Multiaxial fatigue behavior and strengthening of riveted double-angle connections in steel bridges using prestressed CFRP rods

A large number of aging road and railway riveted metallic bridges suffer from fatigue problems. Stringer-to-floor-beam double-angle bridge connections are among the most fatigue-prone details due to the many cyclic distortion-induced stresses from axle loads. Increasing service loads and corrosion put these components at a higher risk of fatigue cracking. As the replacement of existing structures is costly, lengthy, and thus in some cases almost impossible, the development of retrofitting techniques is of great importance. However, fatigue strengthening of angle connections in bridges is a challenging task. The geometric complexities of these details hinder the development of applicable retrofitting techniques, and the multiaxial stress state in connections complicates fatigue analysis.The first objective of this work is to develop an applicable retrofitting system for strengthening the stringer-to-floor-beam double-angle connections in bridges, considering the fact that the traditional retrofitting techniques such as stop holes, welding additional elements or the softening of connections are not capable of effectively tackling the problem. In recent decades, the application of carbon fiber-reinforced polymer (CFRP) composites has attracted much interest for the fatigue strengthening of metallic structures owing to their superior properties such as high strength, light weight, high corrosion and fatigue resistance. The strengthening system developed in this study consists of a new adhesive-free mechanical wedgeâbarrel anchor for prestressed CFRP rods and a friction-based clamping system. Extensive finite element simulations are performed to optimize the design of the strengthening system. In addition, the static and fatigue performance of the developed system is experimentally investigated in laboratory tests. The ultimate tensile strength of the strengthening system is found to be greater than the nominal strength of the CFRP rods, which ensures achieving the full capacity of the CFRP rod strength. In addition, the fatigue tests show neither failure of the CFRP rods nor prestressing loss due to slippage in the anchorage components.The second objective is to determine the high-cycle fatigue thresholds for the prediction of crack initiation in angle connections subjected to multiaxial stresses. The critical plane approach is employed as an advanced approach for multiaxial high-cycle fatigue analysis. In addition, the effect of prestressed strengthening is incorporated into multiaxial fatigue models. Using identified proper critical plane-based multiaxial fatigue thresholds, a design approach is proposed for the fatigue strengthening of angle connections subjected to distortion-induced multiaxial stresses. Finally, the third objective is to apply the developed retrofitting system to strengthen the connections of a 92-year-old riveted railway bridge in Switzerland as a demonstration of a field application. The results of the short-term measurements demonstrate the effectiveness of the retrofitting system in improving the high-cycle fatigue behavior of the bridge connections through the reduction of the mean values of the stress components. The post-installation long-term measurements show no prestressing loss in the CFRP rods, indicating that no slippage occurs in the strengthening system components.

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

Mario Fontana, Elyas Ghafoori, Andrin Herwig, 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 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.

2013

CFRP Strengthening and Long-Term Monitoring of an Old Metallic Roadway Bridge in Melbourne

Elyas Ghafoori, Ardalan Hosseini, Masoud Motavalli

This study gives an overview on carbon fibre-reinforced polymer (CFRP) strengthening and wireless sensor network (WSN) monitoring of a 121-year-old metallic roadway bridge in Melbourne, Australia. A flat prestressed unbonded retrofit (FPUR) system was developed to apply prestressed CFRP plates to the steel cross-girders of Diamond-Creek Bridge. The bridge is subjected to daily passenger and heavy truck vehicles. Sets of laboratory tests were performed to examine the efficiency and fatigue performance of the proposed FPUR system, prior to its installation on the bridge. Furthermore, in order to demonstrate the efficiency of the proposed retrofit technique, the bridge was instrumented with different types of sensors (including strain gauges, temperature and humidity sensors), and short- and long-term measurements were performed. As for short-term measurements, the bridge was loaded by a 42.5-tonne semi-trailer before and after strengthening. For the long-term monitoring, a WSN system was used to monitor the prestress level in the CFRP reinforcements for at least one year. The CFRP plates were prestressed up to about 980 MPa (38% of the CFRP ultimate strength), which resulted in about 50% reduction in the maximum tensile stress in the bottom flanges of the strengthened I-girders. The results of the short- and long-term measurements in this study showed that the proposed FPUR system can be very effective for flexural and fatigue strengthening of such bridge girders.

IABSE2019

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

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

Ardalan Hosseini

EPFL2019