Fin Plate Connection Design Theory (EN 1993-1-8)

The theory behind this fin plate (shear tab) calculator: how a beam web bolted to a plate welded to its support is verified to Eurocode 3 (EN 1993-1-8) with the SCI P358 supplementary rules. We cover the eccentric-shear bolt-group method (the alpha and beta factors), bolt bearing, the fin plate (gross/net/block shear, bending and lateral-torsional buckling of a long plate), the beam web (shear, block shear and shear-bending interaction), the weld, the supporting-member local shear and the punching-shear limit, plus the SCI P358 detailing rules that make the connection a ductile pin.

A fin plate connection (also called a shear tab) is the most common simple (nominally pinned) steel connection. A flat plate is welded to the supporting member - a primary beam web or a column - and the web of the supported beam is bolted to it. It carries the beam end reaction (a vertical shear $V_{Ed}$) and is assumed to transfer negligible moment, while still providing the rotation capacity and torsional restraint the beam needs. This page explains the mechanics and every formula behind the checks this calculator performs to Eurocode 3 (EN 1993-1-8) with the supplementary rules of SCI Publication P358.

Fin plate welded to the support and bolted to the supported beam web; the bolt group sits a lever arm z from the weld line, so the shear is eccentric.

The verification framework

Although a fin plate is designed as a shear connection, the bolt group sits a distance $z$ from the weld line, so the bolts and the welds actually carry the shear $V_{Ed}$ plus a nominal moment $V_{Ed}\,z$. Every potential failure mode - in the bolts, the fin plate, the beam web, the weld and the supporting member - must be verified, because the governing mode is rarely obvious in advance. For each, the design action must not exceed the design resistance, i.e. the utilisation $V_{Ed}/V_{Rd} \le 1$.

Bolt group in shear

The shear resistance of a single bolt per shear plane is $F_{v,Rd} = \alpha_v f_{ub} A_s / \gamma_{M2}$, with $\alpha_v = 0.6$ for classes 4.6 and 8.8 ($0.5$ for 10.9). Because the load is eccentric, the bolts do not share $V_{Ed}$ equally - the group is checked with the SCI P358 / SN017 elastic method, which reduces the group resistance through the factors $\alpha$ and $\beta$:

The eccentric shear is resolved into a direct vertical force on each bolt plus a force from the moment V·z about the group centroid - captured by the alpha and beta factors.

For a single vertical line of bolts ($n_2 = 1$), $\alpha = 0$ and $\beta = 6z / [n_1(n_1+1)p_1]$; for two lines ($n_2 = 2$) both factors come from the polar term $l = \tfrac{n_1}{2}p_2^{2} + \tfrac{1}{6}n_1(n_1^{2}-1)p_1^{2}$. The same idea governs bolt bearing, combining the vertical and horizontal bearing resistances of the plate and the beam web through the same eccentricity factors.

Fin plate resistance

The plate is checked for several modes:

• Gross-section shear - $V_{Rd,g} = (h_p t_p/1.27)\,f_{y,p}/(\sqrt3\gamma_{M0})$. The 1.27 factor reduces the shear area for the nominal moment in the connection.
• Net-section shear through the bolt holes - $V_{Rd,n} = A_{v,net} f_{u,p}/(\sqrt3\gamma_{M2})$.
• Block shear (tearing) - a block of plate tears out along the bolt lines, combining a tension plane and a shear plane (EN 1993-1-8 §3.10.2).
• Bending - significant only when h_p < 2.73 z; then $V_{Rd} = W_{el,p} f_{y,p}/(z\gamma_{M0})$ with $W_{el,p} = t_p h_p^{2}/6$.
• Lateral-torsional buckling - a long fin plate (z_p > t_p/0.15) can buckle out of plane; $V_{Rd} = W_{el,p}\chi_{LT} f_{y,p}/(z\cdot 0.6\gamma_{M1})$.

Beam web resistance

The supported beam web is checked for gross-section shear over the rolled-section shear area $A_v = A_g - 2 b\,t_f + (t_w + 2r)t_f$, net-section shear through the holes, and block shear. For a long fin plate an additional shear-bending interaction of the web is required, because the web must carry the moment $V_{Ed} z_p$ about the bolt group while it shears.

Weld and supporting member

The fillet weld connecting the plate to the support is most often specified as full-strength - stronger than the plate it joins - so it never governs and no explicit check is needed. If a smaller designed fillet is used, it is verified for the combined vertical shear and the nominal moment, with the resultant stress $\tau_r = \sqrt{\tau_v^{2} + \tau_h^{2}} \le f_{vw,d}$, where $f_{vw,d} = (f_u/\sqrt3)/(\beta_w \gamma_{M2})$. Finally the supporting member is checked for local shear of its web/column, and a punching shear limit ensures the fin plate yields before it punches through a thin support: $t_p \le t_2\,f_{u,2}/(f_{y,p}\gamma_{M2})$.

Detailing rules (SCI P358)

To guarantee the connection behaves as a ductile pin, SCI P358 recommends:

• place the fin plate as close to the top flange as possible for stability;
• fin plate depth $h_p \ge 0.6 h_{b1}$ for torsional restraint;
• plate/web thickness $\le 0.5 d$ (S275) or $\le 0.42 d$ (S355) for ductility;
• edge and end distances $\ge 2 d$;
• standard details use M20 Gr 8.8 bolts in 22 mm holes.

Frequently asked questions

Ready to check your connection? Run the full EN 1993-1-8 / SCI P358 verification for a fin plate connection in 3D, with step-by-step derivations for every check.