Finite element modeling for fatigue strengthening of metallic riveted bridges using un-bonded pre-stressed CFRP plates

Many old riveted steel bridges remain operational and require retrofit to accommodate ever increasing demands. Complicating retrofit efforts, riveted steel bridges are often considered historical structures and as such structural modifications that affect the original construction are to be avoided. The presence of rivets along with preservation requirements often prevent the use of traditional retrofit methods, such as bonding of fiber reinforced composites, or the addition of supplementary steel elements. In this paper, an un-bonded post-tensioning retrofit method is numerically investigated using an existing riveted bridge geometry from Münchenstein, Switzerland. Pre-stressed un-bonded carbon fiber reinforced plastic (CFRP) plates will be considered for the strengthening elements. By using prestressed CFRP plates, a portion of the permanent loads (such as the dead-weight) will be transferred to the strengthening element. Fatigue critical regions of the bridge are identified, and the effects of the un-bonded post-tensioning method on strength and fatigue susceptibility are explored.

Finite Element Analysis for Fatigue Damage Reduction in Metallic Riveted Bridges Using Pre-Stressed CFRP Plates

Mario Fontana, Elyas Ghafoori, Andrin Herwig, Emmanuel Mayor, Masoud Motavalli, Alain Nussbaumer, Gary Scott Prinz

Many old riveted steel bridges remain operational and require retrofit to accommodate ever increasing demands. Complicating retrofit efforts, riveted steel bridges are often considered historical structures where structural modifications that affect the original construction are to be avoided. The presence of rivets along with preservation requirements often prevent the use of traditional retrofit methods, such as bonding of fiber reinforced composites, or the addition of supplementary steel elements. In this paper, an un-bonded post-tensioning retrofit method is numerically investigated using an existing railway riveted bridge geometry in Switzerland. The finite element (FE) model consists of a global dynamic model for the whole bridge and a more refined sub-model for a riveted joint. The FE model results include dynamic effects from axle loads and are compared with field measurements. Pre-stressed un-bonded carbon fiber reinforced plastic (CFRP) plates will be considered for the strengthening elements. Fatigue critical regions of the bridge are identified, and the effects of the un-bonded post-tensioning method with different prestress levels on fatigue susceptibility are explored. With an applied 40% CFRP pre-stress, fatigue damage reductions of more than 87% and 85% are achieved at the longitudinal-to-cross beam connections and cross-beam bottom flanges respectively.

Mdpi Ag2014

Strengthening of metallic members under mixed mode fatigue loading using prestressed CFRP plates

Ardalan Hosseini

The strengthening of existing steel structures against high-cycle fatigue (HCF) loading has become one of the most important topics of interest in structural rehabilitation. Apart from the conventional strengthening techniques that incorporate bulky and heavy steel reinforcements, the application of carbon fiber reinforced polymer (CFRP) composites has attracted much interest for the strengthening of fatigue-prone (or fatigue-damaged) steel members. Over the last two decades, considerable research studies have been conducted to further understand the static and/or fatigue behavior of CFRP-strengthened steel members. However, existing knowledge on the issue is more focused on the strengthening of small-scale steel members using nonprestressed CFRP patches subjected to the simple mode I (tensile mode) loading condition. Nevertheless, it is quite evident that a combination of modes I and II (shear mode), hereafter referred to as the mixed mode I/II loading condition, often acts on structural members; such a complex loading is responsible for the failure of fatigue-critical details. Although some limited research studies have been conducted on the mixed mode I/II fatigue behavior of bare mild steel, which is relevant for civil structural applications, the existing literature does not provide any solution for the strengthening of fatigue-cracked members subjected to mixed mode I/II loading condition. Therefore, the main objective of the current PhD work is to propose a strengthening design approach to arrest an existing mixed mode I/II fatigue crack in a structural steel member by using prestressed CFRP reinforcements. To achieve this objective, first extensive analytical and experimental studies are conducted to develop a viable prestressing system by comparing the performance of the prestressed bonded reinforcements (PBRs) with that of a novel prestressed unbonded retrofit (PUR) system. Second, a strengthening design approach is proposed for the mixed mode I/II fatigue crack arrest in existing structural steel members using the developed PUR system. Through the analytical formulation of mode I and II stress intensity factor ranges, a design model is proposed to determine the strengthening solution, including the required prestressing level and/or the cross-sectional area of the reinforcement, which would ensure the complete arrest of an existing mixed mode I/II fatigue crack in a steel member. The proposed model is validated by performing sets of stepwise HCF tests on reference unstrengthened and prestressed CFRP-strengthened precracked steel plates of grade S355J2+N under various mode mixities. From a practical point of view, it deemed necessary to develop a reliable system for the prestressed strengthening of real-scale metallic members. Therefore, an alternative configuration of the PUR system, named the flat prestressed unbonded retrofit (FPUR) system is developed for the strengthening of real-scale metallic I-girders using multiple prestressed CFRP plates. An analytical model is proposed to accurately determine the stress levels in a metallic I-girder strengthened with the developed FPUR system, and sets of real-scale pull-off tests, as well as static and fatigue four-point bending tests are conducted to confirm the excellent performance of the system. The practical application of the FPUR system is demonstrated for the strengthening of a 122-year-old roadway metallic bridge in Australia.

EPFL2019

Connections by Adhesion, Interlocking and Friction for Steel-Concrete Composite Bridges under Static and Cyclic Loading

Dimitrios Papastergiou

Steel-concrete composite bridges with twin-I steel girders and prefabricated slabs constitute an economical and competitive solution for small and medium bridge spans. In recent years they have started to gain their share in the construction market. However, the current slab-to-girder connection with groups of headed shear stud connectors is not well adapted for prefabrication and fast erection, and does not respond successfully to the durability of the construction. New types of connections by adhesion, interlocking and friction constitute a conception that aims to answer these demands. The resistance of these connections is based on the development of longitudinal shear stresses in the interfaces which form the connection. The goals of this study are to complete the existing research in this field by proposing scientific tools in order to predict the connection's structural performance, i.e. prediction of the connection's ultimate resistance and deformation capacity, including the post-failure behaviour. In addition, the behaviour of the connection under cyclic loading is also investigated to assess the connection's resistance to fatigue and to propose a design method for composite bridges under fatigue loading. To fulfill theses goals a combinational methodology was applied which included: study of the state of the art: a) in composite bridges so as to define the requirements and b) in interface behaviour in order to emphasize the parameters which are responsible for the resistance and obtain information for the nature of cyclic loading damage mechanisms, experimental investigations: a) to reveal the laws describing the interfaces and the connection's behaviour for static and cyclic loading, and provide data for development of analytical expressions and model validation and b) demonstrate the capacity of a composite beam under cyclic loading and the extent of its remaining resistance at ultimate limit state, analytical studies: a) to produce the laws describing the interface behaviour under static and cyclic loading and b) to evaluate the impact of the cyclic loading to the slip in the interfaces and in the connection and thus propose a damage factor for cyclic loading, finite element analysis in order to enable an analytical description of the confinement effect, caused by the reinforced concrete slab, which contributes to the resistance, the development of a numerical model to predict the connection's structural performance and to propose a design method for composite bridges fabricated with the new connection. Application of the mentioned methodology resulted in several useful conclusions: The interface behaviour can be described by analytical laws. The post failure behaviour of the interfaces was explained by accounting for the kinematics of the failed surface and basic notions of fracture mechanics. The damage due to cyclic loading on the interfaces and on the connection is expressed by the accumulation of a residual slip. Provided that the shear stresses remain lower than an elastic limit and the accumulated slip does not exceed the failure slip for static loading, fatigue failure is avoided. The confinement effect was incorporated, analytically, together with the laws of the interface behaviour in a numerical model to predict the connection's structural performance and allow a parametrical study. A design method for the verification of composite beams under static and cyclic loading was developed.