Doubling Steel Deck for Increased Strength?

[gmf]

Does anyone have information regarding the resistance capacity for 2 layers of 22 ga. steel deck vs. one, and more importantly, are there any suggestions/reference material for connecting both deck layers together to achieve composite action? Thank you.

 

[ishvaaag]

Since the geometry is quite the same for both decks the gain in strength and stiffness by joining the decks will be quite small. It is like when you use two parallel adjacent wood beams, the total strength is the sum of the two. Even if in this case one are atop the other, the gain in depth and overall mechanical properties are so minimal that the gain remains small.

To analyze the point welds you may resort to elastic analysis of the section, in similar way to that done for the web to flanges connection in built-up double tees. A equivalent simple alternative is considering the compression or tension forces at a band at some station, and lay the difference in forces for the bottom deck between the ends (of the band) as the required capacity to be sustained by the connection in the length of the band. This is an elastical approach that only care for the presumed standing forces (use factored forces when determining the so required capacity).

A plastic required capacity in shear may be generated in ways akin to that used to purvey studs in composite beams, i.e., the C-T pairs at ends and centers of beams are determined (when such points are at the respective capacity in accord with the kind of support), and the share of its algebraical sum that pertains to one of the decks is laid as the shear capacity required between ends and centers of the beams, both at the top plate, and at bottom.

 

[JAE]

2 layers of deck = 2 times the strength and stiffness.

I agree with ishvaaag that connecting to form some kind of composite action is a waste of time.

 

[cap4000]

I would think if you could sufficiently weld the 2 sheets
together so that the horizontal shear forces could be transferred, the net effect would be 2 times stronger than the single sheet. Tough to prove how much welding is actually necessary to achieve this 2 fold increase. Could you split up the existing spans in half and forget the welding??
Good Luck.

 

[Bbird]

Can I just correct the myth that doubling the steel deck does not double the strength or stiffness?

In a composite construction the steel deck is equivalent to part of the reinforcement. Thus doubling the steel deck effectively add another 100% reinforcement (assuming steel bars are not used and the two decks act as one unit).

The concrete has not changed. Before the second deck is introduced there exists a layer of concrete above the neutral axis with a compressive resistance equal in magnitude to the tensile force offered by the steel deck. The centroid of the concrete compression to the tensile resistance is the moment arm. The product of this moment arm and the compressive force (or tensile force as the two equal each other) will exactly balance the applied bending moment.

Now when the second deck is introduced the concrete must increase in area to gain twice amount of compression for balancing the doubled tension. It can only go one direction by moving the neutral axis downward. This effectively reduces the moment arm. The net result is an increase in load carrying capacity but a long way from doubling up in strength or stiffness.

I wrote this after checking with a computer program to see the movement of the neutral axis for both elastic and ultimate strength designs.

 

[JAE]

Bbird - I don't disagree with you in regards to concrete-metal deck composite action. I was responding to the original question that just dealt with the deck - no mention of concrete.

It is not a myth for just the deck (say a roof deck) where you use two layers. You get twice the stiffness and twice the load capacity.

 

[boo1]

GMF please provide clarification:
are the 2 sheets inverted?
is concrete cover provided?

 

[Bbird]

I am not familiar with the gauge 22 thickness and have to search the Internet to find out it is only 0.03" or 0.76mm.

Thus the deck has to be a profiles sheet used for composite construction and it would not be much good without concrete on top.

I use profile decking rarely but inverting one profile sheet against another could leave longtidinal voids in the slab and that can be dangerous because the corrosion risk from moisture inside the voids with such a thin metal sheel. The voids could also suffer distortion under load. The steel deck will not develop the expected tensile strength if it is not fully bonded to and restrained by concrete.

I am among those who have serious doubt on welding two profile sheets structurally to satisfy a composite design.

 

[gmf]

The 2 sheets are not inverted, and there is no concrete cover. The original situation described, involves a repair area approximately 5' x 8' with 4' joist spacing. The Contractor proposed the use of 2-22ga. steel deck, for material availability, rather than 18 ga. To consider the suggestion we needed to find a way for the 2 deck layers to sufficiently work together to resist loading.

 

[structuresguy]

Dbird: 22 gauge decking is used extensively for roof decks, particularly in areas without heavy snow loads. A 22 gauge 1.5" roof deck is good for 128 psf uniform load at 5' spans, and over 50 psf uniform load at 7' spans. I would say that this is pretty good for roofs.

I agree with those above who say that 2 sheets = 2 x strength. I don't think that trying to achieve a composite action is worth while. If I really wanted to tie the two sheets together, I would screw them together with TEK screws. Spacing would probably depend on profile and what made me "feel good" at the time. But on a standard "B" type roof deck, I would probably put a screw in the top of every flute and space them at 12" o.c.

gmf: Will the profiles nest together? Not all profiles nest well. if not, you could offset the flutes so that the bottom flute of the top sheet rests on the top flute of the bottom sheet. (Confused yet) Then screw the two sheets together where they meet. This would have teh advantage of doubling the total depth of the decking, combined with a composite action, you would end up with much more than twice the strength/stiffness. However, you would only have the end bearing web cripling strength of one sheet, instead of two if they were nested. In this configuration the interface shear could be guestimated by figuring M=wl2/8 = P * e. P = flange force, e= distance between flanges. This is based on assumption that top deck acts as top flange, bopttom deck acts as bottom flange. Then your shear interface force could be assumed to equal you flange force. This would probably work OK, but really seems way more work than practical.

 

[Bbird]

Just a few clarifications for those reading this post.

Structuresguy's description appears to be consistent with roof profile sheets normally installed directly to purlins. These steel sheets come with high quaity weather protection coating. By superimposing one sheet over the other the strength can be doubled.

The gmf's question suggested composite action. In UK this means concrete and steel acting compositely. My answer is therefor for bare metal profile sheet designed to have a concrete topping and even with small bar reinforcement. The concrete is assumed perfectly bonded to the steel sheet (by the corrugations of the profile). The steel deck is used as a permanent forwork (in addition to the structural consideration) permitting the omission of propping underneath. For composite action concrete area in tension is ignored for strength consideration and hence 2x strength concept fails to hold.

Please be aware of the difference of the two applications.