Why is the reported joist allowable load less than the value in the SJI catalog?

In the SJI Catalog only one deflection limit is considered. The live load that results in an L/360 deflection is generally listed as a red number. Given this limit, the program can easily calculate a modified allowable load for any specified deflection limit under criteria – deflection criteria for non-composite members. In other words, if you set the live load deflection limit to L/180, you’ll get twice the allowable load reported in the catalog.

More commonly, it's the net total deflection limit, which defaults to L/240, that causes a discrepancy. Any time the ratio or (D+L)/L exceeds 1.5, this limit will control over the default Live load deflection limit. L/360. Just set the net total deflection limit for non-composite members to zero (0) leaving only the live load deflection limit of L/360 to match the allowable capacities from the SJI tables, exactly.

If you have additional non-composite steel beams that need to meet some specified net deflection limit, consider using the alternate criteria for those beams (or vice versa). You can assign which members use which criterion in the Modeler.

How can I force the program to select only K series joists?

Go to Criteria – Joist criteria and for the table selection, use “RAMSJIK.JST” (or a custom table that includes only K series joist data). The default table, “ramsji.jst”, includes k, lh and dlh joists.

Why doesn’t the program select a K series joist when the equivalent uniform loads are less than the allowable?

Before converting the applied loads to an equivalent uniform load, the program first checks to see that the actual loads never exceed the upper limit for K series joists, 550 plf. In other words, if you have a triangular load on a joist that tapers from 100 plf up to 560 plf, the program will never select a K series joist, even though the equivalent uniform load might be something like 400+ plf.

Why does the program design the joist as a special (SP)?

There are several reasons why a joist will be designated as special. The most common relates to the loading. If the real load varies significantly from uniform, then the joist will be designated SP. If the load is too far from uniform, the joist will not be designed at all. “xxGSP”. Refer to the RAM Steel Beam manual for details on how the real loads are converted into equivalent uniform loads and how the tolerances set in the joist criteria dialog box determine if the joist is SP.

This also happens when the slope of the joist exceeds ½” rise per 12” run, even if the load is perfectly uniform. Any joist exceeding 45 degrees is not designed at all.

Note: For moderate to severe slopes the user should consider the effects of thrust on the supporting structure as this is not considered within the program.

Why does the program call out joist girders starting with “xx”?

The program does not truly design joist girders, it simply labels them based on the depth and the uniformly spaced point loads applied. Any joist supporting other members (regularly spaced) is automatically deemed a joist girder. The first two characters in the joist girder designation are for the depth of the member. The only way to have these numbers called out is to assign a Max Depth restriction to the member in the Modeler using Layout - Joists - Size Restrictions.

See Also

Using SJI Virtual Joist Girders

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Structural Product TechNotes And FAQs