When are you required to reinforce a Slab on Grade ? 3

[JRLF]

Am not a S.E or a C.E. but an E.E, and I have a question that I would greatly appreciate being answered by knowledgeable Structural Engineers. Namely,in Chapter 19 of the UBC '97 under Seismic Requirements for Plain Concrete it states under Section 1922.10.3 that structural concrete members are not permitted in buildings in Seismic Zones 2,3 and 4, with the exception being "Non-structural slabs supported directly on the ground".

In sum, does this imply that all non-structural slabs supported on the ground,irrespective of the seismic zone do not have to be reinforced - even if they were - say 50 ft away from an active fault - where there have been 95 small to moderate earthquakes in the past 18 years ? If this is indeed the case, viz. that there is no code requirement for a nonstructural slab to ever be reinforced, no matter what the seismicity of the area and the proximity of a home to an active fault ,why is it that some structural engineers specify in their structural plans, that the interior slab of a home has to be reinforced, along with crack control joints ? Are they doing this solely because it is standard practice in the construction industry to reinforce a slab on grade and to install crack control joints, even though it costs several thousand dollars for the rebar or WWM ?

The other question that I have is : when you have a slab on grade with tall 4 ft bent "stem" rebar emanating from the perimeter and interior footings - before the concrete slab is poured- doesn't this tying of the slab to the footings - via this stem rebar -somehow make the slab "structural" such that the slab and the footings are now part of the lateral-force resisting system and hence via Section 1633 of the UBC '97 -provision must be made such that all building components in Seismic zones 2, 3, 4 must be designed to resist the effects of seismic forces ?

In sum,is one ever required by the UBC '97 code to reinforce a slab on grade ? And, if there is bent-over stem rebar coming up from the perimeter and interior footings, does this fact make the slab structural such that 1922.10.3 requires the slab to be reinforced? And, last - do the requirements of Chapter 16 - which take into consideration the seismic zone and the proximity of the site to active seismic sources take preference over what's stated in Section 1922.10.3 of the UBC '97 that non-structural slabs on grade in Seismic Zone 2,3,an4 do not have to be reinforced ?

As an E.E, who only took two C.E. courses - many, many years ago -I would greatly appreciate hearing a reply to my questions above.

Many thanks,
JRLF

 

[zstructural]

Not that this matters, but are they still using the '97 UBC somewhere?

It's tough to answer all your questions, since there are no "quick" answers. But, one underlying factor to what you're asking about is the strength of the base under the slab-on-grade. The strength and durability of a slab-on-grade matters less about the actual concrete than it does the base underneath. The rebar in the slab can serve many purposes. Crack control. To support the slab over a "weak" spot in the base. Rebar will also help the appearance by limiting the cracking. The other factor I would consider is the use of the slab. Is there forklift traffic? Vehicles? Storage racks? You mention a home, so I'm assuming light loading.

As for the seismic aspect, I don't think we know enough about the structure to answer those questions. You have some pretty "blanket" questions, and without knowing more about the structure, I don't think we could give a responsible answer. That being said, I don't regularly design in high seismic areas, so maybe someone else could speak more about that.

 

[JRLF]

Turns out the UBC'97 went out of existence on July 1, 2005,when the IBC 2003 came into effect, but the Bldg permit was pulled before that date to allow construction under the UBC'97.

Guess my major question is:

Does the 4ft stem rebar from the perimeter and interior footings make this slab on grade "structural" such that because this SFR is located in Seismic zone 3 and only a few hundred yards away from major earthquake faults - the interior slab of the home had to be reinforced.

The stem rebar was not bent over from the perimeter footings of the garage, so I would then assume it is simply lying on the ground,and didn't have to be reinforced.

I showed the photo of the foundation- before the slab was poured to a very knowledgeable Univ. Prof in C.E., who saw the stem rebar bent over and being tied to the slab once it was poured and he said that the slab was structural and had to be reinforced because the home is located in Seismic Zone 3, and that it was more Zone 4 than 3 because of the seismicity of the area , but am now hearing from another C.E. that the slab is non-structural - because it is lying on the ground, and doesn't have to be reinforced.

In sum, who's correct ? - and do the structural design requirements of Chapter 16 - which takes into consideration the proximity of the site to active earthquake sources - take precedence over Section 1922.10.3 ?

In the fault hazard report, the geotechnical engineer strongly recommended taking out earthquake insurance !

 

[csd72]

Lesson 1: Some code interpretations are largely up to the judgement of the engineer - the more of them you ask the more different answers you are likely to get.

Lesson 2 - there is not, and never can be, a code clause for everything.

Lesson 3 - the terms structural and non-structural are very ambiguous but I will attempt a definition. If you remove the member and the building can still stand up to imposed loads, wind and seismic events then it is, in my opinion, non structural.

In seismic zones the ground slab is sometimes used to tie the footings together in a seismic event and would therefore be non structural. Personally I would never use unreinforced concrete if I could reasonably reinforce it as the concrete will crack eventually.

also the terms plain concrete and unreinforced concrete are not interchangeable. A very lightly reinforced member may be considered as a plain concrete member in the analysis.

 

[JRLF]

Tnx's CSD72 for your reply. and the thing that makes sense is if the footings are part of the lateral-force resisting system and they are tied- via bent-over stem rebar,to the slab then the combination of the footings and the slab are going to "rock together" in resisting any lateral forces, such as wind and earthquakes. This is the reason why the Univ. Prof. in CE said the interior slab was structural,and had to be reinforced in Seismic Zone 3, but not the garage.

Looking at ACI 318-08, it states this Code does not govern design and construction of slabs on ground, unless the slab transmits vertical loads or lateral forces from other portions of the structure to the soil- which I assume would be the case when the slab is tied to the perimeter and interior footings, via stem rebar.


As you correctly pointed out, there are are many different opinions as to what is structural vs. non-structural,for another very knowledgeable S.E. said " if the slab is moving with time, then it's structural." Thus, some say it's structural and has to be reinforced, and others say the slab is non-structural and doesn't have to be reinforced, yet the S.E. who developed the plans - which called for reinforcement- admitted that he wasn't sure if the slab was or was not structural, and referred me to his mentor, who was the Univ. Prof. in C.E.

What I still don't have an answer to is : Do the structural design requirements of Chapter 16, re: seismic design , take precedence over the requirements of Chapter 19 ?

In that Section 1922.10.3 has a very general exclusion saying that " nonstructural slabs on ground do not have to be reinforced" which includes Seismic Zone 2, 3 and 4 - whereas Chapter 16 takes into consideration not only the seismic zone (which is a 3 here, but very close to being a 4), but also includes the proximity of the site to the active seismic sources, as well as the soil profile characteristics.

Tnx's again for your reply.

 

[csd72]

That would come back to the question of whether it is structural. If it is structural then chapter 19 does not apply and you need to follow chapter 16.

 

[TheLinker]

zstructural,

To comment on your question,are they still using the '97 UBC somewhere? Yes they are, in Bolivia. Just learned this from doing a project there.

And they have bigger issues down there than using an outdated building code.

 

[a2mfk]

What's all this about? If you are an EE, why are you involved? Your house? I took a class in EE in college but beyond rewiring a light switch I would punt to an electrician, know what I mean?

My seismic design knowledge is limited, but a non-seismic reason for bending bars into a slab is to prevent differential movements between the slab and the wall. These movements could be vertical as a result of the slab settling on the fill, or the foundation settling and the slab staying relatively in the same place. These movements often result in annoying cosmetic problems such as finish cracking and separation, but would not be considered a "structural problem".

If you have a house with significant fill where the top of slab is several feet above grade and the CMU stem walls act as a retaining wall, the bars could then act as tie backs that brace the top of the CMU wall and prevent lateral movement. This also can provide a more economic CMU wall and foundation design.

But you said they did this on the interior also, but the bars only extend out 4 ft. I would expect if you are trying to tie together the foundations for seismic design you would need either tie beams between spread/pad foundations or your slab would require continuous bars from footing to footing that would be lap spliced into the bent bars.

FWIW, the Florida Building Code uses this definition of structural more or less, though it should also include lateral loads:

“any part, material or assembly of a building or structure which affects the safety of such building or structure and/or which supports any dead or designed live load and the removal of which part, material or assembly could cause, or be expected to cause, all or any portion to collapse or fail.”

 

[beaming]

You are right that the slab becomes structural if it is tied to something.
The earth moves little some places and much others. If the soil is very uniform and granular and not subject to much change in moisture content, it moves very little. If it is a fine-grained soil, especially a clay whose particles have much surface and little mass, like a Napoleon pastry, It will change a lot. How much?

A paved road, concrete or asphalt, on clay subsoil with no trees around will flap its wings from wet to dry season. The clay near the edges will expand and contract, and the clay in the middle will only expand, because it can't dry out. Several inches is not rare from winter to summer. A house on piers and footings will also flap its wings. The corners will go up and down, say maybe 4 inches, the sidewalls up and down maybe 2 or 3 inches, and footings on the interior will stay still, maybe 2 inches above where they were the day it was all built, after a few years.

Vegetation will change everything, as will sand lenses, i.e. pockets of sand allowing water to come and go.