Calculating Initial Stiffnesses S_j,ini and Automatic Transfer to Global Structure

Moment frame design according to AISC 341-16 is now possible in the Steel Design add-on of RFEM 6. The seismic design result is categorized into two sections: member requirements and connection requirements. This article covers the required strength of the connection. An example comparison of the results between RFEM and the AISC Seismic Design Manual [2] is presented.

In this article, a lap joint of a ZL purlin on a monopitch roof is modeled and designed using the Steel Joints add-on, and compared with the load-bearing capacity table of the manufacturer.

Steel connections in RFEM 6 can be created by simply entering predefined components in the Steel Joints add-on. The collection of these components is constantly being improved to make your work even easier even when modeling steel connections. In this article, the connection plate component is introduced as a component recently added to the add-on's library.

In the Steel Joints add-on, you can classify the joint stiffness.

In addition to the initial stiffness, the table also shows the limit values for hinged and rigid connections for the selected internal forces N, My, and/or Mz. The resulting classification is then displayed in tables as "hinged", "semi-rigid", or "rigid".

Go to Explanatory Video

In the "Steel Joints" add-on, you can consider preloaded bolts in all components during the calculation. You can easily activate the preloading using the check box in the bolt parameters, and it has an impact on the stress-strain analysis as well as the stiffness analysis.

• "Steel Joints" Add-on
• Explanatory Video: Preloaded Bolts for Steel Joints

Preloaded bolts are special bolts used in steel structures to generate a high clamping force between the connected structural components. This clamping force causes friction between the structural components, which allows for the transfer of forces.

Functionality
Preloaded bolts are tightened with a certain torque, causing them to stretch and generate a tensile force. This tensile force is transferred to the connected components and leads to a high clamping force. The clamping force prevents the connection from loosening and ensures safe force transmission.

Advantages

• High load-bearing capacity: Preloaded bolts can transfer large forces.
• Low deformation: They minimize the deformation of the connection.
• Fatigue strength: They are resistant to fatigue.
• Easy assembly: They are relatively easy to assemble and disassemble.

Analysis and Design
The calculation of preloaded bolts is performed in RFEM using the FE analysis model generated by the "Steel Joints" add-on. It takes into account the clamping force, friction between structural components, shear strength of bolts, and load-bearing capacity of the structural components. The design is carried out according to DIN EN 1993‑1‑8 (Eurocode 3) or the US standard ANSI/AISC 360‑16. You can save the created analysis model, including the results, and use it as an independent RFEM model.

The initial stiffness S_j,ini is a crucial parameter for evaluating whether a connection can be characterized as rigid, semi-rigid, or pinned.

In the "Steel Joints" add-on, you can calculate the initial stiffness S_j,ini according to Eurocode (EN 1993‑1‑8, Section 5.2.2) and AISC (AISC 360-16, Cl. E3.4) with regard to the internal forces N, M_y, and/or M_z.

The optional automatic transfer of initial stiffnesses allows for a directly transfer as member hinge stiffnesses in RFEM. The entire structure is then recalculated and the resulting internal forces are automatically adopted as loads in the analysis and design of the connection models.

This automated iteration process eliminates the need for manual export and import of data, reducing the amount of work and minimizing potential sources of error.

Explanatory Video: Calculation of Initial Stiffness Sj,ini