Bolted Connection Theory - EN 1993-1-8 (Eurocode 3)

A bolted steel connection is designed to Eurocode 3, EN 1993-1-8. Each bolt is checked for the actions it carries - shear, bearing, tension, or a combination - and the connection geometry must satisfy the spacing and edge-distance rules. This page explains the resistances behind the bolt data tables.

Bolt property classes

Bolts are graded by a two-number class such as 8.8 or 10.9. The first number times 100 is the ultimate tensile strength $f_{ub}$ in MPa; the product of the two numbers times 10 is the yield strength $f_{yb}$. So a class 8.8 bolt has $f_{ub} = 800$ MPa and $f_{yb} = 640$ MPa. Classes 8.8 and 10.9 dominate structural work; 4.6 is used for light or non-structural fixings.

Shear and bearing resistance

In a bearing-type (non-preloaded) connection the load is transferred by the bolt shank shearing and bearing against the plate. The shear resistance per shear plane is

where $A$ is the shank area $A$ if the shear plane misses the threads, or the tensile stress area $A_s$ if it passes through them; $\alpha_v = 0.6$ for classes 4.6, 5.6 and 8.8, and 0.5 for 10.9 (through the threads). The plate it bears on must also resist crushing:

with $d$ the bolt diameter, $t$ the plate thickness, and $\alpha_b$ and $k_1$ factors that fall as the end and edge distances reduce - which is exactly why spacing rules matter. The bolt resistance is the lesser of $F_{v,Rd}$ and $F_{b,Rd}$.

Tension and punching

A bolt in tension is checked for thread stripping/failure and the connected plate for punching shear of the bolt head or nut:

where $k_2 = 0.9$ (0.63 for countersunk bolts), $A_s$the tensile stress area and $d_m$ the mean across-flats/across-corners dimension of the head or nut.

Combined shear and tension

A bolt that carries shear and tension at once must satisfy the linear interaction of Table 3.4:

The 1.4 in the tension term reflects that a modest tension does not greatly reduce shear capacity.

Preloaded (slip-resistant) bolts

Class 8.8 and 10.9 bolts can be tightened to a controlled preload $F_{p,C} = 0.7\,f_{ub}\,A_s$, clamping the plates so the load is carried by friction rather than bearing. The slip resistance is

with $n$ friction surfaces, slip factor $\mu$ (0.5 for blasted steel, less for as-rolled) and a hole factor $k_s$. Slip-resistant connections are used where movement is unacceptable - fatigue details, reversing loads, or where alignment must be preserved.

Spacing, edge and end distances

Bolt positions are bounded both ways. Minimum spacing ($p_1 \ge 2.2 d_0$, end distance $e_1 \ge 1.2 d_0$) prevents the plate tearing out and keeps the bearing factor up; maximum spacing prevents local buckling between bolts and keeps the parts in contact. $d_0$ is the hole diameter (Table 3.3). The data tables in this tool list the hole sizes, the minimum and maximum distances and the resulting resistances for each bolt diameter.

The partial factors $\gamma_{M2} = 1.25$ (resistance of bolts) and $\gamma_{M3}$ (slip) and the slip factor class are set by the National Annex; category A–E connections (bearing vs slip-resistant) determine which checks govern.