Frequently Asked Questions
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FAQs
The small holes you are referring to are considered penetrations for plumbing or mechanical piping. If possible, these holes should be located about 1/4 – 1/3 of the joist spacing from a joist. For example, if the joist spacing is 6 feet (72″), the holes should be 18″ – 24″ from the joist. If the holes are core-bored after the concrete slab is cured, they must be located far enough away from the joist to ensure the boring does not cut the joist top chord.
The relative stiffness of the concrete slab when considered with the joist makes the concrete slab the structural element that will absorb the lateral loads instead of the CJ joist. The concrete slab provides the diaphragm resistance.
When calculating the span-to-depth ratio for checking against CJ 200-2015, Section 5.2, the joist span and depth are in inches.
The minimum deck height is 1″, reference CJ-Series specifications SJI 200-2015, Table 4.5-1 and the minimum total slab thickness is 2″ per Section 5.9.2. Make sure that the composite joist, deck, and slab system you chose can develop the required load.
No, if joist are composite, composite deck should be used.
The SJI has not done any research on using joists in partial composite design. Although design equations may be extrapolated for this application, the SJI does not have any confirmation of their applicability.
Smaller diameter welded studs (1/2″ and 5/8″) can be used. The welded stud diameter is based on the forces in the top chord as well as the top chord angle size. Table 4.4-1 and Section 4.5.4 of SJI Composite Joist specifications ANSI SJI 200-2015 contain the design requirement for welded studs.
Wind loads are not included because composite joists are generally used in floor systems where there should not be wind loads. They can be used in roofs, however, the EOR should note that the Factored Total Load include the wind load and the magnitude of this wind load.
The CJ Series joist designation consists of five (5) parts. An example is 30CJ1500/800/300. The first number, “30”, is the actual joist depth not including the concrete deck. The second part, “CJ”, identifies the joist series, The second number and third part, “1500”, is the Total Factored Load, in plf, on the Composite Joist. The third number and fourth part, “800”, is the Factored Live Load, in plf, on the Composite Joist. The last number and fifth part, “300”, is the Factored Dead Load on the Composite Joist. Note that the reason the dead load and live load do not add up to the total load is because the dead load on the Non-Composite joist is not included. This number is typically calculated by the joist manufacturer’s design program based on the information given. Note that all loads in the CJ-Series joist designation are FACTORED.
The composite joist load tables are all based on Factored (LRFD) loads.
The CJ-Series joist catalog and Technical Digest 13 Specification and Design of Composite Steel Joists are available on the SJI website:
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.
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.
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.
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.
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.
Yes, contact your local SJI member joist company requesting that they provide a BIM model containing the steel deck and steel joists.