Frequently Asked Questions

Answers to your frequently asked question. Browse by topic or search for your question.


No, KCS joists must be reinforced if the concentrated load is between panel points just like all other joists, unless the load is less than or equal to 100 pounds.

For nominal concentrated loads between panel points, which have been accounted for in the specified uniform design loads, a “strut” to transfer the load to a panel point on the opposite chord shall not be required, provided the sum of the concentrated loads within a chord panel does not exceed 100 pounds and the attachments are concentric to the chord.

There are no published inertias for J-Series joists and each manufacturer used a different array of chord angles, so there could be a small variance. You can use the tabulated live load values for l/360 deflection and back into the required inertia. Or you can physically measure the top chord and bottom chord and calculate a value.


Each member company may use various products and geometry in their designs that will meet the performance requirements. Their proprietary design is submitted and approved by the SJI consulting engineer prior to such approval by the SJI.

This determined by the manufacturer and is design specific.

For K-series the maximum width is 6 inches. Other series vary by size and manufacturer.

See the section in the Specification for the Approximate Duct Opening Sizes chart.

Almost none. Joists are designed for efficiency and a very high strength to weight ratio.

That depends on the bearing depth and type of joist. The manufacturer can provide this information.

The chart below gives the dates for each joist series from the beginning of the Steel Joist Institute in 1928.


That depends on the loads and the configuration of the TCX. Refer to the Top Chord Extension section in the current Specification.

So that a moment is not developed in the joist or joist girder.

Slots are used due to fabrication and field tolerances. Also, hole to hole connections would encourage using joists to plumb the building.

Yes, but the minimum depths are 2.5” and 5” respectively.

Typical seat depth for short span joists (SJ, S, J, H & K) is 2 ½”. Typical seat depth for long span joists (Longspan, L, LA, LJ, DLJ, LH, DLH) is 5″. For DLH-Series with chord section 18 through 25, the seat depth is 7 ½”.

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