The installed strain rosettes at the representative structural component of steel bridges provide the structural response as the strain time-history at the sparse locations of the connection. The field collected responses are in a reasonable distance to the weld toe, providing the nominal strain responses. Consequently, the acquired SHM data can be applied for fatigue assessment of the of the objective critical components using the nominal stress method. The structural hot-spot stress response of at the weld toe of the component is provided through a validated FE model, in this study. The numerical hot-spot stresses at the weld toe are applied for fatigue assessment of the gusset-less connection using the hot-spot stress method. The two applied approaches for fatigue assessment of the investigating connection, the nominal stress method and the hot-spot stress method, are explained in this section.
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5.3.1 The nominal stress method
The nominal stresses at the welded structural components are determined in a distance to the weld toe, as shown in Figure 5-1. In the nominal stress method, the nominal stress as well as the appropriate S-N curve, developed for the category of the investigating component, is applied to measure the fatigue remaining life of the component. The bridge design code, AASHTO, has documented a variety of the welded structural components various structural component into multiple fatigue categories, A-E (12). However, for the complex welded components, that are not considered in the existing fatigue design codes, application of the S-N curves relies on engineering judgments and the assumptions.
The novel gusset-less connection is not cataloged in the standard fatigue design codes, including AASHTO. Therefore, based on the designerโs assumption and the existing studies for the fillet welds, the category C is employed for fatigue assessment of the connection. The determined properties of Category C are applied to measure the fatigue damage index, using the Minerโs rule that is expressed as Eq. (5-1). The required stress/cycles are provided through post- processing the field collected data, using the rainflow cycle algorithm (13).
1๐2
๐2 Eq. (5.1)
5.3.2 The hot-spot stress method
The hot-spot stress method does consider the local stress concentration due to the notch effect at the weld toe, while excluding the non-linear peak stress, as shown in Figure 5-1. The hot- spot stress can be determined by extrapolating the stress responses at the reference points (Figure 5-1). The distance of the reference points to the weld toe depends on the type of the weld and size of the mesh in numerical models. For the investigating fillet weld toe, the reference points at the
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web of the connection, are located at the 0.4t and 1.0t (t is the thickness of the web) in a perpendicular distance to the weld toe, respectively, expressed in Eq. (5.2). The stress responses at the reference points can be achieved using the numerical model and the fine mesh sizes. In the experimental efforts, the hotspot stresses are achieved by placement of the data acquisition system at the reference points (13).
๐9: = 1.67๐D.<Eโ 0.67๐5.DE Eq. (5.2) The ratio of the hot-spot stress range at the weld toe to the nominal stress is defined as the stress concentration factor (SCF) expressed in Eq. (5.3). The SCF, that is frequently determined using the numerical models, can be multiplied to the nominal stresses to achieve the hot-spot stress without the requirement to the reference points. The SCF is frequently applied for fatigue assessment of the structural components using the field collected nominal strain responses. Even if the variable amplitude traffic loads may result in multiple SCFs, a single SCF ratio is applied to the collected responses (14).
๐๐ถ๐น =โ๐9: โ๐W
Eq. (5.3)
The hot-spot stress method applies less S-N curves as compared to the nominal stress method. In IIW (international institute of welding), the fatigue classes (FAT class) and the associated S-N curves are expressed based on the type of the weld as well as the weld geometry (16). For the fillet welds, it is recommended to apply FAT 90 for the load carrying fillet welds and FAT 100 for the load carrying fillet welds (17). In this study, regarding the performance of the weld at the gusset-less connection, FAT 100 is applied for fatigue assessment at the curved fillet welds.
96 Figure 5-1 Hot-spot stress extrapolation at the weld toe (2)
In developing the appropriate numerical model for the hot-spot stress, an extensive study is performed by the researches to specify the requirements of an efficient FE model. The appropriate FE model for the hot-spot stress method requires a careful attention in providing the mesh insensitive models. In the previous studies, it is recommended to apply the fine mesh sizes (maximum of 0.4t) and higher dimensional elements such as the three-dimensional solid elements, as shown in Figure 5-2. Also, the incorporation of the weld geometry in the FE model, is illustrated to have a significant impact in predicting the precise hotspot stresses at the weld toe (110). The weld geometry can be either modeled using the thick shell elements or solid elements, as shown in Figure 5-2. In this study, due to the considerable size of the weld and the geometry of the curved fillet weld, the 20-noded solid elements are applied to model the gusset-less connection and the fillet welds.
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Figure 5-2 FE modeling of the welded component (a)shell element, (b) solid element with weld geometry (18)