Enzo Martinelli

Steel and concrete composite beams with flexible shear connection: “exact” analytical expression of the stiffness matrix and applications

Abstract A displacement-based finite element model for the analysis of steel and concrete composite beams with flexible shear connection is presented in this paper. No approximations are introduced in the displacement field, because the stiffness matrix and the fixed-end nodal force vector are directly derived from the “exact” solution of Newmarks differential equation. Therefore, the present finite element may employ only one element per member, because internal nodes are only needed for variations of geometrical and mechanical properties and in the presence of concentrated forces. A simple criterion to establish if partial interaction effects may be neglected in the analysis of composite beams is also suggested.

Capacity models for shear strength of exterior joints in RC frames: state-of-the-art and synoptic examination

Damage observed in existing structures after recent earthquake events pointed out the key importance of beam-to-column joints in influencing the global response of reinforced concrete structures. In the last two decades several theoretical and empirical models have been proposed for evaluating shear strength of beam-to-column joints. The present paper reports an overview of the models currently available in the scientific literature for evaluating shear capacity of exterior beam-to-column joints. The present study is the first step of a wide analysis aimed at assessing such models and improving them. Moreover, the uncertainties deriving by applying the mentioned models will be also quantified therein, by means of well-established procedures for probabilistic seismic analysis of structures. The final results of that study are reported within a companion paper.

Modelling and verification of response of RC slabs strengthened in flexure with mechanically fastened FRP laminates

Mechanically fastened fibre-reinforced polymer laminate systems are emerging as a promising means for the repair and strengthening of reinforced concrete members. This technology entails the use of pultruded carbon- and glass-vinyl ester fibre-reinforced polymer laminates with enhanced longitudinal bearing strength that are connected to the concrete substrate by means of steel anchors. Attractive applications are those for emergency repairs where constructability and speed of installation are critical requirements. In this paper, an experimental investigation is first presented that included laboratory testing of scaled reinforced concrete slabs strengthened with four different combinations of laminate lengths and fastener layouts to study optimised strengthening configurations. Compelling evidence was gained on the influence of the partial interaction between reinforced concrete slabs and mechanically fastened fibre-reinforced polymer laminates on the flexural response arising predominantly from bearing ...

Indirect Identification Method of Bilinear Interface Laws for FRP Bonded on a Concrete Substrate

The mechanical response of Reinforced Concrete (RC) beams strengthened by an Externally Bonded Reinforcement (EBR) made out of Fiber-Reinforced Polymers (FRPs) is deeply influenced by the interaction between the concrete substrate and the FRP system, either cured-in-place (sheets) or preformed (plates). In particular, the strength of FRP-EBR RC beams is often controlled by debonding phenomena to develop at the adhesive-to-concrete interface. The most recent theoretical formulations and some experimental results obtained in the last years pointed out the differences that characterize the debonding strength of FRP sheets and plates. According to the findings of those studies, the fracture energy is a fundamental parameter governing the debonding phenomenon. However, determining its value is not sufficient for simulating the behavior of the FRP-to-concrete interface and modeling relevant problems such as intermediate debonding in RC beams externally strengthened by FRP. Consequently, formulating and calibrating local bond-slip models, which take into account the different behavior of sheets and plates, is of fundamental importance for modeling FRP-strengthened RC members. This paper is aimed at identifying bond-laws for sheets and plates through an Indirect Identification Method (IndIM), recently implemented and validated by the authors. A wide collection of experimental results obtained by pull-out tests on FRP sheets and plates is first reported and then employed for identifying the previously noted bond-slip laws. Finally, the results of the identification procedure demonstrate that the debonding phenomenon, described as a fracture process in mode II, should be modeled by assuming different bond-slip relationships for FRP plates and sheets.

Capacity models for shear strength of exterior joints in RC frames: experimental assessment and recalibration

Several theoretical models are currently available in the scientific literature for evaluating the shear capacity of both exterior and interior beam-to-column joints in reinforced concrete (RC) frames. A reasonably wide set of those models, based on either analytical or empirical formulations, has been summarised within a companion work. The present paper firstly presents a wide database which collects results obtained in about two-hundred experimental tests carried out on RC joints. Those results are employed for assessing the above mentioned capacity models by considering a set of experimental data much wider than those usually utilised in the original formulation of such models. Accuracy and reliability of the various models are measured by quantifying some statistical parameters actually describing the relationship between the experimental evidence and the prediction of the various capacity models under consideration. Three relevant classes of joints (namely unreinforced, under reinforced and code-compliant) are considered with the aim of emphasising that the various models perform in a rather different way when applied to those different classes. Finally, a possible recalibration of the various models is proposed with the objective of enhancing their predictive capacity with respect to both the database as a whole and the three classes of RC joints mentioned above.

Shear-Flexible Steel-Concrete Composite Beams in Partial Interaction: Closed-Form “Exact” Expression of the Stiffness Matrix

This paper presents a theoretical model for analyzing shear-flexible steel-concrete composite beams in partial interaction. Both concrete slab and steel beam are modeled according to Timoshenko’s theory, and a continuous linear-behaving shear connection is considered between the two connected members. Simplified kinematic assumptions have been considered for the displacement fields of the two connected members to derive a model, which is at one time rather general, but even simple enough to be easily handled and actually solved in closed-form. The analytical formulation of both stiffness matrix and vector of equivalent nodal forces is the key achievement of the present paper. They completely define an “exact” finite element for the mentioned model and can be easily employed for carrying out computationally efficient analyses of steel-concrete composite beams looking after the effect of both shear flexibility of the structural members and slips occurring at the interface between the two connected members. ...

Interface model for fracture behaviour of fiber-reinforced cementitious composites (FRCCs)

ABSTRACT This paper presents a model for fiber-reinforced cementitious composites (FRCCs) based on a cohesive-frictional interface theory. A zero-thickness joint model is formulated for simulating the fracture behaviour of interfaces between the various phases of fiber-reinforced concretes. In particular, the “Mixture Theory” is used for describing the coupled action between concrete and fibers. Numerical analyses have been carried out on both plain concrete and FRCC members. Comparisons between numerical simulation and experimental evidence demonstrate the accuracy and soundness of the proposed formulation.

Cyclic shear-compression tests on masonry walls strengthened with alternative configurations of CFRP strips

This work reports the results of an experimental programme aimed at investigating the in-plane behaviour of clay-brick masonry walls externally strengthened by carbon fiber reinforced polymer (CFRP) strips. Particularly, four different geometrical layouts were considered for the CFRP strips, though keeping unchanged the quantity of composites employed in each wall. Firstly, a preliminary experimental work was carried out on samples of the constitutive materials for quantifying their key mechanical properties and evaluating the bond behaviour of FRP strips on the masonry substrates. Then, eleven cyclic shear-compression tests were performed to observe the response of strengthened walls and the influence of the strengthening layouts under investigation. The proposed experimental report is intended as a contribution to the current state of knowledge about the behaviour of FRP-strengthened masonry walls: it is available to assess the accuracy and possibly improve the predictive capacity of design-oriented capacity models. Finally, the comparison of the reported experimental results with the predictions obtained by applying the analytical relationships proposed by a recently issued guideline for FRP strengthening of masonry structures is proposed.

On the Behavior of FRP-to-concrete Adhesive Interface: Theoretical Models and Experimental Results

Fiber-reinforced polymers (FRP) are more and more commonly employed for strengthening existing structures of both reinforced concrete (RC) and masonry. Since FRP sheets (cured in situ) or plates (preformed) are externally bonded on a concrete or masonry substrate, the issue of adhesion on those materials generally controls the effectiveness of strengthening in members stressed either in bending or shear (Motavalli & Czaderski, 2007). The use of composite materials for structural strengthening of civil structures and infrastructures began with some pioneering application at the middle of the ‘80s (Meier, 1987) of the past century. Plenty of experimental work and theoretical investigations have been carried out in the following years with the aim of demonstrating the feasibility of strengthening civil structures by means of composite materials (Swamy et al., 1987; Meier, 1995). However, composite materials were already widely used in other fields of structural engineering, such as aerospace (Hart-Smith, 1973), aeronautics and, later, automotive. Thus, the initial research activities about the possible use of composites in civil structures were not mainly focused on the behavior of composites themselves. They were rather intended at addressing two main issues regarding, on the one hand, the different behavior of composites with respect to more traditional materials (basically, steel) commonly used as a reinforcement in civil structures (Arduini & Nanni, 1997; Naaman et al., 2001; Triantafillou et al., 2001) and, on the other hand, the aspects related to the adhesive connection of the FRP laminates to the concrete (or masonry) substrate (Taljsten, 1997; Neubauer & Rostasy, 1997). The main findings of the research activities carried out in the ‘90s contributed to guidelines (fib, 2001; CNR-DT200, 2004; ACI 440-2R-08) for designing FRP-based strengthening intervention of RC and masonry members. Bonding between FRP laminates (sheets or plates) and concrete emerged as a cutting-edge issue from the first decade of research activities on composite materials for civil structures. In particular, several failure modes due to loss of adhesion between the externally bonded FRP element and the concrete substrate have been observed experimentally and recognized as specific features of this kind of members (Meier, 1995; Bonacci, 1996).

Interfacial behavior between Mechanically Fastened FRP laminates and concrete substrate

This study addresses the interfacial behavior between Mechanically Fastened FRP (MF-FRP) laminates and concrete substrate. For this purpose, a simplified numerical model is formulated with the aim of developing a suitable bearing stress-slip relationship to model the mechanical behavior of the FRP/concrete interface. The proposed relationship is significantly different from another proposal found in the literature as it was calibrated using experimental results from tests on MF-FRP connections realized with multiple fasteners.