Fatigue strengthening of riveted girders in a historic railway metallic bridge in Switzerland using pre-stressed un-bonded CFRP laminates

In this paper, the applicability of the constant life diagram (CLD) methodology for fatigue strengthening of metallic members is described. The CLD methodology is a method that uses the combined effect of alternating stress, mean stress and material properties to predict the material life-time under high-cycle fatigue loading. The method could be used most efficiently to improve resistance to crack initiation in ‘safelife’ members, which is, in fact, a preventive fatigue strengthening approach. The method can be used to increase the fatigue safety margins of un-cracked metallic members. Furthermore, an innovative pre-stressed unbonded reinforcement (PUR) system has been developed and patented. The advantages of the developed PUR system are: applicable to unsmooth surfaces (e.g., riveted or corroded), fast on-site installation (i.e., no glue and no surface preparation), no damage on metallic member (no weld, hole or grinding) and adjustable carbon fiberreinforced polymer (CFRP) pre-stress level. The system can be easily disassembled and removed, if necessary, which makes it suitable for historical bridges as a temporary or permanent retrofit technique. Lastly, this paper presents details about fatigue strengthening of riveted girders in a 120-year-old railway (wrought iron) bridge using the PUR system such that the requirements of the CLD approach are fulfilled. plied to metallic members using glues, which transfer the stresses from the metallic substrate to the CFRP plate through the adhesive layer by interfacial shear stresses (Al-Emrani and Kliger 2006b, AlEmrani and Kliger 2006a, Al-Emrani and Kliger 2009, Benachour et al. 2008, Czaderski and Rabinovitch 2010, Dawood and Rizkalla 2010, Dawood et al. 2007, Ghafoori 2013, Ghafoori and Motavalli 2013a). However, the positive effect of pre-stressing is assumed to be more significant than the positive effect of the bond. Thus, it can be concluded that, by developing a pre-stressed un-bonded CFRP strengthening system, the advantages of the compressive force (introduced by the pre-stressed CFRP plates) are achieved; however, concerns related to CFRP-to-steel bonded joints are not relevant anymore (Ghafoori 2015). Moreover, strengthening using un-bonded systems does not require surface preparation of the metallic substrate, which reduces the overall time of onsite retrofitting. The authors have done a series of studies to compare the flexural performance of metallic beams strengthened by bonded and un-bonded CFRP laminates (Ghafoori and Motavalli 2015b, Ghafoori and Motavalli 2015c). In this paper, a new system of un-bonded CFRP strengthening is presented wherein conventional gluing and bonding of the CFRP to the existing steel members is not required. A method based on the constant life diagram (CLD) is proposed for predicting the minimum CFRP pre-stress level required to prevent fatigue crack initiation in metallic girders (Ghafoori et al. 2015a). The paper begins by describing background related to the CLD method, after which strengthening of a Swiss historic bridge, the Münchenstein bridge, is discussed. Following measurements from the wireless sensor network (WSN) system are presented and conclusions given on strengthening of existing bridges using the new PUR system. 2 FATIGUE RETROFIT THEORY In this section, a proactive design approach based on the CLD approach for fatigue strengthening of metallic members is developed. The proposed design approach is capable of taking into account the combined effect of alternating and mean stress magnitudes. Analytical models are developed to predict the fatigue resistance of retrofitted metallic girders. CLDs can foresee the fatigue life of metals at different mean stress levels. For a given maximum stress (max) and minimum stress (min) in a sample stress history, the stress amplitude (a) and the mean stress (m) can be shown in the CLD diagram. The authors have described the half-plane CLD regions and different fatigue criteria for the design of metallic members in their earlier works (Ghafoori et al. 2015b, Ghafoori et al. 2015a, Ghafoori et al. 2015c). Based on the results of many laboratory experiments, the modified-Goodman and the Smith criteria have been recommended for design of ductile metals, such as mild steel and wrought iron, and brittle metals, such as cast iron, respectively. 2.1 Goodman Criterion Figure 1 shows the modified Goodman criterion, the modified Johnson criterion and the yielding lines. Sy, Sut and Se are the yield stress, ultimate tensile stress and fatigue endurance limit, respectively. Goodman proposed a straight line through sa=Se and sm=Sut, which is a sufficiently conservative approach for the design of ductile metals. The modified Goodman line in the first quadrant (before yielding) is written as: