Issue 43

E. Maiorana et alii, Frattura ed Integrità Strutturale, 43 (2018) 205-217; DOI: 10.3221/IGF-ESIS.43.16 206 torque, may be used as preloaded bolts in friction joints. In EN 1090-2 [2] requirements for execution of steel structures (included structural bolting assemblies for preloading), are specified in order to ensure adequate levels of mechanical resistance and stability, serviceability and durability. In particular, it summarizes the steel structures that are designed according to all parts of European standards. North American RCSC “Specification for structural joints using high-strength bolts” [3] deals principally with the strength grades of HS bolts, ASTM A 325 e ASTM A490 providing guidance for their design, installation and inspection in structural steel joints. ASTM F3125 [4], which replaces the six previous standards, simplifying bolt specification, covers chemical, physical and mechanical requirements for quenched and tempered bolts manufactured from steel and alloy steel, in inch and metric dimensions, in two strength grades. Tab. 1 shows in a benchmarking nominal values of the yield strength f yb and of the ultimate tensile strength f ub for European and American equivalent grades: i.e., respectively, 8.8 and 10.9, A325 and A490. EN 1993-1-8 ASTM F3125 Bolt grade 8.8 10.9 A325 A490 f ub (MPa) 800 1000 830 1040 f yb (MPa) 640 900 660 940 Table 1: Minimum values for yield and ultimate tensile strength of HS bolt material according to European and North American Standards. In a slip-critical joint, the resistance is due to friction forces developed between the faying surfaces depending on the preloaded force of the tightened bolts as well as on surface treatment. Both American and European Standards require, prior to bolt preloading, the snug-tightening procedure to bring the plies into firm contact and provide four pretensioning methods, without preference: - Turn-of-Nut Pretensioning; - Calibrated Wrench Pretensioning; - Twist-Off-Type Tension-Control Bolt Pretensioning; - Direct-Tension-Indicator Pretensioning. According to RCSC [3] the minimum Bolt Pretension for Slip-Critical Joints is equal to 70 percent of the specified minimum tensile strength of bolts multiplied by bolt stress area as prescribed in ASTM Specifications [4]. Similarly, under the provisions of EN 1993-1-8 [1] and EN 1090-2 [2], the nominal minimum preloading force F p,C shall be taken as: F p,C = 0.7f ub A res (1) where f ub is the nominal ultimate strength of the bolt material and Ares is the stress area of the bolt. The slip resistant force, governed by preload force F p,C , the surfaces-in-contact slip factor  , the number of plane surfaces in contact n , the safety coefficient  M3 , the hole shape factor k s , is given by Eqn.(2) in accordance with EN 1993-1-8 [1]: s s,Rd p,C M3 k nμ F F γ  (2) where k s = 1 for normal holes, and  M3 is equal to 1.25 at ultimate limit state and 1.1 at serviceability limit state. For RCSC [3] the values are 1.5 and 1.0, respectively. The first step in bolted joints is to obtain the snug tightened condition bringing the connected plates into firm contact. To reach the design preload force it is necessary to apply a correct tightening torque M r ; if tightening torque is lower than that necessary to reach the design preload force, the friction joint is not guaranteed and the mechanism is the same as that of shear bolts; on the other hand, overtightening could exceed the yielding point and increase the plasticization of the screw or nut threads and arrive at rupture. The correlation between F P,C and M r is given by the bolt diameter d and the k -factor k m . In terms of preloading force, for the European code EN 14399-2 [5] the tightening torque depends on the surface treatment of bolt that is parameterized by factor k m .