Overview
The three types of internal force that a steel connection has to transmit are axial force, shear force and moment. Most connections carry two or more of them at once, so a connection is classified by the force it is primarily there to carry. That classification is one of the first decisions in connection design, because it sets which limit states govern and how the joint is detailed. The primary internal force is a common classification for steel connections, as discussed here.
Shear (simple) connections
Shear connections primarily carry shear, although they usually carry some axial load as well. They are the most common type of structural steel connection, and AISC 360-16 B3.4 calls them simple connections: they are detailed to transmit the beam end reaction and to rotate at the joint, so no end moment is assumed. Common shear connections include single plates, web angles and seat angles.
Note that "semi-rigid" is not another name for a shear connection, though older references use the two interchangeably. AISC 360-16 B3.4 splits connections into simple and moment, and a moment connection is then either fully restrained (FR) or partially restrained (PR). Semi-rigid corresponds to PR, which is a moment connection with some rotational flexibility, not a shear connection at all. Calling a shear tab semi-rigid implies it was designed to carry end moment, and it was not.
Axial connections
Axial connections primarily carry axial load, and include splices, bracing, truss connections and hangers. They are typically used to connect columns to columns, or beams to beams, sometimes between different section sizes.
The load direction matters more here than in the other two groups, because a member that is in tension under one load case is often in compression under another. A bracing connection sees the reversal every time the wind changes direction, so both cases have to be checked rather than just the larger one.
Moment (rigid) connections
Moment connections primarily carry moment, though they are usually designed to resist shear and axial load as well. They are referred to as rigid or fully restrained connections, and they are what allows a frame to resist lateral load by bending rather than by bracing. Common moment connections include directly welded members, flange plates and end plate connections.
A moment connection also brings the supporting member into the problem. The flange forces arrive on the column as a concentrated tension and compression pair, so the column has to be checked for local limit states such as web local yielding and flange local bending, and stiffened if it cannot take them. A shear connection never raises that question, which is a large part of why it is cheaper.
Designing Each Type
The three groups are checked against different limit states, so identifying which one you have is what tells you which checks you owe. SkyCiv Connection Design covers all three, to AISC 360-16 in ASD or LRFD, and returns every check with its utilization ratio and the code reference behind it.
Pick a configuration, enter your sections, fixtures and loads, and see which check governs. It runs in the browser, with nothing to install.
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