Residential Pile Supported Slab Rein and Thickness

[flgulfcoasteng]

I’m running into some unusual designs and requests and would like to hear your opinions.

Background:
Along the southeast Florida gulf coast it is customary to use pile supported residential ground floor slabs. The most typical slab is a 5" RC slab with #3 at 12" O.C.E.W., centered, spanning between grade beams that in turn are supported on 10" SQ prestressed piles.

Issue:
Lately I have seen a few plans (unsigned, not sealed) that use a 4” pile supported slab with one layer of 6x6 W2.9xW2.9 WWF (half the area of steel of #3 at 12). Through conversations it appears to be somewhat common now. These lightly reinforced slabs span anywhere from 7 to 14 feet. Besides not meeting the min As of a structural slab, they do not, in my opinion, meet strength requirements (40 psf LL, 0 to 20 psf DL + self weight). I have seen this in several jurisdictions but the majority appear to involve projects in Marco Island, FL

Frustratingly, I have had some clients ask me why I detail a more expensive slab compared to what they are seeing next door and I suspect I am loosing some work over this. A concrete contractor acquaintance of mine confirmed that he is seeing this quite often these days from a number of architects and engineers but will not provide plans to me out of fear of loosing clients in a tight market (as well as some possible legal / contractural issues). I have not had the opportunity to get my hands on a sealed set.

Questions:
- Is the 4” slab truly inadequate? I consider pile supported slabs to be structural and to span between grade beams. Perhaps the new practice is to assume some soil bearing capacity for the slabs? I find this hard to apply as the piles are normally necessitated by poor organic soil layers.
- Should I endeavor to locate a set of signed plans and refer it to the board? I have a definite opinion but would like to hear your thoughts and experiences reporting issues / other engineers to the FBPE.
- Any other Florida engineers have experience with residential pile supported slabs? If so, what is customary in your area?

The practice seems obviously inadequate to me. However, the practice being employed by more than several professionals gives me pause, hence why I am here to vet this issue. Maybe I am missing something?

Thanks in advance!

 

[flgulfcoasteng]

FYI I ran this past two other engineers. They both were puzzled on how someone could justify the light slab designs. It appears they ate getting slabs on grade confused with structural slabs formed on grade. I am amazed that a PE would fail to see the distinction. Something is just not right here...

 

[flgulfcoasteng]

Are not ate... You have to love the iPhone autocorrect

 

[bylar]

I am in south Florida and I seldom see residential on piling.
I thought that all residential plans had to be sealed by an Eng or Arch. (I have been in NC the last 10 yeas)
That said the designs you are seeing are inadequate.
If it needed piles and grade beams then the soil does not support the slab.

 

[flgulfcoasteng]

bylar, you are correct. All residential structural plans have to be sealed for permitting.

Almost all of the inland structures are not on piles. The coastal lots are a mix depending on how the canals and lots were constructed. Areas like Marco Island were developed horizontally in the 60's using dredge and fill operations that built up lots using a mixture of sand and organic muck.

I agree 100% that it is either pile suipported or not but not a hybrid. Which leaves me wondering "Why is this comming up so often now"?

 

[ron9876]

I see mostly two way slabs without interior grade beams. Have to be designed to carry the loads.

Did the drawings that you mentioned have engineers or architects doing the engineering?

 

[flgulfcoasteng]

ron, the most recent drawings I was able to look at were from an engineer in Naples, another from an engineer in Lehigh Acres, and most from an architectural designer (not licensed) that offices in Golden Gate estates and Marco Island. It is interesting to note that the previously mentioned contractor told me that some of the plans form the Naples engineer use a 5” with #3 and some plans detail the pile supported slab as 4” w/ 1 layer of WWF. This happens to be one of the largest, if not the largest, structural firms in Collier County which surprises me. The architectural designer is not listed as licensed by the architectural board nor the FBPE and the plans that I have seen have no name or area reserved for signing and sealing. Maybe there is an engineer who ink stamps his info and then seals the plans; I can only guess at this point.

I wish I had possession of a set. The contractors who understand my frustrations are becoming leery of giving me any more information at this point as they are struggling to hang on and are concerned about the projects they have completed using the questionable plans. It is only a matter of time though until I have a set from a GC or homeowner asking for a renovation design.

 

[OHIOMatt]

Perhaps the piles are there to support columns, bearing walls, shear walls, ect. and the slab is simply floating independent of the rest of the structure. I have designed numerous commercial buildings in this manner.

It will settle more than a pile supported slab, but that does not mean that it will not work, if other items are detailed properly.

 

[flgulfcoasteng]

Matt, these foundations are laid out much differently than the commercial plans we would design with a floating first floor slab. The first indication is that the slab is poured monolithically with the grade beams and on top of the exterior masonry stemwall without an allowance for a bond break at the slab edges. More importantly though the grade beams are laid out continuously throughout the foundation, even in areas without bearing walls: i.e. garage, rear decks, and throughout the interior residence. In all regards it is laid out in a manner that has been typical for the area for 30 years with the exception of the thin lightly reinforced slab.

 

[TXStructural]

I think you are correct that a design which is on piers, even if placed on soil, should comply with design as a structural member. If it is not SOG, it has to comply with ACI 318.

A quick look around WRI's website found a SOG publication titled "DESIGN OF SLAB-ON-GROUND FOUNDATIONS" which has an example of a 4 inch thick slab reinforced using 6x6 W2.9xW2.9, spanning between grade beams spaced about 12 ft o.c. This is a likely source of the design. I only scanned the document, but do not see a structural calculation for the slab itself. They show the wire reinforcement to be mid-depth, and seem to be taking full advantage of continuity moments across the grade beams.

The publication is TF 700-R-07 (WRI/CRSI 81): Design of Slab-on-Ground Foundations, and can be downloaded from the page:

http://www.wirereinforcementinstitute.org/pages/pubs/structural.htm

 

[flgulfcoasteng]

TXStructural, thanks for the report link. It looks the charts and recomendations on page 12 for spans between grade beams are based upon experience for a SOG over clays (PI > 15) soils. Interesting document although I agree with you that in our case in FL we would need to comply with ACI 318.

 

[ron9876]

Likely know of several of the firms that you mentioned. A word of caution--don't mention a firm's name without having the opportunity to review their design first.

Nothing wrong with using WWF as reinforcement but it needs to provide the required capacity.

It is fairly straight forward to report something like this to the board.

 

[flgulfcoasteng]

Thanks ron. I'll keep that in mind.

 

[rowingengineer]

What it the ductility of WWF like in the US?

How could you do anything so vicious? It was easy my dear, don't forget I spent two years as a building contractor. - Priscilla Presley & Ricardo Montalban

 

[flgulfcoasteng]

Fairly ductile. Comaprable to standard rebar. Pherhaps ultimate strain at fracture of 6% to 15% if my poor memory is working. ASTM A82.

 

[TXStructural]

ASTM A1064 is the current standard for welded wire reinforcement. While A615 and A706 reinforcing bars have an elongation requirement, based on size and specification, of about 11-14% in 8 inches; welded wire reinforcement must comply with a reduction in area at fracture of 25-30%. These are not directly analogous, but are comparable.

Welded wire reinforcement is manufactured to work interchangeably with deformed steel reinforcing bars in many uses, and is particularly well suited to flat work, such as slabs and walls. Keep in mind that welded wire reinforcement can be manufactured up to about 14' x 50' using wire up to about 5/8" diameter, spaced 4" o.c. (yes, I have seen it, and yes, it is heavy, but it goes in fast.)