Guide

Structural steel connections and seismic detailing in BC

The steel members in a building, beams, columns, braces, get most of the attention, but the connections between them are where structures actually succeed or fail. A W12 beam is just a beam. The shear tab that bolts it to the column, the moment plate that welds the flange to the column face, the base plate that anchors the column to the foundation, those are the details that determine whether the frame behaves the way the engineer intended, especially during an earthquake.

This page is the technical reference for structural steel connections as we fabricate them at our Burnaby shop. The audience is structural engineers, architects specifying steel, GCs reviewing submittals, and technically-inclined building owners who want to understand what's actually in their steel frame. We cover connection types, bolt and weld specifications, seismic ductility requirements under CSA S16, and the fabrication realities that determine connection quality. For a broader overview of our structural steel work, see the residential and commercial steel pages.

Codes and standards governing connections

Every structural connection in a BC building is governed by a stack of codes and standards. The structural engineer designs to these; we fabricate to the engineer's design; and the inspector verifies compliance. The relevant standards for steel connections:

• CSA S16, Design of Steel Structures, the primary design standard. Covers connection design methods (bolt shear, bolt bearing, weld capacity), ductility classes for seismic lateral systems, capacity design requirements, and detailing rules for seismic connections.
• BC Building Code (BCBC), adopts the National Building Code seismic provisions and sets the design earthquake parameters for each location in BC. Metro Vancouver's seismic hazard values drive the force levels that connections must resist.
• CSA W59, Welded Steel Construction, governs welding procedures, welder qualifications, and weld acceptance criteria for structural steel. Specifies CJP and fillet weld requirements, preheat, interpass temperature, and NDT protocols.
• CSA W47.1, Certification of Companies for Fusion Welding of Steel, requires that the fabricator (our shop) be certified for the structural welding being performed. Our CWB certification qualifies us for all connection types on structural steel.
• CISC Handbook of Steel Construction, the practical reference that supplements CSA S16 with worked examples, standard connection details, and design aids. Most Canadian engineers design connections using CISC standard details as a starting point.

Shear connections, the workhorse

Shear connections transfer vertical load from a beam to a column or girder without restraining the beam end's rotation. They are the most common connection type in a steel frame, on a typical commercial building, 70 to 80% of all beam-to-column connections are shear connections.

Shear tab (single plate). A vertical plate shop-welded to the supporting member (column flange or girder web) with fillet welds, then field-bolted to the beam web. The simplest and most common shear connection. Standard CISC shear tabs use 3 to 5 bolts in a single vertical line, sized for the beam reaction. The plate is typically 3/8 or 1/2 inch thick, A325 bolts in standard or short-slotted holes. We fabricate hundreds of shear tabs per year, the cutting and welding is straightforward, the bolt holes are CNC-drilled or punched, and the quality control is primarily visual inspection of the shop weld and dimensional verification of the hole pattern.

Clip angle (double angle). Two angles bolted or welded to the beam web and bolted to the supporting member. More traditional than shear tabs, still used where the engineer prefers the redundancy of two connection elements. Fabrication involves cutting angles to length, punching or drilling bolt holes in both legs, and shop-welding to the beam web if specified (or field-bolting both legs).

Unstiffened seated connection. An angle or tee welded to the column with the horizontal leg projecting outward as a "seat" for the beam bottom flange. Used where erection convenience is a priority, the beam sits on the seat, making it easier for the ironworker to make the bolted connection at the web. Less common in modern fabrication but still specified on some projects.

Moment connections, where lateral resistance lives

Moment connections transfer bending moment in addition to shear, creating a rigid joint between beam and column. In a moment-resisting frame (MRF), these connections are what provides lateral stability against wind and earthquake loads. The frame resists lateral forces through bending in the beams and columns, with the moment connections keeping the joints rigid.

The fabrication intensity of a moment connection is significantly higher than a shear connection. A shear tab might be a single 3/8-inch plate with three bolts. A moment connection on the same beam includes:

• Flange plates or CJP welds connecting the beam flanges to the column flange, transferring the bending moment as a tension-compression couple.
• Shear tab or web plate connecting the beam web to the column, transferring vertical shear.
• Continuity plates (stiffeners) inside the column at the beam flange levels, preventing the column web from buckling under the concentrated flange forces.
• Doubler plates on the column web in the panel zone (the region between the beam flanges), adding shear capacity to the column web where the moment connection induces high panel zone shear.

For seismic moment frames, the connection is designed to develop the beam's probable plastic moment capacity, not just the design load, but the maximum moment the beam can develop at its plastic hinge location, including strain hardening. This capacity design approach means the connection is stronger than the beam, forcing the yielding to occur in the beam (a ductile, energy-dissipating event) rather than in the connection (a potentially brittle failure).

Weld types, CJP vs fillet

Two weld categories dominate structural steel connections: complete joint penetration (CJP) welds and fillet welds. The choice between them is an engineering decision based on the forces at the joint and the ductility requirements.

Fillet welds are triangular cross-section welds deposited in the corner formed by two surfaces. They don't penetrate into the base metal, they sit on the surface. Fillet welds are the default for shear connections, stiffener plates, gusset plates, and any connection where the weld force is less than the full capacity of the connected plate. They're faster to produce, require less joint preparation, and don't need backing bars or backgouging. Most fillet welds on structural connections are 6 to 10 mm leg size, specified on the shop drawing by the standard AWS weld symbol.

CJP welds fill the entire joint cross-section, developing the full strength of the connected material. They require joint preparation (beveled edges, ground root faces), backing bars or backgouging to ensure full root fusion, and mandatory NDT (ultrasonic testing) to verify that the weld is sound through its full depth. CJP welds are specified on moment connections at the beam flange-to-column flange joint, on column splices in seismic frames, and anywhere the engineer needs the weld to develop the full section capacity.

The cost difference is real. A CJP weld on a W18 beam flange takes 3 to 5 times the welding labour of a comparable fillet weld, plus the joint preparation, the backing bar installation, and the UT inspection. On a moment-framed building with 50 moment connections, the CJP welding labour is a significant portion of the total fabrication cost. Engineers specify CJP welds only where the structural demand requires it, but in Vancouver's seismic zone, that demand is present on every moment frame.

Bolt grades and installation methods

Structural bolting in Canada uses two standard high-strength bolt grades:

ASTM A325 (now covered under ASTM F3125 Grade A325), the standard structural bolt. 120 ksi minimum tensile strength for diameters up to 1 inch, 105 ksi for larger diameters. Used on the vast majority of shear connections and on the web-side of moment connections. Available in Type 1 (medium carbon steel) and Type 3 (weathering steel).

ASTM A490 (now covered under ASTM F3125 Grade A490), the higher-strength option. 150 ksi minimum tensile strength. Used where higher bolt capacity is needed, typically on braced-frame gusset connections, heavy moment connections, and situations where the bolt count needs to be minimized for space reasons. A490 bolts are not to be galvanized (the hydrogen embrittlement risk is too high at this strength level), which limits their use on exterior connections.

Installation method matters as much as bolt grade. The three methods defined in CSA S16 and the CISC bolt installation guide:

• Snug-tight, the bolt is tightened to the point where the plies are in firm contact. Adequate for shear connections and bearing-type connections where slip is acceptable. No special verification required.
• Turn-of-nut, after snug-tight, the nut is turned a specified additional fraction of a turn (typically 1/3 to 2/3 turn) to achieve the required pretension. Used on slip-critical connections and connections subject to vibration or load reversal.
• Tension-control (TC) bolts, bolts with a splined end that shears off at the calibrated pretension. The sheared spline provides visual verification that the bolt has been properly tensioned. Increasingly common on commercial projects because they simplify field verification.

Base plates and anchor bolts

The base plate is where the steel frame meets the concrete foundation. Every column sits on a base plate, a flat steel plate bolted to the foundation through cast-in or post-installed anchor bolts. The base plate distributes the column load over a larger area of concrete, and the anchor bolts resist any uplift or shear at the column base.

On gravity-only columns, the base plate design is straightforward: the plate size is governed by the concrete bearing capacity, and the anchor bolts are nominal (two or four bolts for alignment, not heavily loaded). On columns in a seismic lateral system, moment frame columns, braced frame columns, the base plate design becomes a critical connection. The anchors have to resist the tension from overturning under earthquake load, and the base plate has to transfer the moment from the column into the concrete foundation.

Seismic base plate design follows CSA S16 for the steel design and CSA A23.3 for the concrete anchorage. The anchor bolt pattern, edge distances, and embedment depth are all calculated to ensure the concrete doesn't fail in breakout before the anchor yields. Ductile anchor design, where the anchor steel yields before the concrete fails, is the preferred approach because it provides warning before failure and energy dissipation during shaking.

We fabricate base plates to the engineer's specification: plate size and thickness, bolt hole diameter and pattern, shear lugs if specified, and stiffener plates or gussets for moment-resisting bases. The base plate is shop-welded to the column end (or to a short column stub for separate shipping), drilled for the anchor bolt pattern, and shipped with the column. Alignment of the anchor bolt pattern with the foundation pour is one of the most coordination-sensitive details on a structural steel project, if the anchors are out of position, the steel doesn't fit. We provide anchor bolt templates and setting drawings to the concrete contractor to prevent misalignment.

Seismic detailing for Vancouver's high seismic zone

Metro Vancouver sits on the Cascadia subduction zone, one of the most seismically active regions in North America. The design earthquake for Metro Vancouver buildings produces significant horizontal accelerations that every steel frame and every connection must be designed to resist. The BC Building Code adopts the National Building Code seismic provisions, which define the hazard and the performance objectives; CSA S16 translates those objectives into specific steel design and detailing requirements.

The seismic provisions in CSA S16 are built on a hierarchy of ductility classes. Higher ductility classes allow lower design forces (the structure can dissipate energy through controlled yielding) but require stricter detailing. Lower ductility classes use higher design forces (the structure remains essentially elastic) but with simpler detailing. The engineer's choice of ductility class sets the entire tone of the connection detailing for the project.

Ductile moment frames (Type D MRF)

The highest ductility class for moment frames. Connections must develop the beam's probable plastic moment including strain hardening (1.1 R_y M_p). This means the connection is designed to be significantly stronger than the beam, forcing all inelastic action into the beam. Detailing requirements include: CJP welds on beam-to-column flange joints, specific weld access hole geometry, protected zones on the beam where no holes or attachments are permitted, continuity plates and doubler plates sized by capacity design, and panel zone checks. Reduced beam section (RBS) connections are standard practice for Type D MRFs, the reduced flanges create a predictable plastic hinge location away from the column face and the CJP weld.

Moderately ductile moment frames (Type MD MRF)

Similar requirements to Type D but with somewhat higher design forces and slightly relaxed detailing in some areas. Still requires CJP flange welds, continuity plates, and capacity-designed connections. Type MD is the most common moment frame class for mid-rise commercial buildings in Metro Vancouver, it balances ductility performance with fabrication economy.

Moderately ductile braced frames (Type MD CBF and EBF)

Concentrically braced frames (CBFs) resist lateral forces through diagonal braces that act in tension and compression. The connections at the brace-to-frame joints (gusset plates) must be detailed for the brace's expected capacity in both tension yielding and compression buckling. The gusset plate, its welds to the beam and column, and the brace-to-gusset connection are all sized by capacity design. Eccentrically braced frames (EBFs) use short "link" beams between the brace and the column that yield in shear or flexure during an earthquake, the link connections must accommodate the expected plastic rotation without failure.

Limited ductility and conventional construction

Lower ductility classes (Type LD moment frames, Type CC braced frames) use higher design forces and simpler connection detailing. These systems are permitted for shorter buildings and less critical occupancies. The connections still follow CSA S16 requirements but without the full capacity design regime of higher ductility classes. The fabrication is less intensive, fewer CJP welds, less NDT, but the member sizes are larger because the design forces are higher.

Capacity design, the organizing principle

Capacity design is the principle that runs through all seismic steel detailing. The idea is simple: choose where the structure will yield (dissipate energy) and protect everything else. In practice:

• Beams yield before columns, "strong column / weak beam." The column-to-beam strength ratio must exceed 1.0 at every joint.
• Connections are stronger than the members they connect, the connection must develop the member's probable plastic capacity, not just the design load.
• Braces yield or buckle before their gusset plates fail, the gusset is sized for the brace's expected capacity.
• Anchor bolts yield before the concrete breaks out, ductile anchor design ensures a warning mechanism before foundation failure.

For the fabricator, capacity design means that connection components, plates, welds, bolts, are sized larger than a pure design-load analysis would require. The extra material and welding is the price of seismic safety, and it's not optional in Vancouver's seismic zone. Every connection we fabricate for a seismic lateral system is built to the capacity design requirements specified by the structural engineer.

Reduced beam section (RBS) connections, post-Northridge standard

The 1994 Northridge earthquake in California revealed a widespread problem with pre-Northridge moment connections: the CJP weld at the beam flange-to-column flange joint was fracturing in a brittle mode during strong shaking. The welds were technically adequate by pre-1994 standards, but the stress concentration at the column face, combined with weld defects, backing bar notch effects, and high strain rates from earthquake loading, created a fracture-prone condition.

The solution, developed through extensive testing in the late 1990s, was the reduced beam section. By trimming the beam flanges in a radius cut about one beam depth away from the column face, the plastic hinge is forced to form at the reduced section, away from the column face and the CJP weld. The reduced section yields at a lower moment than the full beam section, protecting the weld from the extreme strain demands.

RBS fabrication requires precision. The flange cut is a radius (typically equal to or slightly less than the beam depth), and both flanges must be cut symmetrically. The cuts are made by CNC or plasma profile cutting and then ground smooth to remove notch effects that could initiate fatigue cracks. The remaining flange width at the reduced section is typically 60 to 70% of the original flange width, enough to carry the required moment while ensuring the hinge forms at the intended location. We fabricate RBS connections at our Burnaby shop using CNC profile cutting followed by hand grinding to the specified surface finish.

Weld quality, what we control in the shop

Connection quality is weld quality. A perfectly designed connection with a defective weld will not perform as intended. Our quality control on structural welds follows CSA W59 requirements and our CWB certification program:

• Welder qualification. Every welder performing structural work holds CWB qualification tickets for the specific weld positions and processes they perform. Qualifications are verified and maintained per CSA W47.1.
• Welding procedure specifications (WPS). Every structural weld type used in our shop has an approved WPS that defines the electrode, voltage, amperage, travel speed, preheat temperature, interpass temperature, and number of passes. The WPS is qualified by testing and approved under our CWB certification.
• Visual inspection. Every structural weld receives a visual inspection by a qualified inspector. The inspection verifies weld size, profile, undercut, porosity, spatter, and arc strikes per CSA W59 acceptance criteria.
• Non-destructive testing (NDT). CJP welds on seismic connections get ultrasonic testing (UT) as standard. The UT operator scans the full weld length to detect internal defects, lack of fusion, porosity, slag inclusions, that are invisible to visual inspection. Magnetic particle inspection (MPI) is used on weld surfaces to detect surface-breaking cracks. NDT scope and frequency are specified by the structural engineer or the project specification.
• Documentation. Weld maps, inspection reports, UT scan records, and welder identification are documented and retained as part of the project quality record. On commercial and institutional projects, these records are submitted to the engineer or third-party inspector.

From engineer's detail to shop floor

The path from the engineer's connection detail to a finished fabricated connection involves several translation steps, and errors at any point create problems downstream.

The engineer's structural drawing shows the connection conceptually, connection type, bolt grade and size, weld type and size, plate thickness. Our detailing team translates this into a shop drawing that shows everything the fabricator needs: exact plate dimensions, bolt hole locations (to the millimetre), weld lengths and sequences, cope dimensions, erection bolt locations, and match marks for field assembly. The engineer reviews and stamps the shop drawing. Then the shop floor works from the stamped shop drawing, cutting plates, drilling holes, fitting up members, welding, and inspecting.

The critical translation is from engineer's intent to shop drawing detail. A connection shown as "W18×50 to W14×90 moment connection, CJP flange welds, shear tab web connection" on the structural drawing becomes a multi-page shop drawing showing: column continuity plate thickness and welds, doubler plate if required, shear tab dimensions and hole pattern, beam cope geometry, weld access hole shape and dimensions, CJP weld joint preparation details, and the erection sequence. If the shop drawing misinterprets the engineer's intent, the fabricated connection won't perform as designed, and the error may not be caught until the steel is on site. This is why the submittal review cycle exists, and why we treat it as the most important quality gate in the project.

Getting the connection details right, what to send us

If you're an engineer, architect, or GC starting a structural steel project in Metro Vancouver that involves seismic detailing, here's what moves the fabrication conversation forward:

• Structural drawings showing the lateral system type and ductility class
• Connection details, even schematic-level details help us understand the scope
• Material specifications (steel grades, bolt grades, weld requirements)
• NDT scope and inspection requirements
• Whether the project involves AESS (architecturally exposed structural steel) at any connections

We review connection details for fabricability as part of our submittal process, flagging issues that are difficult to build, suggesting alternatives that achieve the same structural performance with less shop labour, and coordinating with the engineer to resolve questions before steel is cut. Send us the structural package and we'll start the review.

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