Issue 43

E. Maiorana et alii, Frattura ed Integrità Strutturale, 43 (2018) 205-217; DOI: 10.3221/IGF-ESIS.43.16 215 D ISCUSSION n recent experiments of Cruz et al. [10] and in experiments conducted by the authors following the EN standard, the values of the coefficient of friction peaks have been obtained with samples blasted, with Sa2½, brushed, closed and tested. In the case of the use of weathering steel where the sandblasted surface was left unprotected prior to closure, the friction coefficient increased. On the contrary, in the case of carbon steel, to ensure a high friction coefficient of the surface covered by the bolted joint package and simultaneously having a guaranteed corrosion protection before the tightening torque, the alternatives are two. The first is to blast the surfaces and protect them until the closure, possibly treating the surfaces themselves by brushing before applying tightening torque; the second is to use a paint with effective corrosion resistance and adequate roughness after coating. Commercially, products for the protection of surfaces joined by bolted joint packets working with friction mechanism are available. Some products marketed in Italy were tested according to the directions of the previous legal framework, CNR UNI 10011 [20], which was based on earlier standards applicable to the manufacture of bolts and other products were classified according to other standards such as RCSC [3]. Given the current regulatory scenario of reference in Europe and in Italy, DM 14.01.08 [22], which includes the verification procedures according to EN 1993-1-8 [1] and other related European standards, it was necessary to carry out the experimental tests to obtain the friction coefficients in the manner described in EN 1090-2 [2]. Such redevelopment shall take into account the congruence of the results for the friction conforms to the values that can currently be achieved by preloading and tightening torque bolts manufactured and supplied in accordance with applicable European standards. An important observation should be made regarding the values of k min and k max given by the manufacturer that controlled the production by lot, while by [5], as already mentioned, for K 1 , values of k m should be inside the range 0.10  k i  0.16, thus the value for k m has a relevant oscillation. In the tests performed on the specimens painted with product n.1, the tightening torque value was 520 Nm, that is k m = 0.1505. In the tests performed on the specimens painted with product n.2, the tightening torque value was 520 Nm for the first three specimens and 545 Nm for the fourth, that is k m = 0.16. An increase in the  i value was observed with the percent of zinc in the coating component. Alternatively, using grease between the screw and the nut, to consider a lower k i , is suggested, rather the k max suggested by the manufacturer, the application of a torque of 1.1 M s . Fig. 13 shows synthetically all the results found of  in terms of comparison of: factor k m , surface treatment, paints, standards applied (EN [2], RCSC [3] and CNR [20]). In terms of preload force, the European code permits raising by 25% CNR [20] and 10% RCSC [3]. On the other hand, considering the test for the determination of slip factor, contrary to CNR [20] and RCSC [3], which assumes a value  m from four tests, EN [2] adopts the characteristic value  k =  m – 2.05 s  taking into account the standard deviation within the tests, and in conclusion the mean value  m is reduced by about 10%. Figure 13 : Comparison of all results  . 1-blasted surfaces EN [2]; 2-Rusted EN [2]; 3-paint n.1 CNR [20]; 4-paint n.2 RCSC [3]; 5- paint n.3 RCSC [3]; 6-paint n.4 EN [2]; 7-paint n.5 CNR [3]; 8-paint n.6 EN [2]; 9-paint n.7 RCSC [3]; 10-paint n.8 RCSC [3]; 11-paint n.9 EN [2]; 12-Half coated EN [2] Fig. 14 shows test results showing a comparison between RCSC [3] and EN [2] in terms of the ratio of F Si vs.  . For both American and European standards  increases F Si , but with RCSC [3] a greater value of  than that of EN [2] is observed.