L. Dezi

Shrinkage effects in composite beams with flexible connection

Abstract Shrinkage effects in composite steel-concrete beams with flexible shear connectors are investigated. The analysis is performed for simply supported and continuous beams first by taking into account the shrinkage only and then the simulataneous creep effects. The mathematical formulation of the shrinkage problem for continuous beams with r redundant reactions leads to a system of r + 5 equations, one of which is a differential type equation; the solution is obtained numerically by performing a discretisation along the beam axis. The creep and shrinkage analysis introduces two integral-differential equations in the above system and the solution is obtained by applying the numerical step-by-step procedures. Finally, a parametric analysis is performed in order to evaluate the influence of the main parameters involved.

Slab Cracking Control in Continuous Steel-Concrete Bridge Decks

This paper presents a study on the effectiveness of various casting techniques used to control slab tensile stresses during the construction of continuous steel-concrete composite bridge decks. The main features describing the early age behavior of concrete are outlined and available modeling options are considered. A practical procedure is presented for the evaluation of the slab stress state taking place at various construction stages based on the modular ratio approach suggested by Eurocode 4. The method, which can be implemented using commercial software, requires the evaluation of suitable modular ratios and creep coefficients to estimate the time-dependent effects of the concrete slab weight and the thermal, endogenous, and drying shrinkage components. This simplified procedure is then applied to a case study to evaluate, for various casting sequences, the contributions of the concrete weight and shrinkage components to the slab early cracking. Finally, the efficiency of various casting techniques in controlling the slab tensile stresses is presented and discussed.

Dynamic monitoring of an isolated steel arch bridge during static load test

This paper describes the dynamic monitoring carried out by means of vibration measurements during the standard static load test of a half-through steel arch bridge on the Potenza river. The structural design of the bridge is characterized by some notable aspects: the river crossing is obtained by two coupled steel arches, having a span length around 115 m, and a steel-concrete composite deck sustained by thirty couples of steel hangers; the approaching spans are realized with continuous girders on intermediate supports, having cross section with variable height; the arches are supported on rubber bearing seismic isolators. The vibration measurements were nearly continuously acquired during the loading tests to detect the occurrence of anomalies in the structural behavior. In particular, the global dynamic characteristics of the structure, in terms of modal parameters, were determined using the data set of measurements at specific phases of the load test: first on the unloaded configuration, then on two different loaded configurations and finally after the bridge unloading. Measurements of vibrations due both to the ambient and to the impulse produced by a fully loaded truck passing over a bump, were carried out. The experimental results, in terms of modal parameters of the bridge, are in agreement with the theoretical results obtained with the predictive finite element model developed for design purposes and opportunely modified to account for the real conditions of the bridge during the tests.

Operational modal analysis on a r.c. building for the evaluation of the dynamic changes due to retrofitting

The paper deals with the changes of the modal parameters of a r.c frame building sited in Camerino, Italy. Operational modal analysis were performed before and after the seismic retrofitting of the building achieved with an innovative system that uses external steel towers equipped with dissipative devices. Before retrofitting, ambient vibration tests were carried out with the aim of evaluating the actual linear dynamic behavior of the building including the contribution of non-structural components (e.g. external and internal walls); modal parameters, i.e. natural frequencies and mode shapes determined by means of experimental modal analysis, allowed the calibration of a predictive f.e. model of the building which was crucial for the final design of the retrofitting system. Tests were repeated after the retrofitting works, with the main objective to verify that changes of modal parameters are in agreement with those predicted by the f.e. model. Results and comparisons between the modal parameters obtained before and after the retrofitting works demonstrate that the ambient vibration tests and the data processing techniques used allow the identification of the dynamic behavior of r.c. few-storeys buildings under operating conditions and, then, the identification of dynamic changes due to seismic retrofitting works performed with external steel “dissipative towers”. A model updating procedure is carried out to identify the value of the elastic modulus of the different materials constituting structural and non-structural elements.

An integrated Survey Experience for Assessing the Seismic Vulnerability of Senigallia’s Fortress (Italy): Documentation for Conservation and FEM Modeling

The paper presents the results of research carried out by an interdisciplinary team at the SAD in Ascoli Piceno in collaboration with MiBACT to verify the seismic safety of national museums. The object of study was the Rocca Roveresca Fortress complex in Senigallia (Marche, Italy), a unique example of a small 14th-century fortress shaped by a series of successive modifications. It currently houses a national museum. An integrated survey based on the acquisition of 3D laser-scanner data and endoscopic investigation was necessary to outline the traces of the stratifications and therefore to obtain different high- and low-poly 3D models useful for different purposes. The main objective was to propose an ideal workflow in developing 3D models that are useful for finite element method (FEM) analysis to detect hidden vulnerabilities in the fortress by evaluating the behaviour of several substructures in the walls.

Dynamic response of composite frames with rubber-based dissipating devices: experimental tests

Publisher Summary The chapter discusses dynamic responses of composite frames with rubber-based dissipating devices. The chapter analyzes composite moment-resisting space frames where lateral stiffness and equivalent viscous damping are augmented by means of dissipating devices, and presents the results of a preliminary experimental campaign carried out on a real scale composite space frame equipped with a dissipating bracing system endowed by high damping natural rubber devices. Moment-resisting frames made of steel-concrete members provide an efficient earthquake resistant system. The efficiency is entrusted to the formation of plastic hinges located at the ends of the beams or at the beam-to-column joints designed as partial strength connections. This design criterion is satisfactory in preventing the structure from collapse against high intensity earthquakes, but it often leads to a very large lateral deformability which brings on excessive inter-storey drift under low intensity earthquakes. The energy dissipation associated with the ductile behavior of dissipating zones induces a large amount of structural damage which usually calls for very expensive and difficult rehabilitation works.

Assessment of seismic vulnerability of historical defensive walls

Abstract. This paper presents a study on the behaviour of the walls of the Rocca Roveresca of Senigallia in Italy built in the XIV century on the ruins of a former Roman defensive struc-ture. This is a peculiar example of a small fortress that had undergone in the XV century im-portant modifications of the plant in order to enhance its defensive performances. A linear finite element model is first developed in order to understand the dynamic behaviour of a ge-neric portion of the wall and to detect the probable incipient failure mechanisms. A subse-quent static nonlinear analysis is carried out, with the same finite element model, to investigate the formation of the cracking layout and to detect the position of plastic hinges. The last analysis level is carried out with a tailored macro-element constituted by three bod-ies, namely the two external curtains and the inner fill for which a degradation of the behav-iour is considered. The results obtained demonstrates the efficiency of the wall against earthquakes characterized by return times typical for ultimate limit states. Same issues that deserve further investigation are highlighted.

Asymptotic behaviour of imperfect viscoelastic beams

Abstract In this paper the problem of the lateral stability of imperfect viscoelastic beams is analysed. The problem is examined by means of the quasi-static approach and leads to a system of integro-differential equations which can be resolved by a series expansions and the Laplace transforms. The viscous critical load is defined according to the asymptotic behaviour of the beam and can be evaluated by introducing the assumption of weak fading memory, solid material and thermodynamic compatibility. The critical load does not depend either on the type or the entity of the imperfections. Furthermore, some characteristic aspects of the problem, like the faster progress of the torque moment with respect to the progress of the lateral bending moment, are underlined. For a three-element model it is possible to reach a closed-form solution. In the case of the Poisson ratio constant in time and three-element model, an interesting analogy with the asymptotic behaviour of the imperfect column is observed.

Stochastic Seismic Analysis and Comparison of Alternative External Dissipative Systems

This paper deals with the seismic retrofit of existing frames by means of external passive dissipative systems. Available in different configurations, these systems allow high flexibility in controlling the structural behaviour and are characterized by some feasibility advantages with respect to dissipative devices installed within existing frames. In particular, this study analyzes and compares the performances of two external solutions using linear viscous dampers. The first is based on the coupling of the building with an external fixed-based steel braced frame by means of dampers placed horizontally at the floor levels. The second is an innovative one, based on coupling the building with a “dissipative tower,” which is a steel braced frame hinged at the foundation level, and activating the dampers through its rocking motion. The effectiveness of the two solutions is evaluated and compared by considering a benchmark existing reinforced concrete building, employing a stochastic dynamic approach, under the assumption of linear elastic behaviour for the seismic performance evaluation. This allows efficiently estimating the statistics of many response parameters of interest for the performance assessment and thus carrying out extensive parametric analyses for different properties of the external systems. The study results provide useful information regarding the design and the relative efficiency of the proposed retrofit solutions.

Seismic Response Analysis of Continuous Multispan Bridges with Partial Isolation

Partially isolated bridges are a particular class of bridges in which isolation bearings are placed only between the piers top and the deck whereas seismic stoppers restrain the transverse motion of the deck at the abutments. This paper proposes an analytical formulation for the seismic analysis of these bridges, modelled as beams with intermediate viscoelastic restraints whose properties describe the pier-isolator behaviour. Different techniques are developed for solving the seismic problem. The first technique employs the complex mode superposition method and provides an exact benchmark solution to the problem at hand. The two other simplified techniques are based on an approximation of the displacement field and are useful for preliminary assessment and design purposes. A realistic bridge is considered as case study and its seismic response under a set of ground motion records is analyzed. First, the complex mode superposition method is applied to study the characteristic features of the dynamic and seismic response of the system. A parametric analysis is carried out to evaluate the influence of support stiffness and damping on the seismic performance. Then, a comparison is made between the exact solution and the approximate solutions in order to evaluate the accuracy and suitability of the simplified analysis techniques for evaluating the seismic response of partially isolated bridges.