WWF in concrete over metal deck

[jike]

Here is something that I always wondered about...

In the Vulcraft Deck catalog, for a concrete slab poured over metal deck, they sometimes recommend WWF that are less than the minimum ACI requirement for slabs.

Do some engineers use less than the ACI minimum? How is this justified?

Can the area of the composite metal deck be used to meet the ACI minimum steel requirement? What about form deck?

 

[ishvaaag]

I don't know within the specific US practice, but sheet metal forms of specific design (supported or not) are used to meet the flexural requirements of the slabs in building systems. You may find technical literature in the pdf in the web proving so, like the document

CIV 3221 Building Technology Course
Monash University
Australia

Composite Floor Slabs Lecture

 

[FFIELD]

The answer is yes as long as the steel deck forms a composite section with the concrete. It therefore becomes reinforcing steel.

 

[jike]

Perpendicular to the deck span, the composite metal deck cannot really work as rebar (because of the folds in the deck), even though parallel to the span it does. Does that mean you need more steel to meet the ACI minimum perpendicular to the span?

Kind of strange....

 

[mrengineer]

The book "Designing with Steel Joists, Joist Girders, Steel Deck" by James Fisher, pages 34-35 addresses this. It says "This approach to reinforcement may allow negative moment cracking to form over the supports. This is a serviceability concern, not a strength concern."

 

[NCIndiana]

The WWF is strictly to satisfy shrinkage requirements. If the metal deck is a composite member, I would include that in the area of steel resisting shrinkage of concrete. However, if the metal deck is strictly a form, the WWF becomes a more structural component, in the sense that, if the concrete is is not able to span between supports as a plain concrete, you would essentially be needing the WWF to act as required reinforcement, not just satisfying shrinkage. Do not underestimate the importance of WWF in non-composite decks.

 

[JAE]

jike -
I believe that the rows of design capacities given in the deck catalogs are there in case you want to check an older existing slab OR, it is to account for the fact that deck slabs are designed to allow some cracking at supports (as per mrengineer's comment above).

Also, you can always add fiber reinforcing - not for strength - but to miminize shrinkage cracking.

One of the "goals" of the ACI Chapter 7 shrinkage reinforcing is to help to keep the concrete slabs together and distribute concentrated loads across wider areas. The metal deck sort of does this.

The deck specification also addresses the fact that the designer can neglect the concrete in the ribs for temp/shrinkage calcs.

 

[SlideRuleEra]

jike - The reason that ACI reinforcing requirements do not apply is based on history. The Steel Deck Institute (since 1939) and the unrelated Steel Joist Institute (since 1929) have credentials that are probably just as good as the ACI's (for this special type of construction). They also provide a basic industry standard that has stood the test of time. Of course, over time, Nucor Steel (Vulcraft) has been a major contributor the the knowledge base for designing with both steel deck and steel joists.

Take a look at the free .pdf download titled "Designing With Steel Form Deck" at the Steel Joist Institute web site:

http://www.sdi.org/whites.htm

The Steel Joist Institute does not address your question but has the basic steel joist document for free .pdf download at:

http://www.steeljoist.org/publications/

They will make you register to get it, but that seems to be the only requirement.

As JAE says, if you use fiber reinforcement, be sure that it is ADDED not SUBSTITUTED for wire mesh. The referenced SDI paper addresses this subject.

 

[SlideRuleEra]

I "misspoke" in the above - "Designing With Steel Form Deck" is at the Steel Deck Institue web site - the web address is correct however.