Steel Connection Design: From Simple to Complex

Connection design is where steel structures succeed or fail. I’ve seen elegant frames undermined by poorly designed joints and simple structures made expensive by over-complicated connections. Here’s what I’ve learned about getting connections right.

The Connection Philosophy

Before calculating anything, consider:

1. Load path clarity: How do forces flow through the connection?
2. Constructability: Can it be fabricated and erected efficiently?
3. Ductility: How will it behave beyond design loads?
4. Cost: Standard details beat bespoke every time

Connection Classifications

By Rigidity

By Force Transfer

• Shear connections: Transfer vertical loads only
• Moment connections: Transfer moment and shear
• Tension connections: Hangers, ties, bracing
• Compression connections: Column splices, bearing

Simple Connections

Fin Plates

The workhorse of simple connections:

Design checks:

1. Bolt group in shear
2. Bolt bearing on plate and beam web
3. Fin plate shear and bending
4. Block tearing
5. Beam web local capacity

Standard geometry:

• Plate thickness: 10mm (for beams up to 533UB)
• Bolt size: M20 Grade 8.8
• Edge/end distances: 40mm minimum
• Bolt spacing: 70mm
• Plate depth: beam depth - 2 × clearance

Quick capacity estimate: For M20 bolts in double shear (10mm plate):

• 2 bolts: ~150 kN
• 3 bolts: ~225 kN
• 4 bolts: ~300 kN

Flexible End Plates

Better ductility than fin plates, useful when rotation is expected:

Advantages:

• Shop welded, site bolted
• Good tolerance for beam length variations
• Accommodates rotation

Design considerations:

• End plate thickness: typically 10-12mm
• Double row of bolts for higher loads
• Welds: fillet welds, throat = 0.7 × plate thickness

Web Cleats

Traditional but labor-intensive:

• Double angle cleats bolted to beam web and column
• Good for heavy loads
• More site bolts than alternatives

Moment Connections

Extended End Plates

For continuous beams and moment frames:

Tension zone design:

1. Calculate tension force from moment: T = M / (D - tf)
2. Design bolt group for tension
3. Check end plate bending (equivalent T-stub method)
4. Check column flange (may need stiffeners)

Compression zone:

• Typically adequate by bearing
• Check column web in compression
• Add stiffeners if web buckling governs

Welds:

• Full penetration welds to beam flanges (tension)
• Fillet welds to beam web (shear)

Haunched Connections

For portal frames and long-span structures:

Benefits:

• Increases lever arm, reducing bolt forces
• Moves plastic hinge away from column face
• Aesthetically consistent with frame geometry

Design approach:

1. Size haunch for frame analysis
2. Design connection at haunch tip (larger lever arm)
3. Check haunch stability (potential for LTB)
4. Detail haunch-to-rafter connection

Column Splices

Bearing Splices

When columns are in compression:

Components:

• End bearing between machined ends
• Splice plates (or division plates) for stability
• Holding-down bolts for erection

Design:

• Bearing takes 100% of compression
• Splices designed for tension from moment
• Minimum 4 bolts for erection stability

Non-Bearing Splices

When tension or significant moment:

Design checks:

• Flange splices for moment
• Web splice for shear
• All force transfer through bolts (no bearing credit)

Bracing Connections

Gusset Plate Design

For concentric bracing:

Geometry:

• Whitmore width for tension capacity
• Block shear check
• Thornton check for compression (plate buckling)

Clearances:

• 2tp minimum between intersection and column face
• Allows formation of plastic hinge in plate under seismic loading

Worked Example: Diagonal Bracing

Given: 150×150×10 SHS brace, 450 kN tension, 300 kN compression

1. Gusset geometry: 20mm plate, 400mm width
2. Whitmore width: w = 2 × L × tan(30°) + connection width
3. Tension capacity: Npl = w × t × fy = adequate
4. Compression: Check Thornton method for plate buckling
5. Bolt design: 6 × M24 Grade 8.8 in double shear

Software Tools

Hilti PROFIS

Excellent for:

• Anchor bolt design
• Base plate connections
• Post-installed anchors

StabiCa

For complex connections:

• 3D finite element analysis
• Code checking (EC3)
• Automatic weld design
• Comprehensive reports

SCI Connection Tools

Industry-standard in UK:

• Follows Green Book methods
• Comprehensive design checks
• Good for standard connections

Practical Considerations

Tolerances

Design for real-world fit:

• Holes: 2mm clearance (M20 in 22mm hole)
• Beam length: ±3mm
• Column position: ±5mm

Slotted holes: Use in one direction for adjustment

Weld Specifications

Fillet welds:

• Size = leg length
• Throat = 0.7 × leg length
• Minimum size depends on thinner part (typically 6mm)

Full penetration welds:

• More expensive, better quality control needed
• Required for tension flanges in moment connections

Fire Protection

Connections often need less fire protection than members:

• Protected by concrete slab above
• Lower temperature due to thermal mass
• Check with fire engineer for bespoke situations

Lessons from F1 Facility Projects

Portal frame connections in motorsport facilities present unique challenges:

High Load Concentrations

Crane loads create point reactions. Solution: Local stiffening, spreader beams.

Architectural Exposure

Connections visible in design studios. Solution: Concealed bolts, smooth welds, clean geometry.

Future Adaptability

Facilities may be reconfigured. Solution: Connection capacity exceeds initial requirements.

Common Mistakes to Avoid

1. Ignoring prying action in tension connections
2. Forgetting block tearing in bolted sections
3. Under-sizing welds to beam web in moment connections
4. Not checking column in beam-to-column joints
5. Inadequate clearances for erection and bolt tightening

Conclusion

Good connection design balances engineering efficiency with practical construction. Standard details exist for a reason - they’re proven, efficient, and understood by fabricators.

For non-standard situations, apply first principles: understand the load path, check all failure modes, and provide ductility. When in doubt, consult specialists and visit fabrication shops - seeing real connections teaches more than any textbook.