# Steel Connection Design ## Table of Contents - [Introduction](#introduction) - [Bolted Connections](#bolted-connections) - [Bolt Grades and Properties](#bolt-grades-and-properties) - [Bearing Type Connections](#bearing-type-connections) - [Slip-Critical Connections](#slip-critical-connections) - [Bolt Spacing and Edge Distances](#bolt-spacing-and-edge-distances) - [Bolt Group Analysis](#bolt-group-analysis) - [Welded Connections](#welded-connections) - [Fillet Welds](#fillet-welds) - [Butt Welds](#butt-welds) - [Weld Symbols and Specification](#weld-symbols-and-specification) - [Weld Inspection and Quality](#weld-inspection-and-quality) - [Shear Connections](#shear-connections) - [Fin Plate Connections](#fin-plate-connections) - [Flexible End Plate Connections](#flexible-end-plate-connections) - [Web Cleat Connections](#web-cleat-connections) - [Comparison of Shear Connection Types](#comparison-of-shear-connection-types) - [Moment Connections](#moment-connections) - [Extended End Plate Connections](#extended-end-plate-connections) - [Flush End Plate Connections](#flush-end-plate-connections) - [Haunched Connections](#haunched-connections) - [Welded Moment Connections](#welded-moment-connections) - [Base Plate Connections](#base-plate-connections) - [Pinned Base Plates](#pinned-base-plates) - [Moment-Resisting Base Plates](#moment-resisting-base-plates) - [Anchor Bolt Design](#anchor-bolt-design) - [Column Splices](#column-splices) - [Bearing Splices](#bearing-splices) - [Non-Bearing Splices](#non-bearing-splices) - [Bracing Connections](#bracing-connections) - [Practical Notes for Architects](#practical-notes-for-architects) - [Related Topics](#related-topics) - [References](#references) --- ## Introduction Connections are the joints between structural steel members, and their design is as important as the design of the members themselves. Connection behaviour — whether a joint acts as pinned, rigid, or semi-rigid — fundamentally affects how forces distribute through the structure. For the architect, understanding connection types is critical for managing the visual expression of steelwork, coordinating structural depth at connections, and appreciating the cost and programme implications of different connection details. Connection design is typically the responsibility of the structural engineer or the steelwork fabricator's design office, but the architect must understand the design intent, spatial implications, and aesthetic consequences. ## Bolted Connections ### Bolt Grades and Properties Structural bolts are classified by grade, which defines their strength: | Bolt Grade | Yield Strength fyb (MPa) | Ultimate Strength fub (MPa) | Application | |---|---|---|---| | 4.6 | 240 | 400 | Light structures, secondary connections | | 5.6 | 300 | 500 | General non-preloaded connections | | 8.8 | 640 | 800 | Standard structural grade — most common | | 10.9 | 900 | 1000 | High-strength, preloaded connections | | A325 (ASTM) | 635 | 830 | US equivalent of 8.8 | | A490 (ASTM) | 895 | 1035 | US equivalent of 10.9 | **Grade interpretation (metric):** First number × 100 = ultimate strength (MPa); first × second × 10 = yield strength. Thus 8.8: fub = 800 MPa, fyb = 0.8 × 800 = 640 MPa. Common bolt diameters: M12, M16, M20, M24, M27, M30. M20 Grade 8.8 is the default choice for most structural connections. ### Bearing Type Connections In bearing connections (Category A per EC3), bolts resist shear by bearing against the bolt hole walls. Clearance holes (typically bolt diameter + 2mm) are used. The connection relies on bolt shear strength and plate bearing strength. **Bolt shear resistance (per bolt, per shear plane):** `Fv,Rd = αv × fub × A / γM2` Where αv = 0.6 for Grade 8.8 (shear through threaded portion) or 0.5 for Grade 10.9, A is the tensile stress area (threaded) or shank area (unthreaded), and γM2 = 1.25. **Single M20 Grade 8.8 bolt capacities (approximate):** - Shear resistance (single shear, threaded): ~94 kN - Shear resistance (double shear, threaded): ~188 kN - Bearing resistance: depends on plate thickness and end distance ### Slip-Critical Connections Slip-critical (preloaded/friction-grip) connections (Category B or C per EC3) resist loads through friction between the faying surfaces. Bolts are tightened to a specified preload force (typically 0.7 × fub × As), clamping the plates together. **Applications:** - Connections subject to fatigue loading - Connections subject to load reversal - Connections where slip would be detrimental (e.g., slots, oversize holes) - Seismic connections (some codes require slip-critical for critical elements) **Slip resistance per bolt:** `Fs,Rd = ks × n × μ × Fp,C / γM3` Where ks is the hole factor (1.0 for standard holes), n is the number of friction surfaces, μ is the slip factor (0.2-0.5 depending on surface preparation), and Fp,C is the preload force. Surface preparation classes: - Class A (μ = 0.50): Surfaces blasted with shot or grit, rust-free - Class B (μ = 0.40): Surfaces blasted and spray-metallised with aluminium or zinc - Class C (μ = 0.30): Surfaces cleaned by wire brushing or flame cleaning - Class D (μ = 0.20): Surfaces not treated ### Bolt Spacing and Edge Distances EC3 and AISC specify minimum and maximum bolt spacings: | Parameter | EC3 Minimum | EC3 Maximum | AISC Minimum | |---|---|---|---| | End distance (e₁) | 1.2d₀ | 4t + 40mm | Dependent on edge type | | Edge distance (e₂) | 1.2d₀ | 4t + 40mm | Dependent on edge type | | Pitch (p₁) | 2.2d₀ | min(14t, 200mm) | 2.667d | | Gauge (p₂) | 2.4d₀ | min(14t, 200mm) | 2.667d | Where d₀ is the bolt hole diameter and t is the thickness of the thinner connected plate. ### Bolt Group Analysis When a bolt group is subjected to eccentric loading (shear plus moment), the force on each bolt is determined by: 1. **Direct shear:** Equally distributed to all bolts: Fv = V / n 2. **Moment-induced shear:** Distributed in proportion to the distance from the bolt group centroid (instantaneous centre of rotation method for ultimate capacity) The critical bolt is the one with the maximum resultant force. The elastic method (conservative) assumes the moment-induced force is proportional to the distance from the centroid. ## Welded Connections ### Fillet Welds Fillet welds are the most common weld type in structural steelwork. They are triangular in cross-section and are placed at the junction of two surfaces, typically at right angles. **Throat thickness (a):** The minimum distance from the root to the face of the weld: a = 0.7 × leg length (for equal-leg fillet welds). **Design resistance per unit length (EC3):** `Fw,Rd = a × fvw,d` where `fvw,d = fu / (√3 × βw × γM2)` βw is the correlation factor (0.8 for S235, 0.85 for S275, 0.9 for S355). **Minimum and maximum leg lengths:** - Minimum: 3mm (or √t for t > 6mm, where t is the thinner plate) - Maximum: thickness of the thinner plate minus 2mm (for T-joints) or the plate thickness (for lap joints) ### Butt Welds Butt (groove) welds join plates aligned in the same plane. Types: - **Full penetration butt weld:** Weld extends through the full thickness. Strength equals the weaker plate — no further calculation needed - **Partial penetration butt weld:** Weld does not extend through the full thickness. Designed as a fillet weld with the effective throat equal to the penetration depth Full penetration butt welds are more reliable but require edge preparation (V, U, X, or K prep) and are more expensive due to the preparation and inspection requirements. ### Weld Symbols and Specification Weld symbols per ISO 2553 or AWS A2.4 convey: - Weld type (fillet, butt, plug, etc.) - Size (leg length or throat for fillet, penetration for butt) - Length and intermittent spacing - Location (arrow side or other side) - Supplementary symbols (grinding, backing, field weld, all around) The architect should be familiar with basic weld symbols, particularly for exposed steelwork where weld appearance is critical. Specify whether grinding flush is required for visual connections. ### Weld Inspection and Quality | Method | Abbreviation | Detects | Application | |---|---|---|---| | Visual inspection | VT | Surface defects, size | All welds | | Magnetic particle | MT | Surface and near-surface cracks | Ferromagnetic materials | | Dye penetrant | PT | Surface-breaking defects | Non-magnetic materials | | Ultrasonic | UT | Internal defects, lack of fusion | Full-penetration butt welds | | Radiographic | RT | Internal defects, porosity | Critical butt welds | Weld quality levels per ISO 5817: B (stringent), C (intermediate), D (moderate). Level B is typically specified for primary structural welds in buildings. ## Shear Connections ### Fin Plate Connections A flat plate welded to the supporting member (column or beam) and bolted to the web of the supported beam. - **Capacity:** Typically 50-600 kN (depending on plate size and number of bolts) - **Advantages:** Simple to fabricate, excellent erection tolerance (beam can be lowered into position from above), no site welding - **Detailing:** Plate thickness typically 8-12mm; 2-5 bolts in a single vertical line; long-slotted holes may be used for erection tolerance - **Design checks:** Bolt shear, plate bearing, plate bending, weld to supporting member, beam web bearing, block tearing ### Flexible End Plate Connections A thin end plate welded to the end of the supported beam in the workshop, then bolted to the supporting member on site. - **Capacity:** Typically 50-800 kN - **Advantages:** Both connection faces are prepared in the workshop; good erection alignment - **Detailing:** Plate thickness 8-12mm (thin plate ensures rotational flexibility); 2-6 bolts in one or two vertical lines - **Design checks:** Bolt shear, plate bending (yield line), weld, beam web ### Web Cleat Connections A pair of angles bolted to both the beam web and the supporting member. - **Capacity:** Typically 50-600 kN - **Advantages:** Fully bolted (no site welding), proven performance - **Disadvantages:** More bolts required (double-sided), more site-work than fin plates - **Detailing:** Angle legs typically 90×90 or 100×100mm; 2-6 bolts per leg ### Comparison of Shear Connection Types | Feature | Fin Plate | Flexible End Plate | Web Cleat | |---|---|---|---| | Workshop operations | Weld plate to support | Weld plate to beam | Drill holes only | | Site operations | Bolt beam to plate | Bolt plate to support | Bolt both sides | | Erection ease | Excellent (top entry) | Good | Moderate | | Rotational capacity | Good | Good | Good | | Tying force (robustness) | Moderate | Good | Good | | Relative cost | Lowest | Low-moderate | Moderate | ## Moment Connections ### Extended End Plate Connections An end plate extending beyond one or both flanges of the beam, welded to the beam in the workshop and bolted to the column flange on site. - **Configuration:** 4, 6, or 8 bolt arrangements - **Capacity:** Can develop the full moment capacity of the beam - **Design considerations:** End plate bending, bolt tension (prying action), column flange bending, column web in tension/compression/shear, beam flange and web welds - **Column stiffening:** May require stiffeners (horizontal plates welded between column flanges) to resist the concentrated forces from beam flanges ### Flush End Plate Connections Similar to extended end plate but the plate does not extend beyond the beam flanges. This limits the bolt group lever arm and thus the moment capacity. Suitable for semi-rigid or partial-strength connections in braced frames. ### Haunched Connections The beam depth is increased at the connection by welding a haunch (tapered section cut from a beam or plate) to the bottom flange. This increases the lever arm and the moment capacity. - **Common in portal frames** where rafter-to-column and rafter-to-rafter connections must resist large moments - **Haunch length:** Typically 10% of the rafter span - **Haunch depth:** Typically equal to the rafter depth (so the connection depth is approximately 2× the rafter depth) ### Welded Moment Connections Beam flanges welded directly to the column flange (full penetration butt welds) with the web bolted or welded via a shear tab. Common in the US (pre-Northridge connections were found to be brittle; post-Northridge details include weld access holes, improved weld procedures, and reduced beam sections — RBS or "dogbone"). ## Base Plate Connections ### Pinned Base Plates Used where the column base is designed as a pin (simple construction): - Plate thickness: typically 20-40mm - 2 or 4 anchor bolts (for erection stability and small shear/uplift) - Concrete bearing strength governs plate size - Plate area: A ≥ NEd / (0.567 × fck × γc) — where fck is the foundation concrete grade **Bearing pressure under the base plate:** - Effective bearing area considers the plate stiffness — only the area close to the column flanges and web is effective (effective area method per EC3) ### Moment-Resisting Base Plates Used for portal frame columns and moment-frame columns: - Larger plate with more anchor bolts (4-8 bolts minimum) - Bolt tension on one side resists overturning moment - Compression under the other flange - Thicker plate required (30-60mm typical) - Anchor bolts designed for tension (bolt pullout, concrete cone breakout per EN 1992-4) ### Anchor Bolt Design Anchor bolts transfer forces from the base plate to the foundation concrete: - **Cast-in anchors:** L-bolts or J-bolts cast into the foundation (most economical) - **Post-installed anchors:** Mechanical or chemical anchors drilled into hardened concrete - **Holding-down bolt assemblies:** Grade 4.6 or 8.8 bolts with washer plates and levelling nuts Design checks: steel tension, concrete pullout, concrete cone breakout, concrete splitting, steel shear, concrete pryout. ## Column Splices ### Bearing Splices When the upper and lower column sections are in contact and loads are primarily compressive: - Division plates or packing plates align the column sections - Bolts and splice plates maintain alignment and resist any tension or shear - Splice located 300-500mm above floor level for erection access - Cover plates on both flanges and the web ### Non-Bearing Splices When column sections are not in direct bearing contact (e.g., different serial sizes): - All forces transferred through bolts and splice plates - Full-strength splice required if the column is part of a moment frame - Division plate bridges the gap between column sections ## Bracing Connections Bracing members (typically angles, channels, or hollow sections) are connected through gusset plates: - **Gusset plate design:** Whitmore effective width for tension; Thornton method for compression (column analogy) - **Uniform Force Method (AISC):** Distributes brace force to the column and beam through the gusset without secondary moments when the gusset geometry satisfies specific proportional relationships - **Working point:** The intersection of member centrelines should coincide at a single point to avoid secondary moments (but eccentricity is sometimes intentional for ease of detailing) ## Practical Notes for Architects 1. **Connection depth affects floor zone:** Moment connections with haunches or extended end plates add depth at column locations — coordinate with ceiling zones and services 2. **Exposed connections** demand high-quality fabrication — specify weld quality, grinding requirements, and bolt appearance (e.g., countersunk bolts, dome-head bolts, or bolt caps) 3. **Simple vs moment connections:** Simple connections are cheaper and faster to erect but require bracing or shear walls for stability. Moment connections cost more but allow unbraced frames 4. **Fire protection** of connections is often overlooked — connections must achieve the same fire rating as the members they connect 5. **Erection sequence** influences connection type — fin plates allow beams to be lowered from above, which is often faster 6. **Hollow section connections** (CHS, SHS, RHS) have unique failure modes (chord face failure, punching shear, chord side wall) and often require specialist design 7. **Cost impact:** Connections typically represent 30-50% of the total steel fabrication cost ## Related Topics - [[Structural Steel Design]] - [[Steel Frame Systems]] - [[Structural Steel Properties]] - [[Seismic Design Principles]] ## References - EN 1993-1-8: Eurocode 3 — Design of Joints - AISC 360-22: Specification for Structural Steel Buildings - AISC Steel Construction Manual, 16th Edition - SCI/BCSA Publication P358: *Joints in Steel Construction: Simple Joints* - SCI/BCSA Publication P398: *Joints in Steel Construction: Moment-Resisting Joints* - Owens, G.W. and Cheal, B.D., *Structural Steelwork Connections*, Butterworths --- #structures #steel #connections #bolted #welded #moment-connections #shear-connections