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Screws, Fasteners, and the Design of Nonpermanent Joints 435
Table 8–11

Metric Mechanical-Property Classes for Steel Bolts, Screws, and Studs*

Minimum Minimum Minimum
Size Proof Tensile Yield
Property Range, Strength, † Strength, † Strength, †
Class Inclusive MPa MPa MPa Material Head Marking
4.6 M5–M36 225 400 240 Low or medium carbon
4.6

4.8 M1.6–M16 310 420 340 Low or medium carbon
4.8

5.8 M5–M24 380 520 420 Low or medium carbon
5.8

8.8 M16–M36 600 830 660 Medium carbon, Q&T
8.8

9.8 M1.6–M16 650 900 720 Medium carbon, Q&T
9.8

10.9 M5–M36 830 1040 940 Low-carbon martensite,
Q&T
10.9

12.9 M1.6–M36 970 1220 1100 Alloy, Q&T

12.9

*The thread length for bolts and cap screws is
2d + 6 L ≤ 125
⎧
⎨
2d + 12 125 < L ≤ 200
L T =
2d + 25 L > 200
⎩
where L is the bolt length. The thread length for structural bolts is slightly shorter than given above.
† Minimum strengths are strengths exceeded by 99 percent of fasteners.
8–7 Tension Joints—The External Load

Let us now consider what happens when an external tensile load P, as in Fig. 8–13,
is applied to a bolted connection. It is to be assumed, of course, that the clamping
force, which we will call the preload F i , has been correctly applied by tightening the
nut before P is applied. The nomenclature used is:

F i = preload
P total = Total external tensile load applied to the joint