Introduction to Virtual Joist Girders
Steel joists are sometimes used in moment frames for wind and in regions where the seismic loads are small. While the steel joists themselves are often designed by the joist manufacturer, the structural response of the system must be investigated and the columns and footings designed by the structural engineer. This gets complicated when the exact properties of the joists are unknown at the time that the structural analysis and design are performed. To assist in this effort the Steel Joist Institute (SJI) has created a table of Virtual Joist Girders, a series of joist members with a wide range of stiffnesses and capacities. These can be assigned to the frames in the structural model, with the sizes refined to satisfy deflection, drift and strength requirements. Based on the properties of the selected Virtual Joist Girders, the requirements for the design of the actual joist girders can be determined and specified.
The properties of the Virtual Joist Girders are equivalent to an I-shaped structural steel member. In the analysis software these are treated as if they are I-shaped structural steel beams, hence the term ‘Virtual’.
The data in the Virtual Joist Girder table has been converted to a format suitable for use in the RAM Structural System. The data was provided by SJI, and formatted to conform to the table format requirements of the RAM Structural System. This includes both a Master Table and a Steel Beam Design Table. Bentley does not warrant the suitability or accuracy of the Virtual Joist Girder table. The engineer should understand the purpose of the table, the assumptions made in creating the table, the limitations of the use of the table, and should verify the results to his/her satisfaction. Refer to the documentation on the Virtual Joist Girders produced by SJI, currently available on their website at www.steeljoist.org. Technical questions on the SJI Virtual Joist Girder table or the designs produced should be directed to SJI.
It is the engineer’s responsibility to use these tables properly; no warnings are given by the program if they are assigned to the wrong type of members, or to members with unsuitable properties, or with the wrong Criteria selections. It may not be apparent from the designs produced that there was an error in these selections.
To use the Virtual Joist Girder tables in the RAM Structural System, the following process is recommended:
Tables
In RAM Manager select the Master Table containing the Virtual Joist Girders using the Criteria – Master Steel Table command. The table containing those sections is called ramaiscwithvjg.tab; it is identical to the AISC shapes table ramaisc.tab except it contains the additional Virtual Joist Girder sections.
Next, select the Design Steel Table containing the Virtual Joist Girders using the Criteria – Design Steel Tables command. Select the Beam tab. The table containing those sections is called ramvjg.bms; select that table as the Alternate beam design table:
Modeling
In RAM Modeler, model the joist girder as a Noncomposite Steel beam, with Fy = 50 ksi. Do not model it as a Steel Joist (that will use the program’s Joist Girder selection routines, not the special Virtual Joist Girder table). Note that if the beam is assigned to be a Composite beam the resulting design will be unconservatively incorrect.
Using the Layout – Beams – Steel Table command, assign the girder to Use Alternate Steel Table. In this model only joist girders or Frame beams should be assigned to use this alternate table.
For best results assign a Maximum Depth restriction using the Layout – Beams – Size Restriction command, otherwise the sizes selected may be excessively deep (the table has joists with depths ranging from 20” to 120”). Setting a Maximum Span/Depth Ratio in RAM Steel Beam may also be helpful, although that Criteria option affects all beams, not just the Virtual Joist Girders. SJI recommends that the depth be no greater than 1/12 of the span and no less than 1/24 of the span (span/depth ratios between 12 and 24, with the most economical between 12 and 18).
SJI recommends that the member be designed assuming an unbraced length of zero. This can be approximated by assigning very close brace points to the girder using the Layout – Beams – Brace Points command. Note that if the actual distance between brace points is less than the value of Lp calculated for the size being investigated, the design is the same as if unbraced length is zero. This means that brace point spacing of between 2’ and 4’ (or more) is likely to be sufficient. Alternatively, for Frame Beams investigated in RAM Frame Steel – Standard Provisions mode the Criteria – Flange Bracing command can be used to specify that the Top and Bottom Flange are Continuously Braced. Be careful with this approach, however, because that criteria will apply to all Frame beams, not just those that have been assigned to use the Virtual Joist Girder table.
Gravity Steel Joist Girders
Using the Virtual Joist Girder tables is not generally recommended for standard steel joist girders carrying only vertical gravity loads. Normally these should be modeled as Steel Joists and then the program will determine the label, such as for example 18G4N2.5k. For irregularly load girders the program will merely call out the girder as xxGSP, for which the engineer is expected to supply the girder loads to the joist manufacturer. However, if there are cases where specific joist properties are needed, these Virtual Joist Girder tables can be assigned to gravity beams, and designs obtained.
In RAM Steel Beam select the steel code using the Criteria – Steel Design Codes command.
The table is only valid for AISC 360-05 and AISC 360-10; no other design codes should be specified.
Perform Design All. Unless explicit sizes have been assigned, optimized sizes based on gravity loads will automatically be selected:
These size labels do not directly represent the final Joist Girder design. See Specifying Steel Joist Girders below for information on specifying Joist Girders.
If assigned to gravity members, Virtual Joist Girder sizes will be included in the Takeoff, but it should be noted that the weight listed for these sizes does not include the weight of the web members and miscellaneous materials. These add approximately an additional 15% to the total weight.
Steel Joist Girders as Frame Members
The most common application of these Virtual Joist Girders will be as steel joists in moment frames. Create the model as instructed above. If preliminary Virtual Joist Girder sizes have not yet been assigned to the Frame beams, do so using the Assign – Beam – Size command in RAM Frame.
Or if preferred use the Layout – Beams – Assign Size command in the Modeler, which functions similarly.
Note that the Virtual Joist Girder sizes are included in the listing of I-section sizes as explained previously. The first number in the label is the girder depth. The second number in the label is a series number and has no physical meaning, although higher numbers within the same depth mean heavier joists. For example, VJG24-45 is 24” and is the 45th entry in the 24” girder series.
Note: In the 3D view, these Virtual Joist Girders will be displayed as Steel Beams, they will not be displayed as Steel Joists.
Analyze the frames. Verify that the drifts are acceptable; if not, assign larger Virtual Joist Girder sizes using the Assign Size command. Repeat until the drifts are acceptable.
After the drift requirements have been satisfied, the next step is to verify that the Virtual Joist Girder has adequate strength. To do this, go to the Steel – Standard module. For the steel design code select AISC 360-05 or AISC 360-10, ASD or LRFD. None of the other codes should be selected, as they may not produce correct results for these Virtual Joist Girders. Perform the Code check using the Process – Member Code Check command. This will identify the demand-to-capacity for that size. You can investigate the design of individual joists using the Process – Results – View/Update command. With that command different sizes can be Analyzed and investigated until the desired size is determined, which can be assigned back to the model using the Update Database command in View/Update, or by using the Assign Size command.
The details of the design can be seen in the Member Code Check report. Remember that the results shown here are as if the joist was an I-shape, but the data in the table has been manipulated such that the results from the Virtual Joist Girder being designed as a beam will closely approximate the design of the real joist girder of the same weight under those same loads.
Note that the Steel – Seismic Provisions mode is not applicable to Virtual Joist Girders.
Specifying Steel Joist Girders
As explained previously, these Virtual Joist Girders do not represent any specific Steel Joist Girder design. Rather, they are equivalent-beam section properties based on top and bottom chord material sizes commonly available from most Joist Manufacturers, and on common relative stiffnesses of chord members to web members, and they do yield reasonably close approximations of the final Joist Girder chord area, effective moment of inertia, and weight, for use in the structural models.
Once an appropriate Joist Girder depth selection is made using the Virtual Joist Girder tables, the SER must specify the Joist Girder design requirements using conventional Joist Girder nomenclature, as directed by SJI Standard Specifications and Code of Standard Practice, as well as any special loading requirements. If the Joist Girder 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 Girder moment of inertia as the “target” Joist Girder effective moment of inertia, along with directions to notify the SER if the final Joist Girder design moment of inertia varies by more than 10% from the “target” value.
On the structural drawings do not use the Virtual Joist Girder label; that may not have any meaning to the joist manufacturer. Call out a steel joist as required by the SJI Standard Specifications, indicate the depth and the required moment of inertia, and provide all of the pertinent loads and end moments so that the manufacturer can produce a joist of the necessary stiffness and strength. The gravity loads can be obtained by looking at the Gravity Loads report and the end moments can be obtained using the Member Forces report. The properties of the Virtual Joist Girder can be obtained by looking at the ramaiscwithvjg.bat file (which is located in the RAM Tables directory and can be opened using Notepad). The values in that table look like this:
The Area can be used to calculate the approximate weight of the steel joist girder that will be fabricated.
Again, it is important that the engineer understands the proper use of the Virtual Joist Girder tables. Refer to the documentation on the Virtual Joist Girders produced by SJI, currently available on their website at www.steeljoist.org.