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STEEL CONSTRUCTION FASTENERS
9
1.4
k factor and tightening methods
The k factor describes the linear relationship between the tightening torque (M) and the preloading force (F) achieved. The
manufacturer indicates the applicable k factor for the assemblies. For example, the k factor for K-class k1 is 0.10 < k < 0.16.
Connections must be tightened according to the manufacturer’s instructions. There are two primary tightening methods: the
torque method and the combined method. Both methods are aimed at producing the most accurate preloading force on the
bolt assembly.
The torque method is a two-stage operation, bedding in followed by tightening. Tightening is always done in two steps using
a calibrated manual or machine-powered torque wrench. The first stage involves tightening all assemblies to the torque of
0.75 X M
A
. The final torque in the second tightening is 1.10 X M
A
.
Nominal diameter
The required preloading
force F
v
[kN] is calculated
with the following formu-
la:
Fp,C* = 0.7 X f
yb
X A
s
Torque method
Torque method M
A
for achieving the required
preload force Fv [Nm]
Hot galvanised and lubricated
a
uncoated and lubricated
a
M12
50
100
M16
100
250
M20
160
450
M22
190
650
M24
220
800
M27
290
1250
M30
350
1650
M36
510
2800
a
The manufacturer always delivers prelubricated nuts (e.g. molybdenum disulphide MoS2). The torque is
identical for hot dip galvanised and plain assemblies.
The combined method also involves two stages of tightening. In the first stage, the assemblies are tightened to a preload
torque indicated by the manufacturer. Final torque is achieved by rotating the nut by the indicated degree.
Combined method
Nominal diameter
M12
M16
M20
M22
M24
M27
M30 M36
Preloading
Fp,C* = 0.7 X f
yb
X A
s
[kN]
59
110
172
212
247
321
393 572
Preload torque M
A
[Nm]
75
190
340
490
600
940
1240 2100
Final tightening by rotating; depending on the clamp length ∑ t
Total clamp length connection
components ∑ t (incl. any spacers
and washers)
Rotation in degrees
Rotation in part-turns
1
∑t < 2d
60
o
1/6
2
2d ≤ ∑t < 6d
90
o
1/4
3
6d ≤ ∑t < 10d
120
o
1/3