What is the difference between a simple and a moment connection?

A simple (shear) connection transfers vertical load while letting the beam end rotate, so it carries no meaningful moment. A moment connection holds the angle between the members, transferring bending as well as shear. AISC 360 classifies joints as simple, fully restrained, or partially restrained — the classification is a design decision, and the detail has to actually deliver the behaviour assumed.

Which steel connection is most common?

The shear tab, or single plate connection. A plate is shop-welded to the supporting member and the beam web is field-bolted to it. All the field bolts are on one side, which makes it fast to erect, and it covers the majority of beam-to-column and beam-to-girder framing in commercial buildings.

Are A325 and A490 bolts still current?

The bolts are current; the specification names changed. ASTM consolidated A325 and A490 into ASTM F3125 in 2015, so they are now F3125 Grade A325 and F3125 Grade A490. Both legacy designations remain in everyday use on drawings and in the shop, and the products are the same fasteners.

Can A490 bolts be galvanized?

No. A490 bolts are not to be galvanized because of the risk of hydrogen embrittlement. Where a high-strength bolt is needed in a corrosive environment, that constraint drives the material choice and needs resolving at detailing, not in the field.

What is a gusset plate?

A plate that connects bracing members into a beam-column joint and carries the brace force into the frame. Gussets are where braced-frame geometry most often goes wrong in detailing — the work point location determines whether the joint attracts moment it was not designed for, and the free edge has to be checked for buckling.

Reference

Structural steel connections

Steel members are sized by an engineer, but buildings are held together by the connections — and that is where most fabrication cost, most detailing time and most field trouble actually lives. This is every common connection type, what it does, and what goes wrong with it.

Simple (shear) connections

Transfer vertical load and let the beam end rotate. The connection is deliberately flexible — that flexibility is what keeps it from attracting moment.

Shear tab

Also called Single plate connection

A single plate shop-welded to the supporting member, with the beam web bolted to it in the field.

How it behaves: Transfers vertical shear while allowing the beam end to rotate. The plate's flexibility is what makes the connection behave as a pin.
Used for: The default beam-to-column and beam-to-girder connection in most commercial buildings. Fast to erect because the bolts are all on one side.
Watch for: Bolt edge distance and plate thickness govern. Conventional configurations have specific limits in the AISC manual — outside those, it becomes an extended configuration requiring separate checks.

Double angle

Also called Clip angles, framing angles

A pair of angles bolted or welded either side of the beam web, connecting to the supporting member.

How it behaves: Transfers shear with genuine rotational flexibility from the angle legs. One of the most predictable simple connections.
Used for: Beam-to-girder and beam-to-column framing, especially where the beam must be dropped in from above between existing members.
Watch for: Needs access to both sides of the web. Angle leg thickness controls flexibility — too thick and the connection starts attracting moment it was never designed for.

Single angle

One angle connecting the beam web to the support, bolted or welded.

How it behaves: Shear transfer with an eccentricity, because the load path is offset from the beam centreline.
Used for: Light beams and joists where a double angle is more connection than the load warrants.
Watch for: The eccentricity is real and must be designed for. Generally limited to lighter loads.

Shear end plate

A plate shop-welded across the beam end, then field-bolted to the supporting member.

How it behaves: Transfers shear. Kept thin so the plate can flex and the connection stays simple rather than becoming a moment connection.
Used for: Situations where shop welding is preferred over shop bolting, and beam length control is tight.
Watch for: Beam length becomes critical — the plate is welded at a fixed dimension, so there is no field adjustment. Fabrication tolerance matters more than with a shear tab.

Seated connection

Also called Seat angle, stiffened seat

An angle or bracket under the beam bottom flange carrying the reaction, with a top angle for stability only.

How it behaves: The seat carries vertical load in bearing. The top angle restrains the beam laterally and carries no design load.
Used for: Erection-friendly framing, connections into column webs, and heavier reactions where a stiffened seat is used.
Watch for: The top angle is a stability element, not a load path — it is a common field mistake to treat it as structural.

Moment connections

Hold the angle between members, transferring bending as well as shear. These are what make a moment frame resist lateral load.

Welded unreinforced flange

Also called WUF, WUF-W

Beam flanges welded directly to the column with complete joint penetration welds, web welded or bolted.

How it behaves: Fully restrained. The flange welds carry the moment couple; the web carries shear.
Used for: Moment frames resisting lateral load, and any frame where drift control depends on joint rigidity.
Watch for: Weld quality is the whole connection. CJP welds here are demand-critical in seismic applications and carry specific detailing, backing bar and inspection requirements.

Bolted flange plate

Also called BFP

Plates shop-welded to the column flanges, then field-bolted to the beam flanges.

How it behaves: Fully restrained, with the moment couple carried through the flange plates in tension and compression.
Used for: Moment frames where field welding is undesirable — cold weather, inspection cost, or schedule.
Watch for: Plate and bolt group sizing must develop the required moment. More material and more fabrication than a welded connection, bought in exchange for field simplicity.

Extended end plate

A plate welded to the beam end extending past one or both flanges, field-bolted to the column.

How it behaves: Fully or partially restrained depending on plate thickness and bolt layout. Prying action in the bolts is a governing consideration.
Used for: Moment frames, and portal-frame style construction where the whole connection can be shop-fabricated.
Watch for: Plate thickness and bolt pitch drive behaviour. Thin plate plus prying is a classic failure path.

Reduced beam section

Also called RBS, dogbone

Beam flanges deliberately trimmed a short distance from the column so a plastic hinge forms there.

How it behaves: Fully restrained at the joint, but designed to yield in the reduced section rather than at the weld.
Used for: Seismic moment frames. Developed in response to weld fractures observed after the 1994 Northridge earthquake.
Watch for: Cut geometry is prescribed, not arbitrary — the shape and location of the reduction are what make it work. It is a prequalified connection with specific requirements.

Base plates, gussets and splices

The joints that do not fit the beam-to-column pattern but appear on nearly every drawing set.

Column base plate

A plate welded to the column base, anchored to the foundation with anchor rods.

How it behaves: Transfers axial load in bearing on the grout and concrete. May be detailed as pinned or fixed depending on plate thickness and anchor layout.
Used for: Every column that meets a foundation.
Watch for: Anchor rods are typically ASTM F1554, in several grades. Setting tolerance and grout detailing cause more field problems than the plate design itself.

Gusset plate

A plate connecting bracing members into a beam-column joint, carrying brace forces into the frame.

How it behaves: Transfers axial brace force. Work-point geometry determines whether the joint attracts unintended moment.
Used for: Braced frames, both concentric and eccentric configurations.
Watch for: Buckling of the free edge, and the geometry of the work point. Gusset design is where bracing layouts most often unravel in detailing.

Column splice

A joint where one column section continues into another, usually a few feet above a floor level.

How it behaves: Transfers axial load, and moment where the frame requires it. May bear directly with finished ends, or transfer entirely through the splice material.
Used for: Multi-storey frames where column size steps down with height, and anywhere shipping length limits the piece.
Watch for: Whether the splice is a bearing joint or a full-transfer joint changes everything about the detail. Erection stability during the splice is a real hazard.

Structural bolt grades

ASTM consolidated the old A325 and A490 specifications into F3125 in 2015. Both legacy names are still in daily use on drawings and in the shop — the fasteners did not change, the specification number did.

Designation	Legacy name	Min. tensile	Notes
F3125 Grade A325	ASTM A325	120 ksi	The standard high-strength structural bolt. Medium-carbon steel, quenched and tempered.
F3125 Grade A490	ASTM A490	150 ksi	Higher strength alloy steel. Not to be galvanized — hydrogen embrittlement risk.
ASTM A307	—	60 ksi	Low-carbon. Secondary framing, non-structural attachments, and anchor applications — not a high-strength structural bolt.

High-strength bolted joints are governed by the RCSC Specification. Installation condition — snug-tight, pretensioned, or slip-critical — is a design requirement, not an installer's preference, and it belongs on the drawing.

Common questions

What is the difference between a simple and a moment connection?: A simple (shear) connection transfers vertical load while letting the beam end rotate, so it carries no meaningful moment. A moment connection holds the angle between the members, transferring bending as well as shear. AISC 360 classifies joints as simple, fully restrained, or partially restrained — the classification is a design decision, and the detail has to actually deliver the behaviour assumed.
Which steel connection is most common?: The shear tab, or single plate connection. A plate is shop-welded to the supporting member and the beam web is field-bolted to it. All the field bolts are on one side, which makes it fast to erect, and it covers the majority of beam-to-column and beam-to-girder framing in commercial buildings.
Are A325 and A490 bolts still current?: The bolts are current; the specification names changed. ASTM consolidated A325 and A490 into ASTM F3125 in 2015, so they are now F3125 Grade A325 and F3125 Grade A490. Both legacy designations remain in everyday use on drawings and in the shop, and the products are the same fasteners.
Can A490 bolts be galvanized?: No. A490 bolts are not to be galvanized because of the risk of hydrogen embrittlement. Where a high-strength bolt is needed in a corrosive environment, that constraint drives the material choice and needs resolving at detailing, not in the field.
What is a gusset plate?: A plate that connects bracing members into a beam-column joint and carries the brace force into the frame. Gussets are where braced-frame geometry most often goes wrong in detailing — the work point location determines whether the joint attracts moment it was not designed for, and the free edge has to be checked for buckling.

Scope of this page

This is orientation material for fabricators, detailers and project managers — what each connection is and how it behaves. It is not design guidance and carries no capacities or design equations. Connection design belongs to the engineer of record, working from AISC 360, the AISC Steel Construction Manual and the RCSC Specification.

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