Slab on Metal Deck Placement - Maximum Allowable Concrete and FL Requirements 4

[TylerM94]

I’m working on a project with a senior PM in our office that’s composite steel framed construction with slab on metal deck. During the pre-construction meeting we had for slab placement the contractor took issue with our specification that states: the placing contractor should assume to add an additional 1.5” of concrete to account for deflection of the steel structure within a given bay and meet the desired floor elevation. The contractor said if we are placing concrete and we add an additional 1.5” of concrete in a bay and top of slab is not met, how much more can we add? It is their opinion the deck will locally deflect. My PM does not want to answer this question and wants the contractor to refer to the metal deck shop notes which say maximum unshored spans are based on SDI construction loading criteria of 20 PSF. My question is, isn’t this note referring to an allowable live load for the contractors and finishing equipment on the deck? The contractor insists this is a safety issue (how much concrete can we add - 2,3,4”) to meet top of slab.

The contractor also took issue with our FL requirements for slab on metal deck and said they can’t measure FL for unshored construction. My PM took the lead on the discussion and argued against this - insisting on a laser level to be used as opposed to the “stick method.” Since I’m new to the constructability side I was wondering if someone could help explain this to me as well as opine on what you would do if a contractor was pushing back on your anticipated deflection of the supporting structure.

 

[r13]

I didn't get deep into this article, but seems it spread the responsibility to maintain a leveled floor to both the designer (engineer) and the contractor. Also, adding concrete does not seem a good idea, as it might cause more problems down the road. Please read on. [link file:///C:/Users/Kai%20PC/Downloads/Concrete%20Construction%20Article%20PDF_%20Controlling%20Deflection%20of%20Composite%20Deck%20Slabs.pdf]Link[/url]

 

[JStructsteel]

1.5" additional seems like alot. Whats the spans of the deck and beams?

 

[Celt83]

...and top of slab is not met,...

First and foremost I would address this statement with all parties, there should be no expectation that the floor will have 0 deflection.

Floor Levelness (FL) is not applicable to unshored elevated construction, ACI 117 notes this, however Floor Flatness (FF) is.

My understanding is over pour of the slab on deck can serve two purposes:
1. To allow the contractor to get the minimum slab thickness everywhere understanding that the deck and beams will all deflect, while also not overloading the non-composite beams or deck.
2. To make-up some of the overall deflection in the pre-composite phase, and again while also not overloading the non-composite beams or deck.

If after over pouring the deck within the limits set the floors need additional leveling compound to meet floor finishing requirements then a lightweight self leveling compound is usually called for, in my experience. Since this compound is being added after the slab and beams are composite the additional weight is usually pretty well tolerated, just make sure the connection designs have taken it into account.

My Personal Open Source Structural Applications:

https://github.com/buddyd16/Structural-Engineering

Open Source Structural GitHub Group:

https://github.com/open-struct-engineer

 

[dik]

If you want less deflection, spec a L/360 or whatever...for a flat floor, best way is to spec a Face floor flatness. The issue is on the table, then the contractor should just follow the construction documents. From my project notes (I prefer notes to specs and have accumulated 400K of them over the decades)...

SLAB FLATNESS

SLAB FLATNESS SHALL BE [ 20/15 | 25/20 | 35/25 | 45/35 | 65/40 | 100/50 | FM1 | FM2 | FM3 ]

U/N FLOORS SHALL HAVE A DEGREE OF FLATNESS DEFINED BY THE FACE FF/FL NUMBER THAT SHALL NOT BE LESS THAN:
-20/15 MECHANICAL ROOMS, PARKING STRUCTURES, ETC.
-25/20 CARPETED AREAS OF COMMERCIAL OFFICE BUILDINGS
-35/25 TYPICAL WAREHOUSE FOR MODERATE/HEAVY RANDOM TRAFFIC, FACTORY, ETC.
-45/35 CONSIDERED 'FLAT' TYPICAL WAREHOUSE WITH AIR PALLET USE, ICE RINKS, ETC.
-65/40 CONSIDERED 'VERY FLAT'
-100/50 CONSIDERED 'SUPER FLAT'

ALTERNATIVELY,

THE MAX VERTICAL DEVIATION OVER A [24" | 600MM] WIDTH SHALL NOT EXCEED
-2.5mm (quality FM1)
-3.5mm (quality FM2)
-5.0mm (quality FM3)

THE MAXIMUM VERTICAL DEVIATION OVER A [10FT | 3.0M] WIDTH SHALL NOT EXCEED
-3.0mm (quality FM1)
-6.0mm (quality FM2)
-8.0mm (quality FM3)

FLATNESS TESTING MAY COMMENCE 16 HOURS AFTER SLAB FINISHING. FLATNESS TESTING SHALL BE COMPLETED WITHIN 72 HOURS OF SLAB FINISHING


Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[KootK]

I mostly agree with your contractor on this.

The contractor insists this is a safety issue (how much concrete can we add - 2,3,4”) to meet top of slab.

I agree that is a safety issue and, if your strategy is going to be to basically allow ponding to happen en route to achieving levelness, then you guys should do whatever leg work is required to ensure that that the amount of ponded concrete required to achieve that doesn't get anybody hurt. If there's a "note" somewhere that covers this, it must be one heck of a note.

My PM took the lead on the discussion and argued against this - insisting on a laser level to be used as opposed to the “stick method.”

Your PM's proposed method is theoretically possible. One could do laser leveling from a rigid datum like a column and then have the contactor just keep adding and leveling concrete until the desired FL criterion was reached. One can specify an FL number (ASTM) without reference to ACI 117. But, then, you'd also have to get into specifying related tolerances for the steel framing in addition to the concrete work in order to make it all come together. While all of this is possible, it would be costly and rarely justified when one could just, instead, use a leveling compound after the fact.

My gut feel is that your contractor is more knowledgeable about this than your PM and is probably just trying to gently tell you guys to switch course on your floor flatness strategy.

 

[TylerM94]

All, thank you much for the very helpful feedback.

I agree that is a safety issue

this is the most concerning aspect for me as a young engineer. The contractor has asked us two RFIs on this and we have referred them to the metal deck manufacturer. I am hoping the manufacturer can weigh in on this and provide the contractor guidance as my firm does not seem to want to qualify the amount of concrete that can be added, and I agree with others that 1.5” or 19 PSF is a significant added load to the deck. I’ll keep you posted on how this gets resolved and plan on attending our first slab placement.

 

[KootK]

The metal deck manufacturer can certainly tell you how much the deck will contribute to the ponding. Trouble, is, the beams and girders contribute to the ponding as well and I'd wager that the deck guys will be kicking that part of things back to your office.

 

[dik]

How doe the contractor propose to construct the floor? This is really a means and method item. How will he establish the finished floor elevation? by laser or by screed? and, if by screed, are they going from beam to beam. Most steel deck floors I've been involved with during the decade or two have used laser... and, it comes out pretty flat. It's not a tricky problem... a kazillion sq. ft. of floor, like this, are constructed annually. I don't think it's a safety issue... you've simply estimated an overage for him due to deflection/pondage... that can be an issue, but 1-1/2" of concrete over the area will go a long way to accommodate pondage.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[TylerM94]

How does the contractor propose to construct the floor

they are going to use a screed and motorized trowels. I believe their main concern is this: if the supporting steel structure deflects 2” at mid span and the 6” slab now needs to be 8” at mid span to account for the deflection of the steel frame, is the deck designed to support this? The deck manufacturer has stated that the deck is manufactured according to SDI Standard and referred us to Section 5 of the code on deflection. Commentary on section 5 states: “ "If the designer wants to include additional concrete loading on the deck because of frame deflection, the additional load should be shown on the design drawings or stated in the deck section of the contract documents.”

The way I interpret our Spec is that we are allowing for a max of 1.5” of concrete. Nothing over this. If the top of slab elevation is not met then leveling compound will be required...

 

[Enable]

This is really not that complicated of a situation. The contractor needs to be able to build the floor to meet the specified flatness requirements. The means and methods by which they achieve that is their sole responsibility. However, if the assembly has been designed to accommodate only 1.5” of deflection during placement, and the in-field deflection is greater than that, the contractor cannot both meet the specification for flatness and maintain maximum concrete thicknesses. Presumably the 1.5” was provisioned such that deflection in the assembly should not exceed that amount based on placement. If it does, that is on the designer.

Here is the way the conversation should go:
1. What is the required floor flatness requirements?
2. Can the contractors proposed method of placement theoretically meet such requirements?
3. What is the allowable amount of concrete beyond the specified thickness that we can add to make up for deflection?
4. If the deflection exceeds allowable what do you want the contractor to do (add as required or maintain 1.5” max and deal with later)?

BTW: in specifying such tolerances it is wise to consider method of placement because deflections can be materially different depending on how things are placed (more distributed placement as compared to bay by bay, etc). Also, leveling compounds have weight (often not much less than concrete) so saying you will level later with a compound is a bit strange (unless you have a very lightweight product in-mind…they exist but $$$).

 

[Settingsun]

I read your 1.5" specification as a cost allowance to avoid a claim for the extra concrete. It doesn't read to me as a prohibition on more, though the contractor doesn't know what the structural maximum/limit is.

 

[haynewp]

I have successfully required leveling before on a project and it went well. I added in for the additional concrete weight and was conservative in the additional weight for the deck and beam designs. I tried to require a laser screed level and they said they ended up using a spinning laser level instead of the Copperhead laser screed that I wanted them to use. It still came out very well for that floor and everyone was happy. The max amount to be added for that project was stated to be 1.25". I also required a pre-pour meeting for the procedures to be discussed and and make sure things were understood.

You have to figure in the deck deflection and check the deck for the added concrete along with the rest of the floor system. I kind of overrode ACI 117 and stated a deflection per length that is the goal (such as 1/8" difference per 10 feet of slab length), with a not to exceed amount of added to be concrete. I provided a note stating this and warned that exceeding the stated allowed additional amount could cause a collapse. However, I also had a good concrete sub on that project so a simple note like this may not always go as well. But I don't think that requiring this type of leveling is all that unusual anymore.

Without leveling, l/360 seems to be the most common deflection that is designed for in my experience. Since this is not codified, you may have problems convincing an unreasonable owner that the deflection is acceptable and somewhat industry standard. I had one contractor say that it was going to cause him problems installing the stud partitions due to the l/360 beam deflections. AISC Design Guide 3, however, suggests l/360 or a 1" max accumulated total in a bay (either accumulated applies). The 1" total accumulated can require some heavy beam sizes in my experience. Ponding is stated to still needed to be checked separately.

You could do the self leveling compound after the floors are poured like is suggested above, but I don't have experience in how well that works. I may try that in the future as it is still somewhat concerning that a crew may get over eager and cause a ponding collapse when attempting to level during the initial pour.

This is one of the better article I have found on the topic.

https://www.constructionspecifier.com/specifying-and-achieving-a-level-composite-steel-floor/

The below paper has a procedure at the end to determine deflection convergence of the ponding beams if you are interested.

https://ascelibrary.org/doi/pdf/10.1061/(ASCE)0733-9445(2002)128:9(1099)