FAQs – All Questions

Return to FAQ

What are the bridging requirements for double pitch joists?
For any joist where the center of gravity is above the support point, all rows of bridging should be bolted cross bridging.  The number of rows will vary with length, pitch and weight of joist.  A joist manufacturer can give direction on this.


What is the correct way to specify the depth of a special profile joist?
Single pitch = depth at mid-span
Double pitch (gable), bowstring, scissor, arch = depth at ridge


Can I use the bridging on special profile joists as bracing for the structure?
Bridging is only designed for stability of the joist system. Additional bracing requirements shall be designed and sized by the engineer of record. Bracing material is not supplied by joist manufacturer.


Can I use a designation from the SJI standard load table for my special profile joist?
SJI designations (e.g. 20K4, 44LH09) shall only be used for single pitch and double pitch joists if the pitch does not exceed 1/2 inch per foot.


How do I specify a special profile joist if an SJI designation is not valid?
Provide a detailed load diagram or specify the joist as ‘LOAD / LOAD’.


Should special profile joists be specified with standard SJI camber?
Standard SJI camber my not be ideal. Consider the transition between special profile joists and other areas. Specify zero camber, or specific camber requirements, as needed.


How do I resolve horizontal reactions on the structure from scissor and arch joists?
To remove this thrust force, a slip connection should be used at one end of the joist. If diaphragm forces collected in the joist top chord need to transfer to the structure, alternate which end of the joist has a slip connection.


Do I need to provide more than just the pitch or radius of the roof for special profile joists?
It is best if the structural drawings provide all necessary dimensions and joist depths. This ensures there are no questions or misunderstandings.


Can I use standard seat depths no matter the slope or pitch of my joist?
Seat depths at the high end of joists shall be increased according to SJI tables 2.2-2 & 2.2-3. This ensures proper clear bearing and working point.


What is the maximum depth and length for a special profile joist?
There are many factors that affect these limits, such as production shop capacity, job site access, and shipping limitations. Therefore, discuss options with your preferred joist manufacturer.


Can bolted bridging be welded in lieu of bolting?
Yes, Erection bridging must be installed prior to the hoisting cable being released, consequently if welding is being placed on erection bridging, the welding must be done prior to the hoisting cables being released.


How much force can a bridging resist?
Bridging is only designed for the lateral stability forces in the joists.  In general those values are in the SJI bridging tables in the specification.  If bridging is intended to brace other components of the structure the size and connections must be indicated on the contract documents.  Normally these systems are independent of the joist system.


What are some suggestions for Composite floor joist so that vibration is not a problem?
Floors with joist spacing’s of 4’ to 10’ tend to have increased total concrete slab depths and greater mass to resist floor vibration. SJI’s Technical Digest No. 5 Vibration of Steel Joist – Concrete Floor Slabs discusses in detail, methods for calculating vibration characteristics of CJ-Series steel joists. Calculation of vibration properties for SJI’s CJ – Series joists can readily be determined utilizing FloorVibe v3.0 by Structural Engineers, Inc.


Is there a difference in the vibration characteristics of a composite vs. noncomposite joist supported floor system?
Noncomposite and composite joists have similar vibration characteristics. Attachment of the steel deck to the supporting steel joists via self-drilling screws, pneumatic driven pins, or puddle welds provides composite action between the steel joist and concrete slab when the floor is subjected to human excitation. When calculating the vibration properties for a noncomposite joist per SJI’s Technical Digest No. 5 Vibration of Steel Joist – Concrete Floor Slabs, you assume the joists act compositely.


My CJ-Series joist spacing and total concrete slab depth are not shown in the CJ-Series Weight Tables. How can I estimate the weight and required number of shear studs for this joist?
There are several solutions to this question. One option is to contact a SJI member company providing the desired joist depth, span, spacing, floor deck, total slab depth, concrete unit weight, concrete compressive strength, and loading. A SJI CJ-Series joist design can easily be ran for you.

A second option is to go the SJI website and download the free SJI Floor Bay Analysis Tool. Utilizing this Excel based spreadsheet, you can input your specific CJ-Series design parameters and obtain an estimated CJ-Series weight and quantity of shear studs required.


Please offer suggestions on a preferred location for vierendeel openings within a CJ-Series steel joist.
Locating the vierendeel opening near the mid-span of a uniformly loaded CJ-Series joist will provide the most economical joist design. Differential shear across the joist chords will be at a minimum as one approaches the joist mid-span.


What is the minimum concrete cover on studs attached to composite joists?
It is generally suggested that the shear stud after welding have a minimum of 0.5 inch of concrete cover over the head of the shear stud.


Can SJI manufacturers provide BIM models for CJ-Series joists to assist with mechanical and plumbing coordination?
Yes, contact your local SJI member joist company requesting that they provide a BIM model containing the steel deck and steel joists.


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 is the primer/paint applied?
The typical shop applied primer is a dip applied, air dried paint.


What is the thickness of the primer?
The paint should be 1.0 Mil thick but may vary from as little as 0.8 mils to as much as 2.0 mils. However, since most open web steel joists and joist girders are painted with a dipping process the coating may not be uniform and may include drips, runs and sags.


Is the primer compatible with fireproofing coatings?
It has been found that the SSPC-15 joist primer allows for the adhesion of fireproofing coatings to the joist components. However, check with the specific joist manufacture if the primer coating used is compatible with the fireproofing materials.


Are you able to weld thru shop applied joist paint?
In AWS D1.1 Section 5.15 Welding to Base Metal, it states that one of the exceptions allowing welding is, “Mill scale that can withstand vigorous wire brushing, a thin rust-inhibitive coating, or anti-spatter compound may remain…”. This thin rust-inhibitive coating is essentially the same description of the paint and should be interpreted as such.


Can the joists be provided with different color shop paints?
The SJI member companies have moved to a standard gray shop primer that is a temporary rust-inhibitive coating.


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?


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.


Return to FAQ