How to design steel connections

Most steel structures that fail, fail at a connection. Members are designed by tables; connections are where loads change direction, pass through bolts and welds, and squeeze through thin plates. Designing a connection means following the load path from one member to the other and checking every link in it.

#The connection types

#Bolts: shear, bearing and bolt groups

A bolt in shear can fail by shearing across its shank, or by tearing the plate it bears on. Eurocode 3 (EN 1993-1-8 Table 3.4):

When the load doesn't pass through the centre of a bolt group, the group also has to resist a moment V·e. The elastic method adds the direct share V/n to a twisting share proportional to each bolt's distance r from the centroid: F = V·e·r / Σr².

#Worked example: a fin plate (Eurocode 3)

A beam end shear V_Ed = 120 kN goes through a 10 mm S275 fin plate welded to a column, with 3 M20 class 8.8 bolts in one vertical line at 70 mm pitch. The bolt line is 50 mm from the weld (eccentricity e = 50 mm). End and edge distance e₁ = 40 mm, hole d₀ = 22 mm.

#Bolt group

Bolts at y = −70, 0, +70 mm: Σr² = 2 × 70² = 9800 mm². Moment on the group V·e = 120 × 50 = 6000 kN·mm.

#Bolt shear

#Bearing on the plate

α_b = min(e₁/3d₀, p₁/3d₀ − ¼, f_ub/f_u, 1) = min(0.61, 0.81, 1.86, 1) = 0.61; k₁ = 2.5.

#Plate in shear

Bolt shear governs at 0.62. The weld to the column must carry the shear and the moment V·e — a 6 mm fillet on both sides is typical.

#Gussets and the Whitmore section

A brace force entering a gusset spreads out at about 30° on each side from the first bolt row. The effective width at the last row — the Whitmore section — is the width checked for yield, rupture and buckling of the plate between the brace end and the joint.

#Base plates

A column base plate spreads the column load onto concrete. The concrete bearing strength under the plate rises when the supporting area A₂ is larger than the plate A₁ (√(A₂/A₁) ≤ 2 in ACI). The plate bends as a cantilever beyond the column profile; anchors carry shear and any uplift. A fixed base also carries moment through anchor tension on one side and bearing on the other.

#Design it in Struxt3D

Struxt3D designs a standard connection at every steel member end, by classic component checks from its connections engine, with each governing check citing its clause. It steps through standard sizes — bolt diameter and number, plate thickness, anchors, weld legs — until every check passes for every combination.

1. Open a steel model — the Portal frame shed has column bases, rafter-to-column joints, purlins and bracing, so most connection types appear.
2. Make sure connections are on in Design settings › Steel connections (base plates, end plates, fin plates, gussets, splices).
3. Set the member ends. Pinned ends (Pinned, Pin start, Pin end) get fin plates; rigid beam-to-column ends get moment end plates. See Supports and releases.
4. Run the design (F6).
5. See them in 3D. Show › Steel connections draws the plates, bolts, anchors and welds on the members.
6. Inspect a joint. Click a member or joint: the End connections card expands to the dimensioned sketch, the parts and materials, the largest forces with their combination and every check with demand, capacity, formula and clause.
7. Review all of them in the Connections table, and open the joint's Calculation report. Shop drawings include the connection details.

#Common mistakes

• Forgetting eccentricity in fin plates and bolt groups.
• Designing a "pin" that is actually stiff (or the reverse): the connection must match the analysis assumption.
• Edge distances too small — bearing and block tearing drop sharply.
• Welds sized for shear only when they also carry the moment V·e.