Issue 43

E. Maiorana et alii, Frattura ed Integrità Strutturale, 43 (2018) 205-217; DOI: 10.3221/IGF-ESIS.43.16 207 The equation that gives the relationship between tightening torque and preload force is , (1 1.65 ) r k m p C M v k F d   (3) Approximated with: , 1.10 r m p C M k F d  (4) S LIP FACTOR he decisive parameter for the operation of the friction mechanism in the bolted joint is the slip factor μ which depends on the roughness of the plate, which is associated with the surface treatment of plates closed by the bolted joints. However, the surfaces of the steel components should be protected, as all the other surfaces, to avoid the development of corrosion phenomena between the manufacturing and the erection phase, but also to guarantee the greatest possible friction. In general, the surfaces are cleaned, blasted, followed by the application of inorganic zinc. The grade of sandblasting is usually Sa2½ as described in international standard ISO 8501-1 [6]. In practical applications, the slip factor for short-time loads may be necessary to sustain dynamic loads. For example, Fig. 1 shows a steel bridge girder where the bolted joints surfaces are specifically prepared for friction connections. Figure 1 : Painted beam with inorganic zinc coated surfaces for friction joints. The slip factor tends to decrease with time due to the creep phenomena in coated surfaces. Several studies have been developed to establish adequate slip factors for different conditions; these studies are in general very time consuming due to the wide range of parameters involved. In this context, reference should be made, for example, to the studies reported in the publication n.37 of ECCS [7]. Also, the results of an extensive research work are collected in Kulak et al. [8]. Tab. 2 shows the slip factor value assumed with different surface treatment as in EN 1090-2 [2] while, for an useful comparison, Tab. 3 shows the prescription in prEN 1090-2 ( draft new version of EN 1090-2 ). In other international standards, different systems of friction classes are specified; for instance, in “Specification for structural joints using high-strength bolts” RCSC [3] used in North America, three surface classes are established (Tab. 4). A comparison among European, American, Australian, Japanese, Italian and British Standards for design of bolted joints in steel bridges is reported in Maiorana and Pellegrino [9]. T