Steel Connection Design (EN 1993-1-8)

An overview of how steel connections are designed to Eurocode 3 (EN 1993-1-8): the difference between simple (pinned), moment-resisting and semi-rigid joints, the main bolted and welded connection types (fin plate, end plate, base plate, hollow-section and bracing connections), and the design philosophy of verifying every failure mode in the bolts, plates, members and welds. Each connection type has its own interactive 3D calculator.

Run this method in the free calculator and export the result to Excel (.xlsx) or PDF.

A steel connection transfers forces between members - beams, columns, braces - through bolts, welds and plates. Eurocode 3 (EN 1993-1-8) governs their design and requires that every credible failure mode is verified, because a connection is only as strong as its weakest load path. This page is an overview of the connection types these tools cover, how they differ and how to choose between them; each type has its own interactive 3D calculator that runs the full EN 1993-1-8 / SCI P358 check set.

Three load paths: bolts, welds and plates

Whatever its shape, a connection routes force through three kinds of component, and each has its own family of failure modes:

• Bolts carry shear ($F_{v,Rd} = \alpha_v f_{ub} A_s/\gamma_{M2}$ per shear plane), bearing on the plate they push against ($F_{b,Rd} = k_1\alpha_b f_u d t/\gamma_{M2}$), tension($F_{t,Rd} = 0.9 f_{ub} A_s/\gamma_{M2}$), or a combination. A bolt group under an eccentric load shares the force unequally, so it is checked by the elastic (polar) method - not by dividing the load by the number of bolts.
• Welds (usually fillet) are checked on their effective throat$\,a = 0.707\,s$ by the directional method of §4.5.3 - the resultant of the normal and shear stresses against $f_u/(\beta_w\gamma_{M2})$.
• Plates and the connected members are checked for gross- and net-section shear, bending, block tearing (a block of plate tearing out around the bolt group) and, for slender plates, buckling.

The connection resistance is the lowest of all these, and it is rarely the mode you expect - which is why each calculator checks every path and reports the governing one.

Simple vs moment connections (joint classification)

EN 1993-1-8 §5.2 classifies a joint two ways. By stiffness it is nominally pinned, rigid or semi-rigid; by strength, nominally pinned, full-strength or partial-strength. A simple (pinned) joint - a fin plate or a flexible end plate - transfers shear and only a small nominal moment, so the beam is designed as simply supported. A moment connection - an extended end plate, a welded haunch - transfers shear plus a design moment, making the frame continuous and cutting the mid-span moment. The detail you draw must match the joint type the global analysis assumed: a joint modelled as pinned but actually stiff attracts moment it was never sized for, and a joint modelled as rigid but too flexible lets the frame drift.

Connection types

• Fin plate (shear tab) - a plate welded to the support and bolted to the beam web. The cheapest and most common simple shear connection; the eccentricity between the bolt line and the weld drives the bolt-group and weld checks.
• End plate - a plate welded to the beam end and bolted to the support. A partial-depth plate is a flexible shear connection; a full-depth or extended plate with the bolts grouped at the flange becomes a moment connection, designed by equivalent T-stub theory (§6.2.4).
• Hollow-section connection - an open beam or brace onto a CHS / RHS tube column, where the load spreads into the relatively flexible chord face. Chord-face plastification, punching shear and side-wall failure govern, following the CIDECT / §7 rules rather than ordinary plate checks.
• Bracing / gusset - a gusset plate transferring an axial brace force, checked over its Whitmore effective width for tension and buckling, plus the weld and block tear-out.
• Beam & column splice - cover plates rejoining two lengths of the same member. A bearing splice passes compression through direct contact; a non-bearing splice carries everything through the plates and bolts.
• Base plate - a column welded to a plate held down by anchors cast into concrete; the plate spreads bearing over an effective area and the anchors resist tension and shear to EN 1992-4.

The design philosophy

Whatever the type, the method is the same: identify every load path - the bolts, the welds, the plates and the connected members - and verify each design resistance is at least the design action, $E_d \le R_d$. Because the governing mode is rarely obvious in advance, a connection calculator checks them all at once and reports the lowest margin, so you size the detail by its true weakest link rather than by the check you happened to remember. Pick a connection type above to open its calculator and see the full worked check set.

Frequently asked questions

Pick a connection type and run the full EN 1993-1-8 verification in 3D, with step-by-step derivations for every check.