PhD thesis

Advanced analysis and modeling of strategic infrastructures subjected to extreme loads

PhD Domenico Asprone - Università Federico II, Napoli.

Recent terrorist acts have contributed to change the design approach to critical infrastructures; in fact, malicious disruptions, blasts, or impacts have unfortunately become part of the possible load scenarios that could act on constructed facilities during their life spans. Hence, a sustainable design aims to ensure the satisfactory performance of the structure during its entire lifetime considering all the possible critical actions, which the structure could be subjected to, including severe dynamic load conditions. The evaluation of the actions on the structure in case of such events is fundamental but represents a critical concern, since uncertainty related to loads definition is often quite high, especially for blast actions. Furthermore, structural response in case of such severe dynamic actions represents a critical issue, since both mechanical properties of materials and dynamic behavior of structural elements under severe dynamic loads can be very different from that exhibited under static actions. Moreover, numerical procedures used to simulate high dynamic loading conditions on structures can suffer of lack of accuracy, due to the rate and the intensity of deformations occurring on structural elements. Hence specific investigations become necessary for all these concerns. In particular, the present work addresses the assessment and design of strategic structures which are to be subjected to multiple hazards during its lifetime, including severe dynamic events, especially blast. At this aim, the most critical issues related to the assessment and design of strategic infrastructures potentially subjected to high dynamic conditions, are discussed and analyzed. Given the uncertainty involved in characterizing the load conditions, it seems inevitable to address the design based on a probabilistic framework. The design can be addressed by limiting the probability of failure below a certain de-minimis risk level that is deemed acceptable by the society. Inevitably, evaluation of the probability of failure requires taking into account possible actions or hazards that the structure could be subjected to; in other words, it needs to be evaluated based on a multi-hazard approach. In details, a multi-hazard framework is proposed and implemented for a strategic reinforced concrete buildings subjected to both seismic and blast hazard. The methodology is described in Chapter I and applied to a case study. Then, a deep investigation is presented on mechanical properties of construction materials in case of dynamic loading conditions. In particular, the strain rate sensitiveness of such material is investigated through a wide experimental activity conducted at Dynamat Laboatory at University of Lugano, Switzerland. In details, results of research activities are presented for: • concrete, in Chapter II, • steel for concrete internal reinforcement, in Chapter III, • Neapolitan yellow tuff, a natural stone widely used in Neapolitan area for masonry structures, Chapter IV, • GFRP (glass fiber reinforced polymer), in Chapter V. A further critical issue related to numerical simulations in case of high dynamic loading conditions on structures. In fact, to address dynamic loading conditions on structural elements, a variety of numerical methods have been recently proposed in the literature; the objective is to address advanced mechanical problems, such as those involving rapid deformations, high intensity forces, large displacement fields. In many of these cases, in fact, classical finite element methods (FEM) suffer from mesh distortion, numerical spurious errors and, above all, mesh sensitiveness. Hence, to overcome such issues, a number of numerical methods, belonging to the family of the so-called meshless techniques, have been widely investigated and applied. The objective of employing these methods is to avoid the introduction of a mesh for the continuum, preferring a particle discretization, with the goal of obtaining an easier treatment of large and rapid displacements. Recently, a number of researchers have tried to extend meshless methods also to solid mechanics problems. Among the several meshless numerical methods proposed, particle methods and in particular Smoothed Particle Hydrodynamics (SPH) has been widely implemented and investigated. A revision of the most common SPH methods is presented in Chapter VI and a rigorous analysis of the error is conducted, focusing on 1D problems. A novel second-order accurate formulation is also proposed for 2D and 3D applications. A further issue is addressed in Chapter VII and is related to protection interventions to be introduced in structural design to minimize disruptive effects in case of malicious blast actions and guarantee the safety of the occupants. In particular, a GFRP porous barrier is developed as fencing structure to prevent malicious disruptions, provide a standoff distance in case of blast actions, and reduce the consequences of an impact. The proposed barrier provides protection through two contributions. First, its geometrical and mechanical characteristics ensure protection against intrusions and blast loads. Second, its shape provides a disruption of the blast shock wave, adding additional protection for structures and facilities located beyond it. The efficacy of the proposed barrier under blast loads is presented by showing the results of the blast tests conducted on full-size specimens with a focus on the reduction of the blast shock wave induced by the barrier. A simplified model is also proposed to predict the reduction of the blast pressure due to the porous barrier, providing a procedure to design the geometrical characteristics of the barrier.

PhD Domenico Asprone - Università Federico II, Napoli.

High strain-rate Uniaxial Constitutive Behaviour in Fibre Reinforced Cementitious Composites

PhD Alessio Caverzan - Politecnico di Milano

Fibre-reinforced cementitious composites were developed starting in the eighties when their ability in energy absorption was first highlighted. The increasing rele- vance of the problems related with the damage of socially-sensitive structures (e.g. as high-rise buildings, bridges, and tunnels) has led to the study of fibre-reinforced composites also under fire and blast conditions. In the present framework, a re- search aimed at giving a contribution to the understanding of the behaviour of ad- vanced fibre-reinforced cementitious composites subjected to low and high strain rates, as well as, exposed to high temperatures, was designed. A wide experimental investigation was carried out both in static and dynamic fields, and the results are compared. In the whole research a high performance cementitious composite opti- mized with steel fibres (HPFRCC) has been taken into account. Steel fibres were high carbon straight fibres, 13 mm long with a 0.16 mm diameter (aspect ratio lf/df equal to 80); their content was equal to 100 kg/m3. The maximum aggregate size was equal to 2 mm. The HPFRCC obtained had a compressive strength (fc) equal to 115 MPa. First, tests at low strain rates were performed to characterize the material from the static point of view. Particular attention was turned to the investigation of the effects of the increasing temperature and of the casting procedures. Static tests were carried out on notched samples and on unnotched samples. The unnotched beam samples were cast in order to identify the material behaviour in bending at room condition and after thermal treatment. Samples were all extracted from a slab 1.6 m x 0.60 m in plane, 30 mm thick. The slab was cast by applying a unidirectional flow. In order to guarantee a certain fibre orien- tation the proprieties of the self compacting material were used taking advantage of the flow direction. Temperature influence was then studied for the unnotched samples by thermally treating the specimens prior to testing up to a temperature of 200°C, 400°C, and 600°C, respectively. In order to perform the mechanical char- acterization of the material according to National Recommendations (UNI11039) three prismatic notched samples were cast. Specimens were 150 mm x 150 mm cross-section and 600 mm long. Due to the high viscosity of the material at fresh state, it was not possible to cast them as suggested by the code, but they were cast in steel formworks by applying a flow at right angle with the longer formwork side obtaining a fibre random distribution. Both the unnotched and the notched beam samples were tested on bending. Finally, the bending and the uni-axial response of the materials were compared by performing uniaxial tensile tests on cylinders sampled from the bent specimens. With reference to the material self compact- ing properties, a good fibre alignment was obtained by imposing a unidirectional casting flow. This result is confirmed by the low scatter observed in the material response. A low scatter was observed in the overall test series carried out (bending tests and tensile tests carried out on samples with a good fibre orientation), and at all the strain rates investigated. The material so cast, and thus characterised by a good fibre alignment, showed a very high performance compared with other cementitious composites at comparable costs. The comparison between the results obtained from the material with a good fibre orientation and those from the ma- terial characterised by a random fibre distribution, pointed out the influence of boundary conditions (e.g. cast procedure). Boundary conditions in the structural manufacturing process were shown to dramatically change the material response. Hence, it is needful to characterize the material behaviour by exploiting appropri- ate tests and specimens. The advanced fibre reinforced cementitious composites are usually considered as convenient when a significant reduction of the structural weight can be obtained. Thin structures are the first and most important applica- tion of these innovative materials. However, it is recalled that particular attention must be turned to the tests used to characterise the material behaviour. Exploiting standardised samples could in fact lead to a not correct evaluation of the material behaviour. For these reasons a “structural” sample, which is strictly related to the structure considered and to the boundary conditions adopted during casting opera- tions, should be used to define the material behaviour. The static characterisation of the material highlighted a good response also at high temperatures; a bending hardening behaviour was shown also when previously heated up to 400°C. Though at 600°C the material performance is significantly reduced, a tensile strength of 15 MPa is however measured. The material behaviour at 600°C shows a strongly softening behaviour and fails due to fibre rapture. This change in the material response is assumed to be caused by the degradation of the physic-mechanical properties of the fibres. This was confirmed by tests on the steel wire used to produce the fibres, which highlighted that at 600°C the microstructure of the ma- terial changes. The resistant cross sectional area of the wire reduces, favouring the development of an oxide film on its lateral surface, and leading to a decrease in its strength of 75%. The material was then characterised also from the dynamic point of view. Samples, always sampled following bending tests, were tested under intermediate strain rates (0.1-1 s−1) with a hydro-pneumatic machine. On the other hand, high strain rates were studied by exploiting a modified Hopkinson bar (MHB) present in the DynaMat laboratory of the University of Applied Sciences of Southern Switzerland of Lugano. The MHB consists of two circular aluminium bars, called input and output bars (with a diameter of 20 mm and having length of 3 and 6 m, respectively) between which the HPFRCC specimen is glued using a bi-component epoxy resin. The input bar is connected to a high strength steel pretension bar (having 6 m length and 12 mm diameter), used as pulse generator. A test with the MHB is performed as follows:
a first, a hydraulic actuator (of maximum loading capacity of 600 kN) pulls the high strength steel bar; the pretension stored in this bar is assured by the blocking device;
b the second operation is the rupture of the fragile bolt in the blocking device, which gives rise to a tensile mechanical pulse of duration 2.4 ms and with a linear loading rate during the rise time. The pulse then propagates along the input and output bars, leading the specimen to failure.
Comparison between static and dynamic tests was then performed allowing to highlight several relevant aspects. First, at room temperature the comparison between static and variable strain rate tests, carried out by means of three different mechanical devices, exhibits high values of the Dynamic Increase Factor. For strain rates up to 0.1 s−1 the tests results seem to be well predicted by the trend proposed in the Model Code 2010 for plain concrete. Nonetheless, the DIF values for HPFRCC start to increase at a lower value of the strain rate (between 0.1 and 1 s−1) than that suggested by different models (1 s−1 for Malvar and the Model Code 2010, 30 s−1 for the Model Code 1990). Between 1 and 150 s−1 a transition zone with a lower slope with respect to that expected for a plain concrete was observed. By increasing the strain rate up to 300 s−1 the DIF increases, but the models considered overestimate the slope also in this range of strain rates. On the other hand, analysing DIF behaviour at increasing temperatures allowed to highlight that it is not substantially influenced up to 400°C. On the contrary, at 600°C a high increment in the values of DIF was observed with particular reference to strain rates of 150 s−1. At higher strain rates (300 s−1) this parameter decreases as a function of the magnitude of the previous thermal treatment.

Mechanical behaviour of metallic materials under dynamic loads

PhD Nilamber Kumar Singh - Indian Institute of Tecnology, Delhi