Summary
The paper presents a lumped parameter model for the approximation of the frequency‐dependent dynamic stiffness of pile group foundations. The model can be implemented in commercial software ...to perform linear or nonlinear dynamic analyses of structures founded on piles taking into account the frequency‐dependent coupled roto‐translational, vertical, and torsional behaviour of the soil‐foundation system. Closed‐form formulas for estimating parameters of the model are proposed with reference to pile groups embedded in homogeneous soil deposits. These are calibrated with a nonlinear least square procedure, based on data provided by an extensive non‐dimensional parametric analysis performed with a model previously developed by the authors. Pile groups with square layout and different number of piles embedded in soft and stiff soils are considered. Formulas are overall well capable to reproduce parameters of the proposed lumped system that can be straightforwardly incorporated into inertial structural analyses to account for the dynamic behaviour of the soil‐foundation system. Some applications on typical bridge piers are finally presented to show examples of practical use of the proposed model. Results demonstrate the capability of the proposed lumped system as well as the formulas efficiency in approximating impedances of pile groups and the relevant effect on the response of the superstructure.
Soil-structure interaction is a complex problem to address in structural design due to difficulties related to the modelling of the soil-foundation behaviour, which is frequency-dependent and ...affected by uncertainties. The substructure method is an efficient approach to include the soil-foundation behaviour in the superstructure response, since it allows addressing separately the soil-foundation and the superstructure analysis exploiting different tools and expertise. A deterministic approach is usually adopted for the soil-foundation system analysis selecting properties of foundation and soil based on an engineering judgment, despite uncertainties due to the intrinsic variability of soil parameters are largely recognised by both scholars and practitioners, as well as confirmed by experimental campaigns and laboratory tests. This paper presents a probabilistic perspective of the dynamic behaviour of pile foundations in homogeneous soils, focusing on the effects of the uncertainties in: (i) the frequency-dependent impedance functions; and (ii) the kinematic response factors necessary to derive the foundation input motion from the free-field motion. Single piles and square pile groups are considered. Uncertainties are described through the probabilistic distributions of parameters governing the soil-foundation dynamic response, while the samples are generated using the quasi-random sampling technique. Probabilistic analyses are performed utilizing an efficient numerical model that has been developed, and the variability of the output quantities is presented and discussed. The latter reveals to be strongly affected by frequency. In addition, sensitivity analyses are performed to investigate the influence of each variable uncertainty on the system response. The response quantities are highly sensitive to the shear wave velocity while the soil density and the pile elastic modulus may have a significant role, depending on the foundation layout.
This paper presents finite elements for a higher order steel–concrete composite beam model developed for the analysis of bridge decks. The model accounts for the slab–girder partial interaction, the ...overall shear deformability, and the shear-lag phenomenon in steel and concrete components. The theoretical derivation of the solving balance conditions, in both weak and strong form, is firstly addressed. Then, three different finite elements are proposed, which are characterised by (i) linear interpolating functions, (ii) Hermitian polynomial interpolating functions, and (iii) interpolating functions, respectively, derived from the analytical solution expressed by means of exponential matrices. The performance of the finite elements is analysed in terms of the solution convergence rate for realistic steel–concrete composite beams with different restraints and loading conditions. Finally, the efficiency of the beam model is shown by comparing the results obtained with the proposed finite elements and those achieved with a refined 3D shell finite element model.
This paper analyses the new Research Centre designed for the University of Camerino and entirely financed by the national Civil Protection Department (DPC), following the seismic events in Central ...Italy in 2016. The building has been designed to guarantee speed of execution as well as a high level of safety, especially regarding seismic actions. The structural solution was to create an isolated system with a steel braced super-structure with pinned joints and r.c. sub-structures able to adapt to the complex morphology of the area. As described in the first part of the paper, design choices have been made to achieve a high level of resilience and robustness, i.e., to limit damage to structural and non-structural components and equipment under moderate and design seismic actions and to avoid disproportionate consequences in the event of extreme actions, larger than the design ones. In the second part of the paper, specific risk analyses have been carried out to evaluate the real performance of the building under increasing intensity levels, with reference to both serviceability and ultimate conditions. To this purpose a site-specific hazard study was first conducted, then non-linear analyses were performed using a hazard-consistent set of records with return periods ranging from TR = 60 years to TR = 10000 years. The main demand parameters of both the isolation system and the super-structure were recorded and capacity values corresponding to different ultimate and damage limit conditions were defined. The results obtained in terms of demand hazard curves show that the building performances in terms of robustness and resilience are very high, confirming the efficacy of the strategies adopted in the design.
This work investigates the effects of soil-structure interaction and spatial variability of seismic motion due to nonlinear site amplification on the seismic behaviour of long multi-span bridges ...founded on piles. An analysis framework able to include the spatial variation of ground motion induced by specific geological and geomorphological scenarios in the seismic soil-structure interaction analysis of long bridges is adopted, exploiting advantages of the substructure approach. The methodology is applied to a case study constituted by a pile-supported multi-span bridge founded in a soft clay deposit overlaying a stiff bedrock with three different configurations: horizontal, inclined and wedge-shaped. The reference input motion at the outcropping bedrock is represented by a set of real accelerograms and different seismic response models are used to compute site amplification effects, discussing the contribution to the free-field ground motion of both the two-dimensional configuration of the deposit and the nonlinear soil behaviour. The ground motions obtained from the different models are then used for computing the foundation input motion accounting for the pile–soil kinematic interaction; thereafter, inertial interaction analyses are performed on structural models with either fixed or compliant base, considering the non-synchronous seismic actions at the piers foundation. The results, compared in terms of piers head displacements, ductility demand and deck transverse bending moments, finally show the relative importance of bedrock morphology, soil nonlinearity and soil-structure interaction on the structural response.
This paper presents a static equivalent approach to estimate the maximum kinematic interaction effects on piles subjected to lateral seismic excitation. Closed-form expressions are reported for the ...evaluation of the maximum free-field soil movements and for the computation of maximum pile shear force and bending moments. Firstly, modal analysis, combined with a suitable damped response spectrum, is used to evaluate the maximum free-field response. Secondly, the pile is schematised as a Winkler's beam subjected to equivalent static forces defined according to soil vibration modal shapes and amplitude. The method may be applied by using response spectra suggested by National Standards or those obtained with accelerograms. The procedure proposed may be conveniently implemented in simple spreadsheets or in commercial finite element programs and easily used by practicing engineers. Method accuracy is demonstrated by comparing the results with those obtained with a more rigorous model. Good results may be achieved by considering only the first soil vibration mode making the procedure straightforward for practical design purposes.
This paper presents an analytical model for the short- and long-term analysis of composite steel-concrete beams with partial shear interaction and accounting for shear-lag effects. The material ...properties of the concrete have been assumed to be time-dependent and have been modelled by means of the algebraic methods while the remaining materials forming the cross-section have been supposed to behave in a linear-elastic manner. The global balance condition of the problem has been obtained by means of the principle of virtual work and, integrating this by parts, the governing system of differential equations and corresponding boundary conditions have been determined. Analytical expressions for both short- and long-term solutions have been derived and, to outline their ease of use, a number of case studies relevant for bridge applications have been proposed.
Composite steel–concrete structures represent an efficient and economical form of construction for building and bridge applications. This paper presents the current state of the art on the ...time-dependent behaviour of composite steel–concrete members, i.e. columns, slabs and beams, and how this influences both service and ultimate conditions. In the case of beams, only H-shaped or box steel sections with solid and composite slabs have been considered. In the initial part of the paper, a brief outline of the main aspects related to the time-dependent behaviour of the concrete is provided. This is followed by the description of the work carried out to date on the long-term response of composite columns, slabs and beams considered separately. In the case of composite columns, particular attention has been devoted to the influence of time effects on the ultimate response, role of confinement at service conditions and possible occurrence of creep buckling. Very limited work has been carried out to date on the long-term response of composite slabs. Because of this, only brief considerations are provided on this solution while still presenting recent research dealing with the development of shrinkage gradients through the slab thickness when cast on steel decks. The work outlined on composite beams has been categorised according to different design issues, which include shear-lag effects, the shear deformability of the steel beam, influence of time effects on the ultimate response, prestressing, time-dependent buckling, and sequential casting of the slab. Recommendations for possible future research work are provided in the concluding remarks.
► Influence of the time-dependent behaviour of concrete on composite members ► Service and ultimate response of composite systems affected by time effects ► Research needs on time effects in composite construction
•An innovative steel frame with reinforced concrete infill walls is presented.•A seismic design procedure for the presented structural system is proposed.•Numerical and experimental studies validate ...its structural behaviour.
Steel frames with reinforced concrete infill walls (SRCW) are an interesting seismic-resistant structural solution. However, an effective seismic design of SRCWs is not easy due to the current lack of specific capacity design rules that allow controlling the formation of a proper energy dissipating mechanism. In order to overcome such an issue, a ductile design procedure is presented in this paper. The proposed procedure leads to innovative SRCW systems where energy dissipation is expected to take place only in the vertical elements of the steel frame, which are subjected mainly to axial forces. The non-ductile components, i.e. reinforced concrete wall and steel-to-concrete connections, are expected to suffer negligible damage. Accordingly, the system is designed to control the formation of diagonal struts in the infill walls and behaves as a lattice brace instead of a shear wall. Experimental test results and nonlinear finite element analyses are illustrated to support the developed ductile design approach and highlight the advantages of SRCWs.
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