FAQ

What kind of information is provided on the joist tag and is it helpful in identifying the capacity of the joist?
Typically, the joist tag provides information that is useful to the erector in matching a particular joist in the field to a location on the placing plan. If the placing plan or approved field use drawings from the joist manufacturer are available, then the tag information could be useful. If plans are not available, it is possible that the manufacturer may still have helpful information. When all else fails, contact the manufacturer.

 

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 the bridging be removed after the joists are erected, the deck is in place and construction of the building is completed?
The bridging that is required only for erection stability can be removed. However the bridging needed for lateral stability during the life of the joist cannot be removed without an analysis. Generally the deck provided the lateral stability of the top chord so it might be able to be removed. The bridging generally provides the lateral stability of the BC. The removal of this bridging will require an analysis of the loading as well as consultation with the manufacturer.

 

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.

 

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

 

When is uplift bridging needed?
Uplift bridging is required whenever the bottom chord goes into compression. This will be true if there are net concentrated up loads.

 

Do all bridging lines / rows need to be anchored?
All horizontal bridging lines need to be anchored at the ends either by attaching to a beam, wall anchors or with X bridging termination. Anchorage of the bridging is critical to its performance. We are often asked after the joists have been installed if the bridging can be removed to clear the way for mechanical ducts. In most all cases it can, especially if the bridging is X welded or X bolted and as long as the bridging in the adjacent spaces remains to supply the lateral stability to the joist. In the case of horizontal, removing the bridging and terminating the discontinuity with X welded is acceptable.

 

Is it required that top and bottom chord bridging lines be aligned?
No. The design by the manufacturer may not require that the top and bottom chord bridging be aligned. Special attention should be addressed to bridging anchorage / termination when lines are not aligned.

 

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.

 

What should happen if the bearing seat bolts are not installed?
The seat should be welded.

 

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 long does the primer last?
The standard shop paint is intended to protect the steel for only a short period of exposure in ordinary atmospheric condition and shall be considered an impermanent and provisional coating.

 

Is the shop-applied paint on a joist a universal primer?
Typically, no. But this is a question for the painter who will be applying additional paint to the joists. The joist manufacturer can supply product information to the painter for his use in determining the applicability of additional paint types and finishes.

 

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.

 

When bridging interferes with mechanical ducts, can I remove it?
It is possible but an analysis is required. The manufacturer can provide assistance with an alternate bridging layout.

 

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.