A new design model for adhesive joints used to bond FRP laminates to steel beams

The strengthening and repair of existing structures using bonded carbon fiber reinforced polymer, CFRP, laminates has attracted a great deal of attention in the past two decades. Investigations clearly indicate the great potential of this method for restoring the capacity of corroded steel beams and improving their fatigue life. One important issue regarding the use of this technique in strengthening steel structures is the design of adhesive joints used to bond FRP laminates to steel substrates. Very limited research work has been conducted in this area and, at the present time, there is a lack of suitable design models for FRP-strengthened steel members. This paper is mainly concerned with a proposal for and verification of a new design model for adhesive joints used to bond FRP laminates to steel beams for strengthening and repair purposes. Quasi-static tests were performed on steel plate and full-scale beam specimens bonded with CFRP laminates to evaluate the new design model proposed in this study. The failure, in all specimens, took place at the steel-adhesive interface. The new design model presented in this paper was found to be accurate in terms of predicting the ultimate load and failure mode of the joints

Fatigue design of plated structures using structural hot spot stress approach

In most fatigue design codes, the nominal stress method is the predominant approach for fatigue design of structures. However, the known limitations of this method along with new advanced computational possibilities, have paved the way to search for more accurate stress based fatigue design approaches. The structural hot spot stress approach (SHSS) is one of these methods which has drawn a wide-spread attention since its advent. The SHSS designates the basic stress by taking into account the geometrical variations of the detail at the location of expected fatigue crack initiation (hot spot). In this paper, the fatigue strength of several frequently used structural details is investigated using both nominal stress and SHSS approaches. The aim of this investigation is to establish an equivalency between these two approaches with reference to the fatigue strengths of the studied details. A large database including available fatigue test results (from 1950s till present) is built up and used to produce hot spot stress S-N curves for the studied details

Fatigue assessment of metallic structures under variable amplitude loading

Many types of mechanical structures (cars, air-planes, trains) and civil engineering structures (bridges, wind turbines, offshore structures) are subjected to random in-service loading. Normally the load effects in these structures are composed of different stress ranges and mean stresses which are within the elastic limit of the material. One main problem with fatigue assessment under variable amplitude loading is the performes of fatigue life regarding load sequence effects. In this study a model deals with the problem of fatigue life assessment under variable amplitude loading composed of differing stress ranges and mean stresses which are within the elastic limit of the material take into-account the load sequence effects is developed. Computing of the stress range and the mean stress, in specific time based on the preceding load histories, consider load sequence effects. Using Palmgren-Miner damage rule and the new computed stress range and mean stress lead to predict the fatigue life. An evaluation of the model using data of five diffrent metals availbe in the literature has been investigated. The results showed that the proposed model describes the effect of the load sequence in elastic loading well

Preliminary Study on Plate Girders with Corrugated Webs

Corrugated web beams have been studied extensively in this report. Previous design models have been collected from the literature and compared to Eurocode design models. Different design considerations were recommended based on the findings

High-cycle variable amplitude fatigue experiments and design framework for bridge welds with high-frequency mechanical impact treatment

Fatigue enhancement by way of high-frequency mechanical impact (HFMI) treatment can enable effective design and construction of steel bridges. However, bridges may experience high and varying mean stresses, the effects of which are not covered today by any design recommendation or in the literature on HFMI-treated joints. In this study, fatigue experiments were conducted with realistic in-service bridge loading, which revealed the same high fatigue performance as for constant amplitude loading. The effect of mean stress in spectrum loading was quantified and a method to account for it in an equivalent manner is proposed. A design framework has been developed for design and engineering purposes

Mean Stress Effect in High-Frequency Mechanical Impact (HFMI)-Treated Steel Road Bridges

High-frequency mechanical impact (HFMI) is a post-weld treatment method which substantially enhances the fatigue strength of steel weldments. As such, the method enables a more efficient design of bridges, where fatigue is often the governing limit state. Road bridges are typically trafficked by a large variety of lorries which generate load cycles with varying mean stresses and stress ranges. Unlike conventional welded details, the fatigue strength of HFMI-treated welds is known to be dependent on mean stress in addition to the stress range. The possibility of considering the mean stress effect via Eurocode’s fatigue load models (FLM3 and FLM4) was investigated in this paper. Moreover, a design method to take the mean stress effect into account was proposed by the authors in previous work. However, the proposed design method was calibrated using limited traffic measurements in Sweden, and as such, may not be representative of the Swedish or European traffic. In this paper, larger data pools consisting of more than 873,000 and 446,000 lorries from Sweden and the Netherlands, respectively, were used to examine the validity of the previous calibration in both countries. The comparison revealed no significant difference between the data pools with regards to the mean stress effect. Additionally, the previous calibration provided the most conservative mean stress effect and was considered adequately representative for both countries. The proposed design method was further validated using four composite case study bridges. It was also found that the mean stress effect was mainly influenced by the self-weight, while variation in the mean stress due to traffic had a minor influence on the total mean stress effect. Furthermore, it was found that the mean stress effect could not be accurately or conservatively predicted using FLM3 or FLM4

A comparative study of different fatigue failure assessments of welded bridge details

Five different welded joints frequently used in steel bridges have been selected to investigate the accuracy and applicability of three fatigue assessment methods. The first method, also categorised as the global method, is the nominal stress method, while the more advanced methods are the hot spot and the effective notch stress methods. Solid element based finite element models for welded bridge details were created by following the modelling requirements of each fatigue assessment method. A statistical evaluation based on the results of the finite element analyses and the fatigue test data collected from the literature was performed to determine the mean and characteristic fatigue strength. In addition, the standard deviation for each data series was also determined to conclude how well each method describes the fatigue strength of each welded detail. A method with a lower standard deviation is regarded as more accurate. Moreover, the evaluated results from each method were compared with the recommended fatigue strength values in the Eurocode 3 (EN 1993-1-9:2005) and IIW codes. In the light of the test results in this study, it appears that the codes are in reasonable agreement with the test data, even though a few examples of the opposite occurred. The conclusion based on the revised results in this article indicates that the nominal stress method yields satisfactory results, despite its simplicity. When considering the effort involved in creating FE models for numerical analysis, it seems clear that the choice of the nominal method is fairly acceptable

Evaluation of HFMI as a Life Extension Technique for Welded Bridge Details

Published by Elsevier B.V. In this current study, HFMI technique is used to study the possibility to extend the fatigue life of pre-fatigued flange gusset welds typically found in girder bridges. The results from the study are also compared with results found in the literature for other more conventional techniques for retrofitting, e.g. cut-outs. The study also aims to investigate if the IIW HFMI recommendations could be applied for existing steel structures and that equal fatigue strength improvement could be claimed for prefatigued structures. Furthermore, new recommendations for structural hot spot stress type B are suggested for HFMI treated welds, applicable to flange guest welds. The results indicate that the HFMI could be used for welded bridge details rehabilitation as a competing technology with conventional cut-out. Furthermore, the results indicate that the IIW recommendations for HFMI fatigue strength improvement could also be applied for pre-fatigued welded details. \ua9 2019 The Authors

Assessment of in-service stresses in steel bridges for high-frequency mechanical impact applications

The application of high-frequency mechanical impact (HFMI) treatment to improve the fatigue performance of composite steel and concrete road bridges was studied through a state-of-the-art review in conjunction with simulations of variable amplitude in-service stresses in four case-study bridges in Sweden. Empirical stress range spectra with associated mean stresses were characterised for HFMI-treated bridges. It was shown that the fatigue-critical locations in HFMI-treated bridges remain unchanged compared with conventional bridges and that compressive overloads pose no detrimental effect that requires additional attention in the fatigue assessment. Calculations also showed a considerably better fatigue performance if HFMI treatment is performed on-site, after the application of self-weight stresses

Mean stress effect in high-frequency mechanical impact (HFMI)-treated welded steel railway bridges

The need for new railway bridges is driven by the growing volume of transportation demands for both passenger and freight traffic on railway networks. In the design of these bridges, the fatigue limit state is a criterion that usually limits the allowable applied load level and thus also the utilization of the high strength of the steel material. Therefore, improving the fatigue performance of welded details by high-frequency mechanical impact (HFMI) treatment leads to a more efficient design. However, the fatigue performance of HFMI-treated welds is known to be affected by the mean stress and this needs to be considered in the design of treated welded details in steel bridges. This is rather straightforward if the bridge is subjected to cycles from one type of train but becomes cumbersome when several different sets of trains (e. g. axle loads, axle distances) cross the bridge. In this article, a factor to take the mean stress effect (including self-weight and traffic load variations) into account is derived from traffic data measured in Sweden. Moreover, the mean stress effect is also predicted using the different fatigue load models in the Eurocode. These models either consist of one-load patterns such as LM71, SW/0, and SW/2 or are composed of different trains with different combinations. It was found that the mean stress effect is underestimated by the first group of models. On the other hand, the mean stress predicted by the light traffic mix is found to be close to that calculated using real traffic data, while other mixes (standard and heavy) underestimate the mean stress effect. Therefore, a correction factor to account for the mean stress effects in real traffic is derived (called here λHFMI). This factor can be used to correct the design stress range for fatigue verification of HFMI-treated welded details in railway bridges