Development of Mechanical Strengthening System for Bridge Connections Using Prestressed CFRP Rods

Stringer-to-floor beam web-to-web double-angle connections are among the most fatigue-prone elements in old riveted bridges. These connections are often designed to carry only shear loads. However, in these elements, fatigue damage occurs because of the out-of-plane deformation of the connections, which is ignored in the original design. In this study, a new retrofitting system is developed to reduce the out-of-plane deformation of the connections using prestressed carbon fiber-reinforced polymer (CFRP) rods. The proposed system consists of a mechanical wedge-barrel anchor to hold the prestressed CFRP rod and a clamping system to attach to the parent structure and to transmit forces via friction. A series of finite-element (FE) simulations was conducted to optimize the size and performance of the retrofit system. Laboratory static pull-off tests were conducted and different failure modes were studied and discussed. A novel test setup (with four supports) was designed for testing the steel connections. The effect of the geometrical imperfections during the installation of the connection was carefully investigated using the FE models and was verified through laboratory the tests. Laboratory fatigue tests were conducted on steel connections with the same dimensions as those in a railway bridge. The designed retrofit system was found to be capable of reducing the stresses at the angle connections by more than 40%. The results of the fatigue tests demonstrated that the designed system could survive more than 11 million load cycles without any fatigue damage or any indication of a loss in the CFRP prestressing level. (C) 2020 American Society of Civil Engineers.

Development of prestressed unbonded and bonded CFRP strengthening solutions for tensile metallic members

Elyas Ghafoori, Ardalan Hosseini, Masoud Motavalli, Alain Nussbaumer

In this study, a novel unbonded mechanical clamping system was developed for the strengthening of tensile metallic members using prestressed carbon fiber reinforced polymer (CFRP) plates. The system clamps a pair of prestressed CFRP reinforcement to a metallic substrate and provides an almost uniform contact pressure over the CFRP plate along the anchorage length. A finite element simulation was used to optimize the design of the mechanical components of the system. Subsequently, a set of static and fatigue tests was performed to evaluate the performance of the optimized design. Experimental results revealed that the proposed mechanical clamping system is capable of transferring the entire tensile capacity of the CFRP plates to the steel substrate, even after experiencing 10 million fatigue cycles. The comparative performance of the developed clamps was further investigated by a set of static tests on steel plate specimens strengthened with the prestressed bonded reinforcement (PBR) and the newly developed prestressed unbonded reinforcement (PUR) systems. Furthermore, simple analytical models are proposed to formulate the stress state in prestressed unbonded and bonded CFRP-strengthened tensile metallic members. The accuracy of the proposed analytical formulations was verified by the experimental results obtained during the current study. Experimental results revealed that the efficacy of having relatively high prestressing forces in the normal modulus (NM) CFRP reinforcements is much higher than the stiffness improvement obtained by using ultra-high modulus (UHM) CFRPs. However, the available capacity of the PBR system before debonding failure is far lower than that of the developed PUR solution.

2019

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

Strengthening of Steel Connections in a 92-Year-Old Railway Bridge Using Prestressed CFRP Rods: Multiaxial Fatigue Design Criterion

Elyas Ghafoori, Hossein Heydarinouri, Masoud Motavalli, Alain Nussbaumer

This paper presented a new retrofit system for strengthening the stringer-to-floor-beam double-angle connections in a 92-year-old riveted railway bridge in Switzerland, using prestressed carbon fiber-reinforced polymer (CFRP) rods. The strengthening system transmitted the forces purely through friction, with minimum interference with bridge traffic. The system consisted of two components: a newly developed mechanical wedge-barrel anchor to hold the prestressed CFRP rod and a clamping system attaching the stringer flange. The strengthening system reduced the out-of-plane deformation of the connections, resulting in a reduction in the distortion-induced stresses in the connections. The short-term bridge measurements showed that the application of a total prestressing force of 100 kN reduced the dominant mean stress in the connection hotspot by 47% (from 22.9 to 10.9 MPa) under the passage of passenger trains, whereas the stress range remained unchanged. Using the modified Wohler curve method (MWCM) as a critical plane-based multiaxial fatigue model, the strengthening system was observed to reduce the multiaxial mean stress parameter by 30% under the passenger train loads. The long-term monitoring of the strengthening system, using a wireless sensor network (WSN) system, indicated no prestress loss in the CFRP rods during the seven-month period since its installation.

ASCE-AMER SOC CIVIL ENGINEERS2021