novel one-sided push-out test for shear connectors in composite beams

Push-out tests (POTs) have been widely exploited as an alternative to the more expensive full-scale bending tests to characterize the behaviour of shear connections in steel-concrete composite beams. In these tests, two concrete slabs are typically attached to a steel section with the connectors under investigation, which are then subjected to direct shear. The results allow quantifying the relationship between applied load and displacements at the steel-concrete interface. Since this relationship is highly influenced by the boundary conditions of POT samples, different experimental setups have been used, where the slabs are either restricted or free to slide horizontally, as researchers have tried to reduce any discrepancy between POT and full-scale composite beam testing. Based on a critical review of various POT configurations presented in the dedicated literature, this paper presents an efficient one-sided POT (OSPOT) method. While OSPOT and POT specimens are similar, in the proposed OPSPOT setup only one of the two slabs is directly loaded in each test, and the slab is free to move vertically. Thus, two results can be obtained from one specimen, i.e. one from each slab. A series of POTs and OSPOTs have been conducted to investigate the behaviour and the shear resistance of headed stud connectors through the two methods of testing. The results of this study than were compared with those of different POTs setups conducted by other researchers. The new OSPOT results show in general an excellent agreement with the analytical predictions offered by both British and European standards, as well as the estimated shear resistance proposed other researchers in the literature. These findings suggest that the proposed one-sided setup could be used as an efficient and economical option for conducting the POT, as it has the potential not only to double the number of results, but also to simplify the fabrication of the samples, which is important in any large experimental campaign, and to allow testing with limited capacity of the actuato