Structure
Bolted and Welded Steel Connections: Following the Load Through the Joint
A load-path field guide to steel joints: every bolt, weld, plate and stiffener a force must cross to leave one member and enter the next.
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Follow the force, not the drawing
A connection is not a picture on a sheet; it is a relay. Something upstream is pushing, pulling or twisting, and that force has to leave one piece of steel, travel through everything the detailer put in its way, and arrive in the next piece without stopping. Every failure a steel joint has ever had is the same failure: the load reached a place it could not cross.
So walk it. Pick the biggest number at the joint - the beam end reaction, the brace axial, the flange couple from a moment frame - and follow it hand over hand: out of the member, into the angle or plate, through the bolt or the weld, into the plies that bear against it, across the plate body, through the second set of fasteners, into the supporting flange, and finally into whatever the column stands on.
Each heading below is one crossing. Detailers, fabricators and erectors each own a different stretch of the same path, which is why a joint can be drawn correctly, made correctly and still fail - the handoff between two competent parties went unchecked. Read a connection as a chain of crossings and the checks arrange themselves.
The first handoff: member face into connecting element
Force leaves the member through whatever the connection actually touches. A shear tab welded to a column flange takes the beam reaction out of the beam web only - the flanges contribute nothing there, which is why a coped beam with its top flange cut away still works and a beam with a damaged web does not. Single-angle and double-angle connections take it out of the web as well; a seated connection takes it out of the bottom flange in bearing.
Fit-up governs this first crossing more than any calculation. A gap behind a shear tab, an angle sprung open by a bent leg, a bolt group forced into position with a bull pin instead of a drift pin - each one means the plies are not sharing the way the detail assumed. Erectors know the sound of it: bolts that will not run down by hand, plates that spring when the wrench comes off.
Gauge lines and edge distances are the physical record of that assumption. When a fitter shifts a hole line to clear a stiffener, the load path moves with it, and eccentricity the design never carried appears in the plate. Fabrication and erection tolerances - straightness, plumbness, mill variation, the permitted setting of an anchor rod - are published in AISC 303, Code of Standard Practice for Steel Buildings and Bridges, and in EN 1090-2, Execution of steel structures and aluminium structures. Field fit that lands outside them is an RFI, not a judgement call.
Through the shank: bolts, plies and the hole they sit in
Once force is in the connecting element, the shank has to carry it. A bolt in a double-angle connection sees two shear planes; the same bolt in a single plate sees one, and it moves twice the stress for the same reaction. Whether the threads fall in the shear plane changes the strength again - the X and N designations on a bolt schedule are not decoration, and a longer grip with a shorter thread run is what puts the shank where the drawing says it is.
Bearing and tearout govern the plies, not the bolt. Steel in front of a bolt hole either crushes or tears out to the free edge, and which one happens depends on ply thickness, hole type, spacing and how much material sits between the hole and the plate edge. A short edge distance is the single most common thing that turns a nominally adequate bolt group into a plate that unzips - and it is nearly always created on site, by a fitter trimming a plate to clear something.
Hole type is part of the path. Standard, oversized, short-slotted and long-slotted holes each change the bearing area and the amount of movement before load is picked up, and slotted holes have an orientation the erector must respect. Bolt grades, assemblies and their marking are covered by ASTM F3125, Standard Specification for High Strength Structural Bolts and Assemblies; installation, joint type and faying-surface condition by the RCSC Specification for Structural Joints Using High-Strength Bolts. In a slip-critical joint the force crosses by friction before it ever touches the shank, so paint overspray, faying-surface condition and a missing hardened washer are structural defects, not housekeeping.
Through fused metal: what the throat actually carries
Welded crossings carry force through fused metal whose cross-section nobody can see. For a fillet, the throat does the work, not the leg the inspector measures - which is why an undersized leg costs proportionally more capacity than it looks like it should, and why a convex bead does not make up for a short one. Length matters equally: a return that was never wrapped, a crater at the end of a run, or a stop-start that left slag are all missing throat.
Transverse fillets are stronger than longitudinal ones, and several codes permit taking credit for the direction of loading. That credit belongs to the engineer, not the welder, and a field weld sized on the assumption of transverse loading in a joint actually loaded along its length has no reserve left. Where a bracket carries an eccentric load, the weld group behaves like a bolt group in the same geometry - the far end of the run does most of the work.
Procedure controls whether the metal is sound at all. Preheat, interpass temperature, filler classification and prequalified joint geometries live in AWS D1.1, Structural Welding Code - Steel; in Canada in CSA W59, Welded Steel Construction; in Australia and New Zealand in AS/NZS 1554.1, Structural steel welding. Which of those governs is set by the jurisdiction on the permit, not by the fabricator's habit. Weld access hole geometry, backing bar treatment and run-off tab removal are the details most often skipped in the field, and each of them removes throat or leaves a notch exactly where the stress concentrates.
Argument about a fillet weld happens with a tape and a gauge in hand at this exact crossing, so the capacity the leg and length are worth belongs on the same page rather than back in the office.
Nominal weld shear capacity
323 kN
This is the NOMINAL weld strength Rn from AISC 360 §J2.4. It is not a usable design capacity: apply φ = 0.75 for LRFD, or divide by Ω = 2.00 for ASD, and check the connected base metal separately — the base metal, not the weld, governs many joints.
- Effective throat thickness
- 0.22 in
- Capacity per mm of weld
- 1.64 kN/mm
For the dimensions entered, expect a nominal weld shear capacity of 323 kN. Moderate confidence — sound arithmetic, but allow for the variation any real site introduces. Set for United States. The market selector changes the units and the trade terminology; any standard behind the formula is cited under sources.
Add the equipment this sizes
This result is a specification — 323 kN — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Through the plate body: net section, block shear and the Whitmore width
Having crossed the fasteners, the load now has to travel through the plate itself, and a plate has three separate ways to refuse. Net section rupture takes the line of holes across the width. Block shear tears a chunk out - shear down one leg of the bolt pattern, tension across the other - and it punishes tight edge distances and short bolt patterns hardest. In compression, a gusset buckles over the effective width spreading from the connected length, the conventional thirty-degree spread that every brace connection is checked on.
Coped beams add a fourth. Cut the top flange away and the remaining web behaves like a plate cantilevering off the connection, prone to local buckling and to cracking from the re-entrant corner if the cope was flame-cut without a radius and left unground. A cope extended on site to clear a duct has changed the strength of that crossing, and nobody upstream knows it happened.
Look at the plate the way the force does. Draw the failure line by hand across the bolt pattern and ask what area remains on it; that thirty-second sketch on a scrap of plate catches more real problems than re-reading the connection schedule. Where the remaining area looks thin, the answer is usually another bolt row or a longer plate, not a bigger bolt.
The turn: when the path has to carry moment
Turn the path through ninety degrees and it becomes a couple. A moment connection does not carry moment as moment - it splits it into a tension force at one flange and a compression force at the other, and those are the numbers that actually cross the joint. In a bolted end-plate or flange-plate detail, the tension side is the whole problem: bolts in tension, a plate bending between them, and prying action that adds force the bolts never see on the drawing.
Bolt count on the tension flange sets the geometry of everything else - plate thickness, bolt gauge, whether stiffeners are needed, whether the flange plate will fit between the column flanges at all. Settling that number late means a fabricator re-detailing after material is already cut.
Seismic frames tighten every one of these crossings. Prequalified geometries, demand-critical welds, protected zones where nothing may be attached, and backing and tab removal requirements come from AISC 341, Seismic Provisions for Structural Steel Buildings, AISC 358, Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications, and AWS D1.8, Structural Welding Code - Seismic Supplement. Drilling a hanger through a protected zone severs a load path that was qualified by full-scale testing.
The moment has just become a flange force landing on a bolt group, and the bolt count that force demands is the one number that has to be settled before the fabricator cuts plate, so it is worth running here.
Bolts needed
8 bolts
A simplified equivalent-force method — a full moment connection design also checks bolt group eccentricity, prying action, and plate bending per AISC 360 Chapter J and should be verified by a structural engineer.
- Equivalent force at bolt group
- 375 kN
With the figures above, the bolts needed comes to 8. The method behind this is well established, though site conditions and material batches will move it somewhat. This is presented for United States. Building in another market? Change the selector above and the units and terminology follow.
Add the equipment this sizes
This result is a specification — 8 bolts — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Into the supporting member: flange, web and panel zone
Arriving is not the same as crossing. The supporting member has to accept the force locally: a column flange bends between its own web and the bolt lines, a girder web can crush or crumple under a concentrated force, and a moment-frame panel zone shears as a small independent element inside the column. Continuity plates and doubler plates exist for exactly these crossings, and they are the plates most often value-engineered out and most often missed in the field.
Web crippling and local yielding show up wherever a beam sits on a bearing plate or another beam frames in without a stiffener. On existing structures, look for the tell-tale wrinkle in a web under an old crane rail or a stacked mezzanine beam.
Skewed and sloping members complicate the arrival. Force arriving off-axis puts a component into the supporting member's weak direction, and bent plates, sloped shear tabs and skewed end plates all need the geometry checked in three dimensions rather than on the single elevation the shop drawing shows. Where a brace lands at a beam-to-column joint, three load paths meet in one gusset and the resultant has to be resolved before anyone argues about weld size.
Out of the column and into the concrete
Down the column the same force keeps travelling, and the last steel-to-concrete crossing has its own vocabulary. Axial load spreads through the base plate into bearing on the grout and concrete below, and the plate has to be thick enough to distribute it without dishing at the edges. Grout that was never fully packed under the plate - a common outcome when the shim stacks are tall and the mix is stiff - leaves the load riding on the shims alone.
Horizontal force is a separate question. Friction under the plate, a shear lug, or bearing of the anchor rods against the plate holes each carry it differently, and anchor holes in base plates are deliberately much larger than the rods, so plate washers must be welded to the plate before shear is assumed to cross that way.
Tension in an anchor rod pulls a cone of concrete with it. Breakout, pullout, side-face blowout and edge distance are governed by ACI 318, Building Code Requirements for Structural Concrete, in jurisdictions that adopt it, and by EN 1992-4, Design of concrete structures - Design of fastenings for use in concrete, elsewhere; the concrete design code named on the permit decides which. Anchor rods set out of position and then bent back into the plate holes with a torch have lost both their embedment geometry and their material properties.
Proving the crossing: pretension, procedure and inspection
Nothing above is real until somebody proves it. Snug-tight, pretensioned and slip-critical are three different joints, and only the drawing says which one sits in front of you. Pretension can be installed by turn-of-nut, calibrated wrench, twist-off-type tension-control assemblies or direct-tension indicators, each with its own pre-installation verification and its own field evidence - a matchmark, a snapped spline, a squashed washer bump.
Inspection of welds runs from visual, which catches undercut, undersize, overlap and craters, up through magnetic particle and ultrasonic testing on complete-joint-penetration welds. Acceptance criteria and the extent of testing are set by the welding code in force and by the contract documents together; a fabricator's internal standard overrides neither.
Bolts have field rules worth knowing cold: high-strength assemblies are supplied and stored as a lot with their lubricant intact, galvanised A490 assemblies are not permitted, and reuse is restricted in ways that depend on grade and coating. Anything heated, welded on, or driven with a hammer has left the specification. Documentation matters as much as the act - an inspection record that cannot say which joints were verified is a record a future engineer will treat as no record at all.
Where the path gets cut after erection
Load paths get severed after the structure is standing, usually by trades who have no reason to know they are doing it. Holes burned through a beam web for conduit, a hanger rod welded to a bottom flange, a cope extended for ductwork, a stiffener ground back for pipe clearance - each removes area from a crossing that was checked once and never re-checked.
Temporary work counts too. Erection lugs, tag-welded seats and lifting eyes left in place are notches on a member that may see fatigue or cyclic seismic demand, and the specification usually requires them removed and the surface ground flush. Field welding onto a member without knowing its grade and carbon equivalent risks cracking that will not show for months.
Shims and fills are the quiet one. Adding fillers between plies changes bolt behaviour, and beyond a certain thickness the connection needs extra fasteners or the fill developed by welding - the governing steel design code says which. Stacking finger shims to make a bad fit go away is a repair that hides its own evidence.
Reading a joint cold, in five minutes
Standing at a joint with nothing but a tape and a torch light, work in the same order the force does. Where does the load come from, and what part of the member is it actually leaving through? What is the first element it touches, and does that element sit flat against its neighbour with no gap the eye can catch?
Count fasteners and measure the two distances that matter - hole to hole, and hole to edge - then look for the shortest remaining material anywhere in the pattern. On welds, check length and returns before arguing about leg size; a full-length undersized fillet is a smaller problem than a correctly sized fillet that stops short.
Finish by looking at where the force leaves. Is there a stiffener where a concentrated force lands, is the supporting web unbuckled, does the base plate sit in solid grout with its washers welded? A joint that passes that walk-through will nearly always pass the calculation, and a joint that fails it will fail somewhere the calculation was never asked to look.
Walk the joint before you sign it off
Five checks that follow the same order the force travels, from the member face to the concrete. Take them with a tape, a fillet gauge and the connection schedule, and record what you measured rather than what the drawing said.
- Bolt assemblies, kept as a lot — Grade, length, washers and lubricant condition together; segregated by lot and protected from weather, with the pre-installation verification done on that lot.
- Hole-to-edge and hole-to-hole, measured on the steel — Not scaled off the drawing - any plate trimmed on site has a new edge distance and a new tearout check.
- Fillet length, returns and throat — Check the run end to end before arguing about leg size; craters, unwrapped returns and stop-starts are lost capacity.
- Continuity and doubler plates present — Confirm against the issued-for-construction detail wherever a concentrated force or a panel zone is involved; these are the first plates dropped in a cost exercise.
- Base plate grout and welded plate washers — Grout fully packed rather than bridging the shim stacks, washers welded before any shear is assumed to cross through the oversized anchor holes.
Opens the calculators above on one screen with the dimensions from this article already filled in. Quantities only — this site publishes no price list, because local prices vary too much to publish honestly.
Drawn from
- AISC 360, Specification for Structural Steel Buildings
- AISC 303, Code of Standard Practice for Steel Buildings and Bridges
- AISC 341, Seismic Provisions for Structural Steel Buildings
- AISC 358, Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications
- RCSC Specification for Structural Joints Using High-Strength Bolts
- ASTM F3125, Standard Specification for High Strength Structural Bolts and Assemblies
- AWS D1.1, Structural Welding Code - Steel
- AWS D1.8, Structural Welding Code - Seismic Supplement
- CSA S16, Design of Steel Structures
- CSA W59, Welded Steel Construction
- EN 1993-1-8, Eurocode 3: Design of steel structures - Design of joints
- EN 1090-2, Execution of steel structures and aluminium structures - Technical requirements for steel structures
- AS 4100, Steel structures
- AS/NZS 1554.1, Structural steel welding - Welding of steel structures
- ACI 318, Building Code Requirements for Structural Concrete
- EN 1992-4, Design of concrete structures - Design of fastenings for use in concrete
Guidance, not a specification. Local codes, the engineer of record and the product manufacturer’s instructions govern where they differ from anything written here.