FAQ – Joists

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How can the correct joist series be determined by the date of original construction of the building in question?
The chart below gives the dates for each joist series from the beginning of the Steel Joist Institute in 1928.

1929-1958  SJ-Series 1953-1958 Longspans
1958-1961  S-Series 1958-1961 L-Series
1961-1978  J-Series 1961-1966 LA-Series
1961-1986 H-Series 1967-1978 LJ-Series
1986- K-Series 1970-1978 DLJ-Series
1962-Present LH-Series
1970-Present DLH-Series


Are bearing seat depths helpful in determining the joist series?
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”.


Can bearing seat depths vary from the K series and LH series joists?
Yes, but the minimum depths are 2.5” and 5” respectively.


Why can’t hole to hole bearing seat connections be used?
Slots are used due to fabrication and field tolerances. Also, hole to hole connections would encourage using joists to plumb the building.


Why are the bottom chord extensions not welded?
So that a moment is not developed in the joist or joist girder.


How far can I extend the top chord?
That depends on the loads and the configuration of the TCX. Refer to the Top Chord Extension section in the current Specification.


How large of an additional top chord axial load can I have in my joist?
That depends on the bearing depth and type of joist. The manufacturer can provide this information.


How much extra capacity is built into a joist?
Almost none. Joists are designed for efficiency and a very high strength to weight ratio.


What duct size can I fit through a joist or girder?
See the section in the Specification for the Approximate Duct Opening Sizes chart.


Chord widths for joists?
For K-series the maximum width is 6 inches. Other series vary by size and manufacturer.


End panel lengths and distance between panel points?
This determined by the manufacturer and is design specific.


Does the SJI publish or designate chord member sizes for particular joists?
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.


How do you calculate the maximum moment for a joist?


Where can I find moment of inertia for J-Series joists?
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.


Can K-Series joists accommodate point loads not at panel points?
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.


Can concentrated loads be placed between panel points on KCS joists without reinforcement?
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.


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