FAQ – Composite Joists

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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.