In structural engineering, a moment connection refers to a joint that allows the transfer of bending moment forces between a column and a beam or between any other two adjoining members. In this article, you will learn more about the moment connection, its types, moment connection vs shear connection, and its uses in steel construction.
More About Moment Connections
In addition to allowing the transfer of bending moment forces, moment connections transfer normal and shear forces. Thus, it provides one of the most robust joint types in the steel construction industry. Moreover, these joints attain higher levels of robustness by strengthening the connectivity of flanges using plate stiffeners, welds, and other fixtures.
A strong moment connection increases a joint’s rigidity in all rotational and translational directions, and consequently, its cost. Besides connecting steel members, a moment connection can also join a single member to a foundation. The diversity and robustness of this type of joint make it a popular choice for connections requiring significant strength. However, a designer must ensure that it is only deployed when necessary due to its significant cost difference when compared to other joint types.
Types of Moment Connections
When it comes to moment connections, there are several classifications depending on a variety of factors. Classification can be based on the cross-section of the members such as hollow sections, open sections, or both. Also, it can be based on the connecting elements such as welds, bolts, or a combination of both elements. In addition, it is common to find moment connection classification according to its suitability for seismic loading or not. This section focuses on the general classification of these joints.
Flange Plate Connection
Flange plates often serve in providing moment connections between columns and beams. Generally, there is a weld connection between the flange plates and the column while bolts connect the plates to the beam. However, to achieve full restraint and continuity, a weld connection is used throughout for joining the plates.
When this is the case, then the flange plates and the web shear tab/plate should be thicker than the beam flange and the beam web respectively. Thus, the plastic hinge shifts away from the column. Also, it is important to limit the beam flange thickness to minimize the residual stress in the welded joint. For fabrication of the column-flange plate joint, engineers specify either a double-J or bevel T-joint CJP groove weld. These types of welds minimize distortion, weld shrinkage, and residual stress in the joint. The flange plate to beam connection commonly uses fillet welds.
Through Plate Connection
This moment connection type entails passing a rigid plate through a slot in the column and welding them together. The fabricator then bolts the protruding end of the plate to the beam. This method provides additional stability and allows the joint to take more moment and shear loads than in other types of connections.
However, this comes at an additional fabrication cost. Through plates are like gusset plates, but by passing them through the column, they engage more of its cross-section in load resistance. Hence, it provides higher strength capacity.
Some concerns of note when using through plate connections include:
- The strength of the column can be greatly reduced due to the weld connection to the through plate, so exercise caution in weld execution and NDT.
- Thinner sections of a column are susceptible to shear tear out due to the all-around welding.
- Ensure provision of enough clearance between the web and the bolts to avoid material damage when attaching to the beam.
Collared Plate Connection
A collared plate connection is like a through plate connection in that they are both welded to the column and bolted to the beam. However, the difference is that the collared plate has a hole in which the column fits. So, there is no need for insertion through the column.
As a result, the load-carrying capacity increases without a detrimental effect on the strength of the column. But the collar plate suffers some form of weakening due to the cut out hole.
When using this type of moment connection, it is necessary to consider the overall reduction in stiffness due to the cut-out. Also, engineers should consider the detrimental effects of welding when connecting the plate to the column.
Having a hunched beam is another means of providing a moment connection between a beam and column. A typical hunched beam is one with a varying cross-sectional area, where the wider part is at the joint with the column. Achieving this cross-sectional variation could be via tapering or stepping.
Also, it is common to find this type of beam alongside a plate or other strength stiffeners to improve its performance. Generally, hunched beams replace prismatic beams when the span becomes excessive. In addition, they often serve aesthetic purposes on cantilever beams.
Directly Welded Connection
This is the simplest form of moment connection that involves a direct weld joint between the beam and column. Also, it is the weakest among all connection types. However, it sees wide use in lightweight frames, where design loads are relatively low. The shear tab/plate connection is a common application as shown in the figure below.
Moment Connection vs Shear Connection
Moment and shear connections are featured in nearly all steel and concrete structures, as they are critical to load-bearing. The table below highlights their fundamental differences.
|This transfers both shear and moment forces between adjoining elements.
|Transfers only shear load between elements.
|Moment connections can be described as rigid, stiff, or fixed joints.
|Shear connections can be described as pinned or hinged joints.
|Does not allow translation or rotation of the joint.
|Allows rotation but restricts translation at the joint.
|These are less common in industry because are needed only at joints with complex loading.
|Shear connections are more common in the construction industry, as they satisfy loading needs most times.
|Due to their robust fabrication process, they are more expensive.
|Their simple fabrication process makes this connection available at a relatively low cost.
Application in Steel Construction
Moment connections are common occurrences in the steel construction industry. Moreover, they are prevalent in structures such as multi-story buildings, bridges, and other structures where joints are subject to rotational loads. Although they are more expensive than other load-bearing joints, they offer robust designs and enable engineers to provide safe designs. Generally, designers use a single type of moment connection throughout a structure to simplify and speed up fabrication, as well as save cost.
But if the loading and shape of the joints/members are complex, and the demand for moment transfer increases, then there is a need for a combination of moment connections. For example, some joints deploy hunched beams, alongside flange plates and stiffeners as the diagram below shows.
Some of the common industry standards that provide guidelines for the design of these connections include ANSI/AISC 358, BS 5950, and Eurocode 3.