Simple Connections in Multi-Storey Steel Frames

The most common form of steel connections in steel-framed structures is simple connections. These are connections that are incapable of transmitting significant bending moment between the frame elements. They are designed to only resist shear and tie forces, in other words, they are very similar to pin joints. However, in contrast to purely pinned joints, simple connections have some capacity to resist nominal moments due to their geometry and assembly. This result in some bending moment being transferred through the connection. Thus a simple connection is best defined as a joint that doesn’t transmit significant moment and relatively free to rotate.

From the above definition, a simple connection can also be referred to as a nominally pinned joint. That is, a joint that can transmit nominal moment but is relatively free to rotate. However, a distinction must be made between the terms “Connection” and “Joints” as the two terms are not interchangeable. A joint is a point of intersection in a model while a connection is an assembly of components within a structure that connects elements together.

Classification of Steel Joints

Majority of the current code of practices classifies a joint in terms of it capacity to transmit moment. Simple joint are defined in terms of their inability to develop bending moments. According to BS-EN 1993-1-8:2005 a nominally pinned joint should be:

  • Capable of transmitting the internal forces, without developing significant moments.
  • Capable of accepting the resulting rotations under the design loads

When assessing steel models of steel-framed structures, it is common to assume that the joints within them are pinned or rigid. BS EN 1993-1-8 requires that all joints must be classified by stiffness or by strength. This determines what type of connection is required and by determining the joint category, the appropriate checks can be done.

Classification by Joint

This is based on comparing the initial rotational stiffness of a joint with the classification boundaries given in BS EN 1993-1-8. Alternatively, experiments may be carried out and the joints classified based on the result or based on previous satisfactory performances.

Classification by Stiffness

To classify a joint based on its strength, two criteria must be considered. The design moment resistance of the connection and the ability of the joint to rotate. For a joint to qualify as a simple nominally pinned joint, it design moment must be less than 25% of a fully rigid joint. In addition to this, the connection must be capable of rotating when actions are applied to it. Any joint that stops short of this requirement, is either a semi/fully rigid joint.

Types of Simple Connections

There are typically two types of simple connections used within steel braced frames. The endplates and the fin plate connections.

Endplate Connections

The endplates are the most popular type of simple connections. In these type of connections, an endplate is welded to the supported beam in a workshop. The steel beam is then bolted to the supporting steel beam or column on site.

The image shows a end plate steel simple connections
Figure 1: End Plate Steel Connections

End plate connections are further sub-divided into two types, the partial-depth end-plate or the full-depth end-plate. Full depth endplates extend the whole depth of the steel beam. While a partial depth end plate only extends a certain distance. The main advantage of the end plates is that, they provide 100% shear capacity. However, it disadvantage lies in the difficulty in erecting two sided connections due to problem of shared bolts.

Fin Plate Connections

The fin plate connection is the reverse of the end plate connection. It consist of a length of plate welded to the supporting member in the workshop, to which the supported beam web is bolted on site.

The image shows a fin plate steel simple connections
Figure 2: Fin Plate Steel Connections

The fin plate connection is also very popular as it is one of the quickest connection to erect. Another advantage of the fin connections is that it eliminates the problem of shared bolts in two-sided connections.

Design Procedure for Simple Connections

The design procedure of simple connection is a very rigorous and complex involving numerous design checks. When designing a connection to BS EN 1993-1-8, up to 16 checks will be required. These checks are explained in three steps:

  1. The first step is to ensure that the connection is detailed to behave like a simple connection. In other words, it must be incapable of developing significant moments.
  2. Determine how the actions applied to the connections is transmitted between the frame elements
  3. Check the resistance of every component of the connection relative to the design effect.

To ensure that these requirements for simple connections are met. Certain rules must be followed, this includes a recommendation on standard plate sizes, geometry and bolt sizes.

In summary, this post is just an introduction to the concept of simple connections. Their design will be covered in much detail in subsequent posts.

Also see: Steel Columns in Simple Construction

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