![]() Therefore, a VJG20-1 is assumed to weigh exactly the same as a VJG40-1. The only problem is that Virtual Joist Girders base their cross sectional area solely on the chord area. That shape will then be listed as the suggested shape as part of the redesign or optimization done by RISA-3D. So, if two shapes both satisfy stress or capacity checks, the program will choose whichever shape weighs less as the one which is considered to be more efficient. The RISA optimization is done based on cross sectional area. Note: This only appears when the Material is set to Hot Rolled Steel and the Member Type is set to Beam. A Virtual Joist Girder option will appear in the Design List as shown in the Draw Members dialog to the right. This design list is used as part of the RISA-3D optimization or redesign routines. There is a new design list used referencing all the shapes in the SJI Virtual Joist Girder table. The Virtual Joist Girder database will be listed as one of the database options on the Hot Rolled tab of the Shape Selection dialog as shown to the left. How to Use Virtual Joist Girders in RISA-3D Virtual Joist Girders are included as of RISA-3D v11 released December 2012. Can I Use Virtual Joist Girders in my Version of RISA-3D? This will lead to better communication between the EOR and the joist manufacturer, resulting in a quicker, more efficient design process. This should result in analysis assumptions for the strength and stiffness of the member that are closer to what the joist provider can realistically provide. The engineer should use these Virtual Joist Girders in their analysis model whenever the girders are part of a lateral frame. Link: SJI: Virtual Joist Girders Table When to Use Virtual Joist Girders Therefore, anything I say about RISA or opinions I give on the program should be viewed with the understanding that I have some bias on the subject.The Steel Joist Institute (SJI) has developed a Virtual Joist Girder table which converts common joist sizes into equivalent wide flange beams for use in RISA-3D. But, I now work for one of their competitors. However, my employment with RISA ended somewhat contentiously when they were purchased by a large multi-national company (Nementschuck.sp?). Obviously, I used to work for RISA (for 16 years). During much of my time at RISA, this would not have been an option. There may now be a way to force RISA to ignore these checks (setting the L_torque to zero). My thoughts are the following:Ī) The way the bracing of the top chord is entered is important.ī) SJI may not consider torsional buckling or flexural torsional buckling limit states of the chord. ![]() ![]() It's been a few years now, but my recollection is that there are often differences between what you can get with RISA (which is based on AISC codes) and what the SJI tables show. Instead, I would model it as a pinned-roller support (as you have) where the first diagonals frame right into that support.ĭuring my time as as the manager of the RISA Support group, I saw a number of people who were doing similar things with modeling joists like this. By that I mean that little length of top chord that goes into extreme bending. I probably would not model the little "joist seat" length.
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