Concrete Slabs on Metal Deck governing code 2

[vgillespie]

Which code or standard governs the design for the concrete strength and control joint spacing for elevated slabs on metal deck? What is the minimum strength of concrete that can be used on a composite concrete slab on metal deck? What is the criteria for depth and spacing of control joints? Is all this information in SDI, I don't have the book. Is the SDI manual worth buying? Please note references for advice.

 

[JAE]

There is no specific "code" that covers concrete strength for concrete on metal deck. Usually, you will see between 3000 psi to 4000 psi used, with 3000 being more common.

Composite concrete slab on metal deck is similar but obviously affects your beam design. With a significant project, a 4000 psi mix may prove more economical than the 3000 but you'd have to do some trial designs and compare costs.

Control joints: Normally, we do not specify control joints in elevated slabs.

SDI puts out a Diaphragm Deck Manual is informative and gives you good background on diaphragm design of decks. Many of the deck suppliers put out good catalogs that summarize the info.

 

[vgillespie]

Low strength concrete on metal deck. I am working on a small building that has low concrete breaks. I specified 3000 psi but even after 54 days of curing and taking core samples for testing the strength is about 1800 psi. The beams will still work for stress without composite action but non composite deflection is a problem. I want to use the composite section to control deflection but can I by code design a composite beam with 1800 psi concrete?

 

[JAE]

You can design it, really, with any strength. With the low strength, two things occur:

1. Your concrete f'c = 1800 will result in a lower composite beam capacity and affect the shear strength per shear stud connector. This should all be included in your calculations.

2. Your concrete's low strength will also result in a lower Ec which is usually given as (wc)^1.5 x 33 c sqrt(f'c) where wc is your unit weight of concrete in lb/cu. ft. and f'c is in psi. The lower Ec will result in higher deflections (n = Es / Ec will be higher and your transformed section Ixx will be lower).

The composite section is stiffer than the non-composite section, even with the low strength, but you also may be approaching the compressive strength of the concrete.

 

[tw]

Hey JAE,

How do you handle misc. deck support around a rather large (say 32&quot concrete column in a composite slab? The column is continuous thru the floor. I get various answers on who is responsible for this material and design. Our specs & plans handles openings but I would like to make sure my package has this covered.

TW

 

[JAE]

Our spec (metal deck section 05310 in CSI format) usually includes items such as gage metal closure plates and misc. angles around elements that project through the deck and interupt the closure. We don't usually draw/design anything, such as closure plates around columns, but rather depend on the specs to refer to the need as it arises.

I agree, it can be a gray area and sometimes is missed by not-so-bright contractors - but most of them are aware of these misc. items and include them in their bid.

 

[honat]

Composite concrete deck

Can a girder parallel to the metal deck span be considered as composite and designed as such? How can one consider such girder as part of the shear transfer mechanism?

 

[JAE]

Yes it can be designed as composite.

The direction of the flutes of the deck only affect the shear stud capacity a bit (See AISC's specification for the adjustment factor).

 

[phdunn]

Vgillespie:
I have not see any control joints or shrinkage joionts in elevated slabs. I think this is because the steel and metal deck which is tied by the composite action to the concrete all thermally expands and shrinks at the same rate. WWF, metal or fiberglass fibers that you should add for conrete shinkage. Any construction joints should be off the centers of beams and girders, so the concrete will be in compression on the top. I have seen extra reinforcement placed above girders to distribute/ minimize cracking from the filler beam end rotation.

 

[phdunn]

i use ACI for concrete strength