What is a Virtual Joist Table?
Open web steel joists (joists) are custom designed trusses designed and manufactured for use in specific locations on specific construction projects. The project structural engineer of record (SER) provides the joist designation as well as any special loads or other design criteria. The joist manufacturer designs and manufactures the joist to meet the specified project requirements.
There are many instances where the SER has a need to incorporate the joist into the structural model of the overall building design. For example, in designing a lateral load resisting frame using joists, the SER must include the joist in the frame model, and must provide member end-moments and member end-forces to the joist manufacturer for incorporation in the final joist design.
The process requires good communication between the SER and the joist manufacturer to ensure compatibility between the joist design and the overall frame design model. Historically, the coordination process has been hampered because the SER did not know, at the time of modeling the overall building structure, the design properties for the joist.
The Steel Joist Institute (SJI) has developed a table that provides the approximate section properties for virtual joists for use by the SER in preparing the building structural models.
The virtual joist section properties do not represent any specific joist design. Rather, they are equivalent-beam section properties based on top and bottom chord material sizes commonly available from most joist manufacturers and common relative stiffnesses of chord members to web members.
The tabulated virtual joists do not directly represent the final joist design and cannot be used to specify the joist design requirements. However, they do yield reasonably close approximations of the final joist chord area, effective moment of inertia, and weight, for use in the structural models.
The intent is for the virtual joist tables to be used in the design models through use of the member selection and code check functions of the SER structural design software of choice. Using these functions, the structural design software treats the virtual joists as though they were custom wide-flange beams, and makes appropriate member selections, as well as displaying section properties and calculating approximate weights.
Once an appropriate joist depth selection is made using the virtual joist tables, the SER must specify the joist design requirements using conventional joist nomenclature, as directed by SJI Standard Specifications and Code of Standard Practice, as well as any special loading requirements.
If the joist stiffness (effective moment of inertia) is significant to the overall building structural model (such as in a lateral load resisting frame), then the SER must specify the virtual joist moment of inertia as the “target” joist effective moment of inertia, along with directions to notify the SER if the final joist design effective moment of inertia varies by more than 10% from the “target” value.
These tables were specifically developed in the format used by STAAD, and have been extensively tested using STAAD-Pro. However, our expectation is that they should work equally well with other structural design software, once the table format has been updated, for compatibility. In addition to the text-format file used by STAAD, we are also providing the virtual joist table in Excel format, for easier adaptation to other systems.