Column Splice Types — AS 4100 Clause 9.5

Column splices are required at approximately every 2-3 storeys (8-12 m) in multi-storey steel buildings due to:

1. Transport and erection constraints: column sections longer than 12-15 m are difficult to transport and erect
2. Section change: column size reduces up the building as axial loads decrease
3. Fabrication limits: standard mill lengths for hot-rolled sections are typically 12 m

AS 4100 Clause 9.5 recognises two distinct column splice types:

Type 1 — Bearing Splice (Machined Ends)

The column ends are machined flat and bear directly against each other, transferring the compressive axial load through direct bearing. The splice plates and bolts primarily provide:

• Continuity of the section for tension (if uplift occurs)
• Lateral restraint to the column at the splice
• Nominal bending continuity (typically 25-50% of the section moment capacity)

Key requirement per Clause 9.5.4: The ends must be prepared to achieve full contact. Machining or grinding of the column ends is required; saw-cut ends are generally not acceptable for bearing splices unless the saw produces a surface finish of Ra = 6.3 micrometres or better.

Bearing splices are the most economical column splice type because:

• Bolt groups transmit only nominal tension (not the full axial load)
• Cover plates can be relatively thin
• Fewer bolts are required than a non-bearing splice

Type 2 — Non-Bearing Splice (Cover Plates Only)

The column ends are not prepared for bearing (or a gap is intentionally provided). The full axial load is transferred through bolted or welded cover plates acting in shear. Non-bearing splices are used when:

• A change in column section occurs (different serial sizes, requiring a division plate or packer)
• The column is in tension (uncommon but possible in braced frames or tall slender structures)
• Machining of column ends is impractical (e.g., site modifications)

Bearing Splice Design Method

Step 1 — Check Bearing Stress

The direct bearing capacity of the column end in compression is: phi_Ns_bearing = 0.90 x fy x Ag (same as section squash capacity)

This is essentially the full section compressive capacity — bearing at the machined interface is not a limiting condition for standard columns.

Step 2 — Design Tie Plates for Tension and Moment Continuity

Per AS 4100 Clause 9.5.4, the tie (cover) plates must be designed for:

Axial tension: Ntie = 0.25 x phi_Ns (25% of section capacity — empirical allowance for accidental tension)

Bending moment: Mtie = 0.25 x phi_Ms (25% of section moment capacity — provides nominal continuity)

These requirements ensure the splice is not a complete hinge. In practice, for braced frames where columns carry primarily axial load, the 25% requirement is a reasonable balance between economy and robustness.

Step 3 — Bolt Group Design

Bolts transfer the tie forces (tension and moment) from the upper column to the cover plates, then from the cover plates to the lower column. Bolts are in single shear per shear plane.

Bolt shear check: n_bolts x phi_Vfn >= maximum of (Ntie, shear from Mtie)

For tie plates on each flange, the moment is resolved as a flange force couple: T_flange = Mtie / (d - tf).

Step 4 — Cover Plate Sizing

The cover plate cross-sectional area should approximately match the flange area for flange cover plates. Web cover plates should total at least the web area but need not exceed it if the web contribution is minor for the load path.

Minimum plate thickness: 10 mm for flange cover plates, 6 mm for web cover plates (practical minimum for handling and welding).

Non-Bearing Splice Design Method

Step 1 — Determine Load to be Transferred

The full factored axial load N* (compression or tension), plus any moment M* and shear V*, must be transferred through the splice plates.

Step 2 — Proportion Flange Cover Plates

For compression: the flange cover plates must carry the portion of N* assigned to each flange. The load distribution between flanges depends on the relative stiffness of the flange and web cover plate groups: N_flange = N* x (Af x d_f) / [2 x Af x d_f + Aw x (d - 2tf)/2]

In practice, for UC sections with heavy flanges, approximately 70-80% of the axial load is transferred through the flange cover plates and 20-30% through the web plate.

Step 3 — Proportion Web Cover Plates

The web cover plates transfer the remaining axial load plus any shear V*. All bolts in the web plates must be checked for combined shear and axial effects.

Step 4 — Bolt Group Design

Similar to beam splices, the bolts at each flange and web group must be designed for shear. Bearing checks are required for all plies.

End-Plate Splice — Alternative Detail

For lightly loaded columns (N* < 1,500 kN), an end-plate splice is an economical alternative:

• A flange plate is welded to each column end in the shop
• The plates are bolted together on site through pre-drilled holes
• The plates must be thick enough to prevent prying action (typically 20-32 mm for UC sections)
• 4-8 bolts per column are typically sufficient

End-plate thickness per Clause 9.3.4 (prying check): tp >= sqrt(4 x N* x b' / (phi x fy x bp x L_eff))

Where b' is the bolt-to-flange-face distance, bp is the plate width, and L_eff is the effective yield line length per bolt.

Worked Example — 310UC158 Bearing Splice

Problem: Design a bearing column splice for a 310UC158 column (Grade 300) in a braced-frame building. Axial load N* = 2,600 kN (compression), M* = 45 kN.m nominal moment. The column reduces to 250UC89 above the splice.

Upper column — 250UC89: d = 260 mm, bf = 256 mm, tf = 17.3 mm, tw = 10.5 mm Ag = 11,300 mm^2, fy = 280 MPa

Lower column — 310UC158: d = 326 mm, bf = 316 mm, tf = 25.0 mm, tw = 15.4 mm Ag = 20,100 mm^2, fy = 260 MPa (Grade 300, tf > 20 mm)

Step 1 — Bearing check at machined ends: Upper column to division plate: phi_Ns = 0.90 x 280 x 11,300 / 1000 = 2,848 kN > 2,600 kN. OK. The section change requires a division (packer) plate between the upper and lower column ends to distribute the load from the smaller to the larger section.

Step 2 — Division plate design: The division plate spreads the load from the 250UC89 footprint to the 310UC158 section. Plate size = 316 x 326 mm (matching the lower column). Plate thickness must resist bending from the eccentricity between the upper column centroid and the lower column.

Eccentricity e = (326 - 260) / 2 = 33 mm (maximum, assuming flush flange alignment). Bending moment in plate = N* x e = 2,600 x 0.033 = 85.8 kN.m per flange projection. Required plate thickness: tp >= sqrt(4 x Mtie / (phi x fy x bp)) = sqrt(4 x 85.8 x 10^6 / (0.90 x 260 x 316)) = sqrt(343 x 10^6 / 73,944) = sqrt(4,641) = 68.1 mm. Use 70 mm Grade 300 plate. This thick plate ensures the bearing stress from the upper column spreads through the plate to the lower column without plate bending failure.

Step 3 — Tie plates (25% rule): Ntie = 0.25 x 2,848 = 712 kN. Provide flange cover plates each side: 256 x 16 mm plate (Grade 300). Ap = 256 x 16 = 4,096 mm^2 per plate. Total tie plate area = 2 x 4,096 = 8,192 mm^2. phi_Nt = 0.90 x 280 x 8,192 / 1000 = 2,064 kN > 712 kN. OK.

Step 4 — Bolts for tie plates: M20 Grade 8.8 bolts, threads excluded from shear plane: phi_Vfn = 0.80 x 0.80 x 830 x 245 / 1000 = 130.1 kN per bolt (single shear). Bolts required per flange = 712 / 130.1 = 5.47 → use 8-M20 bolts per flange (2 rows x 4 columns). Total 16-M20 bolts for the splice (8 per column side).

Step 5 — Web cover plate: Provide 10 mm web cover plate each side. 4-M20 bolts per side (2 rows x 2 columns). Design for 25% tension and all shear: small demand, entirely adequate.

Final specification: Bearing splice with 70 mm division plate (Grade 300), 256 x 16 mm flange tie plates both sides, 12-M20 Grade 8.8 bolts, machined column ends to Ra = 6.3 micrometres. Conforms to AS 4100:2020 Clause 9.5.4.

Erection and Detailing Considerations

Column alignment: Provide erection aids (drift pins in oversized holes, or alignment lugs) to align columns during erection before bolting. The column-to-column alignment tolerance per AS 4100 Clause 14.5 is +/- 5 mm horizontally for the upper column relative to the lower column at the splice.

Safety during erection: The splice must be sufficiently robust to resist construction loads (wind, temporary bracing forces, accidental impact) before the permanent structure is completed. The 25% tie force provision provides some of this robustness, but additional temporary restraints may be needed for tall or slender columns.

Fire protection continuity: At column splices, the fire protection system (intumescent coating, vermiculite spray, or board encasement) must be continuous across the splice. Cover plates that project beyond the column envelope should be fire-protected to the same standard as the column.

Frequently Asked Questions

When should a column splice be designed as non-bearing rather than bearing? Use a non-bearing splice when: (a) the column sections above and below have different serial sizes requiring a packer and the difference is too large for a practical division plate (>50 mm thickness), (b) the column carries significant tension (net uplift in tall buildings or braced frames), (c) machining of column ends is impractical due to access or cost, or (d) the splice is located at a site where the column ends cannot be prepared (e.g., modifications to existing structure).

What is the practical maximum section change at a column splice? A section change of one serial size (e.g., 310UC158 to 250UC89) is easily accommodated with a division plate. A change of two serial sizes (e.g., 310UC158 to 200UC59) becomes more complex — the division plate is thicker, bolt groups are larger, and a detailed check of the load path through the plate is essential. Changes of more than two serial sizes are rare in practice and may require a transition piece or separate base plate at that level.

How are column splices handled in moment-resisting frames? In moment-resisting frames (MRFs), the column splice is typically positioned at approximately one-third of the storey height above floor level (where moments are lower under lateral load). The splice must develop the moment from the governing load combination — the 25% tie force provision for bearing splices is generally not adequate. Full moment splices use heavy flange cover plates, bolted or welded, with slip-critical bolts if moment reversal occurs. The splice capacity must equal or exceed the moment demand at that location.

Does AS 4100 require a specific surface finish for bearing splices? Clause 9.5.4 requires that bearing surfaces be "prepared to achieve full contact." While the standard does not mandate a numerical roughness value, industry practice (per the ASI Steel Designers Handbook) is a machined or ground surface with Ra <= 6.3 micrometres. Saw-cut ends may be acceptable for lightly loaded columns (N* < 0.3 x phi_Ns) if the cut is clean and square.

This page is for educational reference. Column splice design per AS 4100:2020 Clause 9.5. All structural designs must be independently verified by a licensed Professional Engineer or Structural Engineer registered with Engineers Australia or the relevant state registration board. Results are PRELIMINARY — NOT FOR CONSTRUCTION.

Disclaimer: This content is for educational purposes only. Results must be verified by a licensed professional engineer. Steel Calculator provides preliminary design tools — NOT a substitute for professional engineering judgment.