Technical assistance

Tightening of preloading screw assemblies

Screw assemblies (screw, nut, 2 washers) for preloading, which are manufactured in accordance with SIST EN 14399-4 or. SIST EN 14399-8 (HV screw assemblies) and are used in preloaded steel structures in accordance with the requirements of SIST EN 1090-2, preloading according to one of the methods described in SIST EN 1090-2 (point 8.5).

The torque reference values M_r,1,, used to achieve the rated minimum preloading force F_p,C are most often determined for each type of k-class screw assembly used by the screw assembly manufacturer in accordance with the relevant part of EN 14399-4 or . EN 14399-8.

The tightening torque reference value is determined as M_r,1, = k_m d F_p,c. In the case where the screw assembly belongs to class K-1, the value k_m = 0,125 can be used for the value of km. In this case, the value of M_r,1 can be determined as:

M_r,1 = 0,125 d F_p,C

M _r,i– tightening torque

k_m – value of k-factor (coefficient of friction)

d - nominal diameter of the screw

F_p,c – nominal bolt prestressing force

Table 19, SIST EN 1090-2 - values FpF_p,C u Kn

Strength class	Screw dimension in mm
Strength class	12	16	20	22	24	27	30	36
8.8	47	88	137	170	198	257	314	458	K1	t is determined by the manufacturer, if there is no measured data, then we take into account the middle value - which is 0,125
10.9	59	110	172	212	247	321	393	572

(F_p,C is important for kv. 10.9 - HV construction screws, in kv. 8.8 only HR construction screws are made)

Example of calculation of the torque reference value for the dimension of the screw assembly M20: HV screw assembly k-class K1 (strength class 10.9):

M_r,1 = k_m x d x F_p,c = 0,125 x 0,020m x 172000 N = 430 Nm

The following two tightening methods are most commonly used for HV class screw assemblies in accordance with SIST EN 1090-2:

combined method of tightening screws: it consists of two steps (the most commonly used method)

1. STEP

Set the torque wrench with a suitable operating range to a torque value of approx. 0,75 M_r,i.

Before starting the second step, the first step must be performed on all bolts in the joint. The table on the right shows the approximate values of the torques for the first tightening step.

Strenght class	Nominal screw diameter d (mm)
	12	16	20	22	24	27	30	36
8.8	53	132	258	351	446	652	886	1548
10.9	67	165	322	439	557	815	1107	1935
Table:	values 0,75 M r,1 (k_m=0,125) u Nm)

2. STEP

Further rotation for a specific partial turn
Further rotation for a specific partial turn
(Table 21, SIST EN 1090-2)
	Rotation (°)	Partial turn
t < 2d	60	1 / 6
2d ≤ t < 6d	90	1 / 4
6d ≤ t≤ 10d	120	1 / 3

d = nominal diameter of screw

For torque wrenches used in the first step of the combined method, ± 10% accuracy with a calibration frequency of once a year is required.

b) torque tightening method

It basically also consists of two steps:

1) the key must be set to a torque value of approximately 0.75 M r,i . Before beginning the second step, the first step must be completed on all bolts in the joint;/p>

2) the key must be set to a torque value of 1.10 M r,i.

The torque wrenches used to tighten the screws by the torque method must be calibrated to an accuracy of ± 4% in accordance with EN ISO 6789 - once a week.

Technical assistance

Tightening of preloading screw assemblies

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