Issue 41

S.M.J. Razavi et alii, Frattura ed Integrità Strutturale, 41 (2017) 432-439; DOI: 10.3221/IGF-ESIS.41.54 433 correlation of loss in the fatigue properties due to the coating thickness [19,20]. By employing the Kitagawa–Takahashi diagram, Vogt et al. [21] specified a threshold value of the coating thickness that could not affect the fatigue behaviour of unnotched components made of structural steels. Valtinat and Huhn [22] published a preliminary study on hot dip galvanized bolted connections. Due to the available gap between the recent and past literature and also limited available data on this topic, the present technical note is aimed to partially fill this gap by presenting a clear explanation for the preparation and final assembly methods of the specimens. In the current paper, the geometry and the procedure for fabrication and assembly of the specimens is described in details. Additionally, the fatigue data are summarized and compared with those taken from the standard in force for the same detail category followed by a comparison between the present data and some recent data by the same authors Berto et al. [23] from notched galvanized specimens weakened by a central hole. M ATERIAL AND GEOMETRY OF THE SPECIMENS he test specimens, made of S355 structural steel, for the bolted connection are shown in Fig. 1. Preloaded M12 bolts of class 10.9, system HR, were used in drilled holes. Hot dip galvanized coatings of fasteners according to UNI EN ISO 10684. The dimensions of the test samples were designed primarily to produce a net section fatigue failure of the middle main plate, and not in the bolts or cover plates (EN 1993-1-8). All the samples were hot-dip galvanized for an immersion time of 14 minutes which is typical in the application. The result was a zinc layer of about 400 µm. This layer is commonly employed in practise in large structures. Subsequently, the joint surfaces were treated according to a light sandblasting process (sweep blasting). In the absence of precise information by the national and European regulations it is proposed a specific procedure which relates to indications in the literature (ISO 8503). In fact, the hot-dip galvanized surface of structural steels requires special handling. The aim was to remove the outer layer consisting of pure zinc notoriously soft and malleable thereby making the surface rough. Moreover, this type of blasting does not severely damage the existing coating obtained by hot dip galvanizing. A specified torque was applied to the nut in two steps using a calibrated wrench capable of an accuracy of ± 4 % according to ISO 6789. Most of these connections are used in steel structures frequently submitted to cyclic loading such as splice joints used in steel and composite bridges. Figure 1: Geometry of the test specimen employed in the research program. D ETAILED PROCEDURE FOR SPECIMEN FABRICATION fter the hot dip galvanization process of steel plates, an adequate surface preparation was performed. The features of the sweep blasting procedure utilized are shown in Tab. 1. All above mentioned data were certified by the sand blaster firm inspection procedure. The results were a roughness equal to 32 µm and a reduction of the zinc layer measured in 12 µm (max. value). The surface preparation grade corresponded to SA1 (light blast cleaning). Fig. 2 T A