Steel Connections: What They Are and an Application Example for Calculation and Verification

In the collective imagination, a structure is almost always identified with a reinforced concrete structure. However, in recent times, an increasing number of designers are being fascinated by metal structures. When designing metal structures, it is necessary to carry out the verification of structural elements, particularly of steel connections, which remain a challenging problem to solve unless one decides to use a steel connection verification software that allows for free modeling of the connection and finite element analysis (FEM) to combine design freedom and calculation rigor.

If you also find yourself facing the daunting task of calculating steel connections, we recommend trying out a steel connection verification software to discover a simple and effective way to perform the calculations and verifications of steel connections.

Below you will find an overview of the characteristics of steel connections, their classifications, and I will provide you with a practical example of how to calculate steel connections.

What are steel connections?

A steel connection is the intersection between the axes of structural steel elements. Steel metal structures arise from the assembly of various structural elements that must be joined together through connections essential for the realization of the three-dimensional building organism. The structure usually consists of one-dimensional elements (beams and columns) and two-dimensional elements (plates) that can be semi-finished, drilled, and assembled in a location different from that of installation.

A classic example of a steel connection is given by the intersection between a column and one or more beams, the intersection of two or more beams, and finally, the connection of the vertical elements with the foundation structure.

Example of a steel connection

The study of steel connections plays a fundamental role in the overall stability of the structure. The joints must be designed and constructed consistently with the assumptions used in the global stability verification, so as to create constraints that perfectly adhere to what was hypothesized during the calculation phase. To this end, it is necessary for the structural designer to focus on the design and verification of such joints.

Types of structural joints: classification of steel connections

Joints are classified both based on the means of joining used and based on the connected structural elements.

Classification by means of joining used

The means of joining used within steel joints are classified as follows:

• bolted connections;
• welded connections;
• friction connections with high-strength bolts.

It is also possible to identify a further classification of steel connections based on stiffness and resistance.

Based on stiffness, joints can be classified as:

• hinged: this type of joint must be designed so that no significant moments can develop that could negatively affect the elements of the structure. A hinged joint allows for the relative rotation of the connected parts without inducing secondary moments;
• rigid: this joint must be designed so that its deformation does not have a significant influence on the distribution of internal forces and moments in the structure nor on the overall deformation. The deformations of rigid connections must be such that they do not reduce the strength of the structure by more than 5%. Rigid connections must be capable of transmitting the forces and moments calculated in the design;
• semi-rigid: a joint that does not meet the criteria for a rigid connection or a hinged connection must be classified as semi-rigid.

Based on resistance, joints can be classified as:

• hinged (joints): when the joint is able to transmit the design forces without inducing secondary moments within the structure. The rotational capacity must be sufficient to allow the development of the plastic hinge;
• fully restoring resistance: when the joint is able to transmit the ultimate stress characteristics of the least resistant element converging at the joint;
• partially restoring resistance: when the joint is able to transmit the design stress characteristics, but not the ultimate ones of the least resistant element converging at the joint.

Classification based on the connected structural elements

In the context of a possible classification of the joints that are created within a steel structure, it is possible to identify:

• beam-column connection: these are typically the site of plastic hinges in a potential collapse mechanism of the structure. Among the two elements converging at the joint, the column represents the weaker element due to instability phenomena. This connection often serves to transmit bending moments from one beam to the adjacent one, assigning the shear forces and a fraction of the bending moment to the column. Beam-column joints subjected to high bending moment values are typical of framed structures. For them, the joints can be:
  • fully restoring,
  • fully restoring only the bending capacities,
  • partially restoring with sufficient rotational capacity.

For the design of a seismic-resistant framed structure, a ductile mechanism must be achieved through the activation of plastic hinges located at the ends of the beams and at the base of the columns on the first floor, avoiding soft story mechanisms. In this case, the rules of beam-column hierarchy can be less stringent than the checks at the limit state of damage, as the seismic design of frames is conditioned by the limitations of lateral displacements, as well as by the limitations of destabilizing effects due to lateral loads. It is therefore necessary to create joints capable of ensuring high ductility and adequate flexural stiffness in cases where the limitation of lateral deformability is particularly restrictive.

• beam-beam connection: among the two elements converging at the joint, it is possible to identify, in most cases, a main beam and a secondary beam. Preferably, a hinged joint is created between the two converging elements using angle brackets and bolts. The connection between the secondary beam and the main beam through the hinged joint is capable of ensuring the transfer of shear stress from the web of the secondary beam to the web of the main beam. Depending on the elements constituting the connection, it is possible to create joints that fully restore both flexural and shear resistance, joints that fully restore only flexural resistance (with the necessary precautions), and joints that partially restore flexural resistance allowing for rotations.
  

  Example of beam-column joint

• bracing connections: some beam-column joints are made more complex by the presence of bracing diagonals. Their shape and type vary depending on the profiles used. Two types of braces can be identified:
  • Concentric braces: depending on the orientation of the diagonal and the type of connection introduced, the post-critical deformation of the diagonal can be characterized by the formation of plastic hinges at the midspan of the diagonal or even at both ends. The connections at the ends of the braces must be designed to promote instability in compression, which represents a typical mode of damage. The seismic energy within braced seismic-resistant structures is dissipated through the plasticization of the diagonals subjected to tension. The main types of connection for concentric braced structures are:
    • Diagonal-beam-column connection;
    • Diagonal-beam connections (inverted V braces);
    • Diagonal-diagonal connection (X braces).
  • Eccentric braces: the dissipation of seismic energy occurs through the plasticization of the link, while the diagonals of the brace must be designed to remain stable in the elastic range. Unlike concentric braces, the connections of the diagonals must be designed to constrain bending deformations both in-plane and out-of-plane. The main types of connection for eccentric braced structures are:
    • Diagonal-beam connection;
    • Link-column connection;
• column-foundation connections: the base joint of the columns can be subjected to compression, bending, and simultaneous shear action. The stresses to be considered in the design of the joint are those resulting from the adopted calculation scheme. A series of issues can be identified in modeling the base joint, such as verifying the geometric dimensions in plan of the base plate, the position and diameter of the anchor bolts, the sizing and thickness of the base plate and any ribs, and the transmission of shear forces.

Classification of steel connections based on transmitted forces

Depending on the stresses transmitted by the joint, it is possible to identify:

• shear connection: it is a joint that allows the transmission of shear forces between two elements through a bolted connection. They are generally used to connect beams with other beams or with columns (stresses in the joint: shear and tension);
• moment connection: it is a joint capable of transmitting the stresses due to the bending moment typically between a beam and a column. This type of connection represents high stiffness in terms of both translation and rotation;
• tension connection: it is a joint that allows the transmission of tensile stresses to the structure. A typical example can be given by the diagonals of bracing elements.

What type of verification should be performed on the elements of the steel joint

The verifications of the elements that constitute the steel joint to be performed are:

• bolt shear verification;
• bolt tension verification;
• verification of the connection plate and the connected element;
• punching verification of the connection plate and the connected element;
• shear and tension verification.

These verifications can be performed using two possible approaches: one that utilizes the formulations reported in the literature and the other that employs FEM analysis of the connection.

It should be emphasized that the simplified approach proposed in the literature is applicable only under predetermined and significantly binding geometric/design conditions and also leads to a non-negligible degree of approximation of the results.

Utilizing a finite element modeling of a steel connection, in addition to allowing maximum geometric freedom of the elements converging within the joint (after all, with FEM analysis “if you can draw it, you can calculate it!”), allows for optimization of the materials involved with significant savings also in economic terms (smaller plates in size and thickness, fewer bolts involved in the connection) and especially a saving in terms of hours of work by the structural engineer to design, calculate, and verify the connections.

Application example: calculation and verification of steel joints

This section illustrates a case study that involves the calculation and verification of a steel joint of a new construction structure. To perform the modeling of the steel structure, structural calculation software was used, while for the finite element modeling (FEM) of the steel joint, NeXus steel joint verification software was employed.

The building consists of two above-ground floors with a floor-to-floor height of 3.00 m. The floors are made of corrugated sheet with a total concrete thickness of 12 cm. The structure in question can be inscribed within a square with a side of 9.30 m.
The vertical load-bearing elements are made with HEA 260 profiles, the perimeter beams are IPE 220, box sections with rounded edges of section 90x90x4.0, and vertical braces with diagonals of section UPN 120 and equal-sided L profiles of section L80x80x10.

Model of the steel structure created with EdiLus FEM calculation software

After performing the structural calculation, we move on to the design and verification of the steel connections. We proceed with the calculation of a beam-column-bracing connection which, being a joint that does not fall within the simplified conditions reported in the literature, requires a thorough analysis for its verification. For this reason, we will make use of the finite element analysis (FEM) software for steel connections NeXus, leveraging the dynamic integration present within EdiLus.

First, we select the joint to be analyzed within EdiLus, after which we choose to integrate with NeXus. This functionality (available soon) allows us to import into NeXus the geometry of the converging members within the joint as well as the acting loads.

Import of geometry into NeXus

Within NeXus, it will be possible to edit the connection by making cuts between the converging elements, inserting plates, bolts, and welds. The revolutionary 3D input allows for the design of any type of steel connections easily and quickly.

Insertion of the elements that constitute the node

1. Insertion of cutting planes
2. Insertion of plates
3. Insertion of bolts
4. Insertion of welds

At the end of the element insertion operations, it is possible to view the final result.

Completed node model

Once the insertion of the objects that define the node is completed, it is possible to perform the calculation of the connection model and observe the calculation results.

Results of the calculation of steel nodes

1. Structural model
2. Calculation results: displacements
3. Calculation results: stresses
4. Calculation results: deformations

From the calculation results, it is evident that all checks are satisfied, fully complying with the regulatory indications (D.M. 2018 and Circular 2019).

Upon completion of the calculations, executive details, calculation tables, and reports will be generated. It will also be possible to return to EdiLus, incorporating the construction details and tables into the general model of the structure, which will be automatically integrated into the project documents.

By leveraging the dynamic integration between EdiLus and NeXus, it is possible to produce all the necessary documents for submission to the competent offices regarding the calculation results and their validity.

The combined use of EdiLus and NeXus by Acca software ensures that the structural designer can design any steel structure and its respective connections, from the simplest to the most complex!

Software Updates

The new version of NeXus brings with it a series of important updates that will make the software even more powerful and efficient.

The main added features include:

• non-linear analysis: thanks to the 3D model of the connection, it is now possible to quickly perform finite element calculations and analyze the elastoplastic behavior of the elements through non-linear calculations;
• selection of Eurocodes as calculation standards: it is now possible to select Eurocodes as calculation standards, applying the guidelines present in the annexes of various countries, including Europe, England, France, Germany, Portugal, Spain, Singapore, Croatia, and Slovenia;
• multicore parallel calculation for non-linear analysis: the new specification allows for the utilization of modern processors’ multicore capabilities, enabling simultaneous calculations across multiple processes. This significantly reduces calculation times, particularly during non-linear analysis.

If you want to fully understand how to leverage these capabilities for the design of steel connections, I recommend requesting a demo of the software for the verification of steel connections right away!