Early-stage deformation localisation in thrust systems: new perspectives from carbonates in the Italian Southern Alps and Oman Mountains

This thesis investigates mechanisms and boundary conditions that steer the early localisation of deformation and strain in carbonate multilayers involved in thrust systems, under shallow and mid-crustal conditions. Much is already understood about deformation localisation, but some key points remain loosely constrained. They encompass i) the understanding of which structural domains can preserve evidence of early stages of tectonic shortening, ii) the recognition of which mechanisms assist deformation during these stages and iii) the identification of parameters that actually steer the beginning of localisation. To clarify these points, the thesis presents the results of an integrated, multiscale and multi-technique structural study that relied on field and laboratory data to analyse the structural, architectural, mineralogical and geochemical features that govern deformation during compressional tectonics. By focusing on two case studies, the Eastern Southern Alps (northern Italy), where deformation is mainly brittle, and the Oman Mountains (northeastern Oman), where ductile deformation dominates, the thesis shows that the deformation localisation is steered by several mechanisms that mutually interact at different stages during compression. At shallow crustal conditions, derived conceptual and numerical models show that both inherited (e.g., stratigraphic) and acquired (e.g., structural) features play a key role in steering deformation and differentiating the seismic behaviour of the multilayer succession. At the same time, at deeper crustal conditions, strain localises in narrow domains in which fluids, temperature, shear strain and pressure act together during the development of the internal fabric and the chemical composition of mylonitic shear zones, in which localisation took place under high-pressure (HP) and low-temperature (LT) conditions. In particular, results indicate that those shear zones acted as “sheltering structural capsules” in which peculiar processes can happen and where the results of these processes can be successively preserved even over hundreds of millions of years.

On the characterization of the mechanical behaviour of bonded steel-CFRP adhesive joints

The use of adhesively bonded carbon fiber reinforced polymers (CFRP) is well established to repair metallic structural elements in the aerospace industry for more than three decades. Despite a few exceptions, this technology has yet not been exploited for the steel construction industry where there is a great need to rehabilitate old metallic bridges. For instance, in Europe more than 30% of the railway bridge stock operated for more than 100 years. These bridges are made of old mild steel or puddle iron that exhibits poor behaviour due to the quality of the material itself and degradation caused by the long-term loading or environmental effects. The modest results for Steel/CFRP joints obtained may be due to the type of adhesive used. In fact, most of the previous studies utilized brittle adhesives specially developed for concrete structures. Recent ductile adhesives that made for the automotive industry for metallic joints should be more appropriate. In this study, an experimental investigation on the behaviour of CFRP/steel adhesively bonded joints is presented. A comparison between brittle adhesives and ductile adhesives is conducted. The results show that the ductile adhesives achieve much higher performance than the brittle ones. The brittle adhesives provide more stiffness to the adhesive joint. In the specimens with the ductile adhesives, the failure pattern started by yielding the steel bars first then the adhesive joint which is promising since it can facilitate the design significantly if the steel yielding can be used as a design criterion. The main disadvantage of ductile adhesives is they are usually more expensive than brittle ones. In order to solve this issue, bi-adhesive joints, in which the joint is mainly made of (low cost) brittle adhesive and ductile adhesive in the stress concentration region, are proposed. The results revealed very high improvement up to the yielding strength of the steel bars and with a balanced stiffness.

Numerical prediction of the lateral displacement capacity of flexural reinforced concrete shear walls

Previous earthquakes showed that shear wall damage could lead to catastrophic failures of the reinforced concrete building. The lateral load capacity of shear walls needs to be estimated to minimize associated losses during catastrophic events; hence it is necessary to develop and validate reliable and stable numerical methods able to converge to reasonable estimations with minimum computational effort. The beam-column 1-D line element with fiber-type cross-section model is a practical option that yields results in agreement with experimental data. However, shortcomings of using this model to predict the local damage response may come from the fact that the model requires fine calibration of material properties to overcome regularization and size effects. To reduce the mesh-dependency of the numerical model, a regularization method based on the concept of post-yield energy is applied in this work to both the concrete and the steel material constitutive laws to predict the nonlinear cyclic response and failure mechanism of concrete shear walls. Different categories of wall specimens known to produce a different response under in plane cyclic loading for their varied geometric and detailing characteristics are considered in this study, namely: 1) scaled wall specimens designed according to the European seismic design code and 2) unique full-scale wall specimens detailed according to the U.S. design code to develop a ductile behavior under cyclic loading. To test the boundaries of application of the proposed method, two full-scale walls with a mixed shear-flexure response and different values of applied axial load are also considered. The results of this study show that the use of regularized constitutive models considerably enhances the response predictions capabilities of the model with regards to global force-drift response and failure mode. The simulations presented in this thesis demonstrate the proposed model to be a valuable tool for researchers and engineers.