FAQ – Engineering and Design

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What is the AISC criteria for analyzing a truss?
The SJI specification has some variations for determining the allowable stresses for the joist components, different from the AISC specification. Many of the software companies include these equations rather than the SJI variations, consequently the results of a SJI and AISC comparison don’t always match exactly – but they are very close.

 

How should seismic and wind loads be specified?
By providing a complete load diagram to the manufacturer. See the SJI Code Of Standard Practice for additional information.

 

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.

 

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?
WL2/8

 

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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