Where to align rebar in concrete slab on metal deck

[Roger Ellingson]

thread507-185763
Hello,
I am placing reinforcing bars during the setup for a 4" total thickness concrete slab on top of 1.5" LVL metal deck. It's a residential project but almost light commercial in scale. There are two trapezoidal shaped areas to pour, about 30'x20' and about 18'x25'. The slab is to function as a diaphragm. The engineer has approved size and spacing of the 3/8" reinforcing bar at 12" O.C. E.W. The deck flutes are 6" O.C. The concrete guy wants to add the fibers.

The metal deck runs the long dimension of both areas. The transverse bars are perpendicular to the long axis. My question is where to align the longitudinal bars? Should they align over a valley in the deck? Over a hat? Since the slabs are trapezoidal, they could even fan out from wide spacing to narrower. Does it matter?

Thank you for any additional insights beyond the referenced thread which was extremely good one on the subject area.
Roger.

 

[JAE]

Why don’t you ask the engineer? If they are the engineer of record you need to communicate with them as it sounds like the engineer isn’t successfully communicating with you.

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[Roger Ellingson]

Thanks JAE for reading. I have sent the request to the engineer also although he is on break and I am not sure he will be back at work before I need to get the bar cut and installed. I have done considerable searching on the question but unable to find much on the question. The Design Guide 18 for parking garages on metal deck which was referred to in the attached thread had the best info. It aligns the bar over the hats in the Appendix design example but does not comment on why the location. I figured there might be others out there interested in thinking about the bar placement so I also posed the question to this group. Again, I am not debating crack control, just want to learn if the bar alignment vs the flutes has significance structurally. Roger.

 

[JAE]

The location of the bars would be either for structural strength OR for crack control.

If structural strength, the location of the bar relative to the flutes probably doesn't matter.
What does matter is the vertical positioning of the bars to get to the desired depth, d, used by the design engineer in their calculations.
No design guide or online resource can tell you what "d" the engineer needs.
Having said that - if the engineer didn't designate on the plans the required vertical position, then the bars are either there only for crack control, or the engineer is a dunce.

If crack control - then generally the bars should be positioned in mid-depth or perhaps in the upper portion of the slab with a required top clearance....3/4" to 1 1/2" generally.



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[Roger Ellingson]

Thank you JAE for your comments. My engineer is great. I've been working with him for over 4 years on this project. He is employed by a reputable west coast engineering firm. Finding a structural engineer who is willing to work with a homeowner/designer/architect/general contractor was not easy. They have all the work they can take on out here in Portland, OR. I am the one who is the dunce, spending my retirement trying to build this concrete structure!

The engineer has specified 6x6 mesh or #4 16" O.C. E.W. I have always stayed away from mesh due to difficulty holding the mesh in position when pouring. With his approval I have substituted #3 bar @ 12" to help prevent telegraphing bars to the surface. The basic structural strength of my floor system is provided by OWSJ/decking/shear studs and concrete. In hopes of improving the aesthetics of the exposed concrete floor and wall appearance, let alone to help achieve an even floor thickness, I have divided the area into multiple pieces using metal formkey as both a screed rail and to help distract the eye from inevitable cracking. 'd' is half way through the slab, so 2" in my case, +/-1/2". The 2" 'd' also is what the 4" formkey was hole punched at.

So getting back to the bar placement question... I think you are correct the engineer has specified the reinforcement primarily to keep the cracked chunks together. There are places the bars are important structurally to pick up some shear loads along the walls where shafts penetrate the diaphragm but the majority of the reinforcement in the slab is to keep the cracked pieces together. But hey, if bar position relative flutes would improve the slab's structural performance, why not take it?

I appreciate you taking the time to respond to my question. In your opinion, bar alignment with the flutes probably does not matter structurally. This will make bar installation simpler.

Thank you.
Roger

 

[hokie66]

Doesn't your engineer carry a mobile (cell) phone? When I was practicing, I never minded a call with a good question about my designs.

 

[Roger Ellingson]

Hi hokie66, I would think he would but I have never considered calling him on one. I have called his office maybe once. I prefer to bother him via email. Even with email I do not expect him to respond immediately. when I first realized the building I had dreamed up and designed was not going to be of prescriptive design and everything was going to have to be stamped, I was scared to death I was not going to find an engineer willing to support my unusual inexperienced strongly design-build project methods. So anyway once I found one I try to respect his work/life situation. So far so good - I think we are both enjoying the project and pleased with how it is turning out. I might have led the reader to believe I was panicking on this issue. I am doing fine. Thinking through JAE's comments really helped me relax! Thanks for writing. Roger

 

[Roger Ellingson]

Lateral rebar position relative shear stud postion question.

Hi, I have another question regarding hte lateral position of the same rebar relative to the shear studs in a 4" metal decked, suspended composite slab. Does running 3/8" rebar right up against or very near 36/4 patterned 3" long 5/16" diameter shear studs have any affect on slab performance?

Since the stud spacing is the same as my bar spacing, there is opportunity to fix the distance between the studs and bars. Is this generally a good thing? Bad thing? Another doesn't matter situation?

Thanks again,
Roger.

 

[BAretired]

I don't recognize the acronym LVL. It appears to be similar to the Canam P-3015 composite deck shown in the attached file. The file indicates welded wire mesh located approximately 1.5" below the top of slab. Composite deck has small deformations pressed into the webs of the flutes which allow the deck to bond with the concrete. In addition to supporting the weight of the slab while curing, the deck acts as reinforcement for the slab after curing. From your photo, it appears that your deck is also a composite deck.



BA

 

[Roger Ellingson]

Hi BA,The Canam looks the same product or pretty much similar. I forgot what LVL stands for also! I think the industry also refers to it as 'B' deck. Mine happens to be from Vulcraft. Maybe it is their acronym. It has the embossments as you note. It's been years since I studied suspended concrete floors. Check out my memory: The shear stud attached to the top chord of the steel joist and subsequently embedded in the slab creates a 'T' structural element out of slab/joist connection. This T structure is much stronger than the simple 'I' type structure of the joist itself. The combined floor(which would included the deck)/joist establishes the composite floor assembly. You are right, the stay in place decking can also support typical construction loading and can reduce or eliminate the need for temporary shoring. Thanks for your interest. Roger

 

[EDub24]

I don’t think the horizontal placement is critical. As long as the bars have the correct spacing you should be fine. For the vertical distance I would place the bars as close to the top of deck as the code allows for concrete cover with the bars parallel to the flutes on the top and the perpendicular bars on bottom. You should verify this with the EOR or if there is a special inspector with him/her. You could also talk with the deck manufacturer.

 

[Roger Ellingson]

Thank you EDub24. I appreciate your thoughts on the matter. Since posting, I have continued to peruse online suspended deck reinforcement pictures also. Based upon yours and JAE's input and what I have seen online, my current plan is to align long bars with the valleys of the deck that do not have shear studs in them. I will run a perimeter bar 2" away from the edge pour stops. Since the 70' sidewalls are 6deg askew from the building centerline, I will deadhead the long bars when they reach this perimeter bar. Full grid pattern.

NEW PROBLEM:
Yesterday, I was mocking up the formkey, deck, slab bolster, and long and transverse bars. Turns out the SBU, Upper slab bolster, I special ordered is actually 1" stuff. I had specified 3/4" stuff. The bolsters were labeled 3/4". So now I will probably have an extra two week delay in setting the bars on the deck as it appears the correctly sized and labeled bolsters are out in Pennsylvania and I am in Oregon.

The SBU I purchased is like three number nine wires with intermediate wire cross/legs. They are 5' long and sit perpendicular to the flutes. My 3/8" long bar sits on them and the 3/8" crossbars hang below the long bars before threading through the formkey holes which are 2" down from the slab surface. I am trying to keep the bars mid slab as per engineer's design and recommendations from the concrete finisher. Any higher and I risk telegraphing and even cracking above the bars according to the finisher's experience. 1" dobies are too tall. I don't know of any other 3/4" risers the would work on my decked application.

Thank you for your interest.
Roger.

 

[Roger Ellingson]

Bolster update:
I am thinking about using 1/2" bar as a bar bolster on top of the deck. I have not seen it done before in any references but don't know why it would not work well and be cheap. The slab is sloped for drainage and within a year the floor will be weathered in. I have a couple hundred unused steel 15" long formkey stakes that are 1/2" thick that would also function well laid across the flutes but the 1/2 bar seems a better solution and is less expensive and easy to get locally. SO I think I am good on the bolster.
Roger.