Virtual joists and virtual joist girders

For Tekla Structural Designer 2025 Service Pack 2 and later, the joist design options for the US market have been significantly enhanced with the introduction of virtual joists and virtual joist girders.

What are virtual joists and virtual joist girders?

Virtual joists and virtual joist girders are approximate structural elements, developed by the Steel Joist Institute (SJI), that represent custom-designed trusses. They provide structural engineers with approximate section properties for use in building structural models, proving especially useful in the early stages of design and modeling to facilitate efficient structural analysis and member selection.

How they function in Tekla Structural Designer

Modeling: A new droplist has been added to the Joist command on the Model ribbon to allow you create the new virtual types.

Select Virtual > Standard to create a virtual joist, or Virtual > Girder to create a virtual joist girder. When either of these types is selected, the "Fabrication" option in the Properties window is set to "Virtual joist", the remaining behavior is similar to that for the existing joist types, with defaults primarily the same, except for the grade, section size, and size constraints.
- A new SJI VJ-50 steel grade has been created for virtual joists and virtual joist girders. This grade has specific properties, including a Young's Modulus of 29,000 ksi and a 15% higher density (575.424 lbf/ft3) than typical structural steel to account for the approximate weight of web members and miscellaneous materials.
- For virtual joists, the size constraints applied to autodesign are a minimum depth of 10 inches and a maximum of 120 inches.
- For virtual joist girders, the size constraints applied to autodesign are a minimum depth of 20 inches and a maximum of 120 inches.
- Continuous LTB restraint is automatically applied to virtual members in Tekla Structural Designer. This restraint cannot be edited and is therefore not visible in the member properties.
- As with existing joists and joist girders, only pin ended members can be designed. Virtual joist girders can have moment releases applied (in member properties) but they get flagged as beyond scope of design in that case.
Extensive section properties: The SJI provides comprehensive tables of approximate section properties. These include a total of 1236 virtual joists across 28 series and 3978 virtual joist girders across 51 series, available in both US and SI units. The properties are ordered by increasing moment of inertia (Ixx), as bending is the primary design component for joists.
Design implementation and checks: In Tekla Structural Designer, virtual joists and virtual joist girders are assumed to be plate doubly symmetric I section beams subject to AISC360 design as follows:
- Strength checks are performed for Major Shear and Major Flexure only.
- Other forces (axial force, minor shear, minor moment, and torsion) are reported with a warning if they exceed predefined "ignore forces below" limits, indicating they have not been designed for.
- Deflection checks are also performed, similar to regular plated beams.
- For virtual joists and virtual joist girders, initial sizing and design requires specific span-to-depth ratio checks (depth no more than Span/12 and no less than Span/24).
- Warnings are also triggered for: net uplift (bending moment reversal), and, steel grade other than SJI VJ-50
Model Connectivity: Virtual joists only permit members to connect at their ends. However, virtual joist girders allow connections along their length, similar to existing steel joist girders.
Graphical Representation: To visually distinguish them, virtual joists and virtual joist girders are shown with a shaded web plane without wires, indicating their virtual nature.
BIM Export (Initial Release): For the initial release, there is no direct transfer of virtual joists and virtual joist girders in BIM.

Important Considerations

Approximate properties: It is crucial to remember that the tabulated virtual joist and virtual joist girder section properties are approximate equivalent-beam section properties. They do not represent any specific final joist design and cannot be used to specify the final joist design requirements.
Communication with manufacturer: Good communication between the structural engineer and the joist manufacturer is vital, especially since these virtual properties are approximations.
Limitations: Please note that these virtual members are not designed for composite action, cantilevered joists, joist seats, double joist configurations, custom KCS joist customization, vibration checks, fire resistance, or joists with sloping top chords. Final designation of joists and joist girders to SJI Specification is not included in the current release.