Hollow Section Connection Theory (EN 1993-1-8)

The theory behind this hollow section calculator: how a fin plate welded to a circular (CHS) or rectangular (RHS) tubular column is verified to Eurocode 3 (EN 1993-1-8 Chapter 7), CIDECT Design Guide 9 and SCI P358. We cover the range of validity, the fin plate (gross/net/block shear and bending), the eccentric weld, the column wall local shear, the punching-shear limit that keeps the plate ductile, and the chord-face plastification of the tube.

A hollow section connection joins an I- or H-beam to a tubular steel column - a circular (CHS) or rectangular (RHS) hollow section. The simplest and most common is a fin plate welded to the tube wall, with the supported beam web bolted to it. Because the tube wall is thin and flexible, two failure modes that never appear with an open column must be checked: punching shear of the wall and chord-face plastification of the tube. This page explains the mechanics and every formula behind the checks this calculator performs to Eurocode 3 (EN 1993-1-8 Chapter 7), CIDECT Design Guide 9 and SCI P358.

A fin plate welded to a CHS / RHS column, with the supported beam web bolted to it. The bolt group sits a lever arm z from the weld, so the shear is eccentric.

The verification framework

First the joint is checked against the EN 1993-1-8 Table 7.1 / CIDECT range of validity (the tube slenderness $d_c/t_c$); the chord-face formulas only apply inside it. Then every failure mode is verified - the fin plate, the weld, the column wall, punching, and the tube chord face - with the utilisation kept at or below 1.0.

Fin plate and weld

The fin plate is verified exactly as for an open-section column: gross-section shear (with the 1.27 reduction for the nominal moment), net-section shear through the holes, block shear, and - for a long plate - bending and lateral-torsional buckling. The fillet weld of the plate to the tube wall carries the vertical shear plus the nominal moment $M = V_{Ed} z$, with the resultant stress $\tau_r = \sqrt{\tau_v^{2} + \tau_T^{2}} \le F_{w,L,Rd}$.

Column wall and punching shear

The tube wall is checked for local shear over the area $A_v = 2 h_p t_c$. The decisive tubular check is punching shear: the plate must yield before it punches through the thin wall, which SCI P358 enforces with the thickness limit

The fin plate force is resisted by the tube wall; if the plate is too thick relative to the wall it punches through, so a thickness limit forces the plate to yield first.

If the plate is thicker than this limit, a backing (doubler) plate is welded around the tube wall or a thicker tube is chosen.

Chord face plastification (CIDECT)

The most distinctive tubular failure is chord-face plastification - the tube wall bends and yields locally under the plate. CIDECT Design Guide 9 gives the axial resistance of a plate welded transversely to a CHS:

which, with the lever arm to the bolts, gives the chord moment resistance $M_{1,Rd} = h_p N_{1,Rd}$; it must exceed the nominal moment $M = V_{Ed} z$. For an RHS column the equivalent chord-face formula uses the wall yielding mechanism with the width ratio. This is usually the governing check for a slender tube.

Detailing notes

• keep $10 \le d_c/t_c \le 50$ and within the AISC non-slender limit so the simple plate connection is permitted;
• use a single vertical bolt line for the fin plate, with M20 grade 8.8 bolts as the standard detail;
• if punching or chord-face plastification governs, add a backing/doubler plate around the tube;
• full-strength fillet welds of the plate to the tube remove the weld from the governing modes.

Frequently asked questions

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