Two-story slab on grade foundation footing in California

[Christopher Kirby]

I just looked all over the internet for a detail for a two-story slab on grade footing. I was wondering if anyone had called out #4 rebar instead of #5.

I am a building designer and always have to listen to my engineer. He calls out 2-#4 rebar top and bottom, 4 rows total, in my 12″ wide x 12″ into grade one story footings. He calls out #5 in my two-story footings.

But I was curious if I could use #4 rebar in a 15″ wide x 18″ into grade two-story footing.

Well, I didn’t find anything. No detail or thread that explicitly discussed this. Then a contractor called and I asked him. He said he has always seen 2-#5 T&B.

I thought I would post this to see if anyone had any comments on the matter. Also, if anyone asks this question in the future, maybe they will find this thread.

Thanks

 

[Mark K]

Listen to your engineer. If you go to a larger footing the code will require more minimum reinforcing.

 

[Msradell]

Rebar and concrete serve very different purposes in footings and you can't compensate for the size of the rebar by adding additional concrete. Engineers are trained to determine which sizes of which material are required for these footings, trust and follow their calculations. Otherwise any liability for future problems will fall on you and/or your company.

 

[Paul Sweet]

ACI 318 (the concrete code) has a requirement for the area of the reinforcing to be a certain minimum percentage of the concrete area. A 15" x 18" footing is almost twice the area of a 12" x 12" footing.

 

[bill1952]

Did you ask the engineer if the #5 is based on code, and if #4 meets code, why he said #5?

 

[Mark K]

This obsession with #4 bars is misplaced It would not surprise me to find that the cost difference is less than the value of the time spent talking about it.

While the engineer designing the building is sensitive to the cost of construction my experience suggests that no engineer's design is consistent with the minimum code requirements in all respects. If the engineer were to attempt to not go beyond the minimum code requirements his fee would be higher because that would take more time. This would also have an impact on the time to complete the work delaying the issuance of a permit. But you probably consider the engineer's existing fee to be too expensive.

 

[Christopher Kirby]

This obsession with #4 bars is misplaced It would not surprise me to find that the cost difference is less than the value of the time spent talking about it.

While the engineer designing the building is sensitive to the cost of construction my experience suggests that no engineer's design is consistent with the minimum code requirements in all respects. If the engineer were to attempt to not go beyond the minimum code requirements his fee would be higher because that would take more time. This would also have an impact on the time to complete the work delaying the issuance of a permit. But you probably consider the engineer's existing fee to be too expensive.

First part: correct, worrying about this detail is not won't the cost savings.

Second: my engineer is an old guy with no time for dumb questions like this. When he says it's gotta be #5 I listen. And his rate is the lowest I will ever find since we've worked together for so long.

I suppose my curiosity is just a general one. Questions like: Why do we do things the way we do? Where does it say it in the code or juris. Muni code? What's the best explanation? Just stuff like that.

Thanks

 

[Christopher Kirby]

ACI 318 (the concrete code) has a requirement for the area of the reinforcing to be a certain minimum percentage of the concrete area. A 15" x 18" footing is almost twice the area of a 12" x 12" footing.

Thanks. Found this on Quora

What is the minimum percentage of steel in slabs, beams and columns?
Solid slabs and stairs:Minimum reinforcement:

high yield bars - 0.13% of gross cross-section

mild steel bars - 0.24% of gross cross-section

Beam:Minimum reinforcement:

Longitudinal steel:

high yield - 0.25% bh

mild steel - 0.50% bh

where b is the width of beam and h is the overall depth of beam

Links:

mild steel: 0.25% of a horizontal section through the web

high yield steel: 0.12% of horizontal section through web

Columns: Minimum reinforcement:

Longitudinal steel:

1% of the necessary concrete area

Links: make the choice to satisfy the following:

size at least one-quarter of the biggest longitudinal bar

spacing: 12 x size of smallest longitudinal bar but not more than 300mm every corner and each alternate longitudinal bar should be restrained by a link in each direction

Walls: Minimum reinforcement:

Vertically 0.4% of cross-sectional area

Horizontally 0.2% of cross-sectional area

https://www.quora.com/What-is-the-minimum-percentage-of-steel-in-slabs-beams-and-columns?top_ans=95591525

 

[mtlogcabin]

I suppose my curiosity is just a general one. Questions like:

Why do we do things the way we do? Sometimes it is simply years of experience and we know what will work.

Where does it say it in the code or juris. Muni code? We have footing and foundation details that have been designed by an engineer that the city has been using for decades. Not adopted or in the code but is code compliant and is now the standard details on residential projects.

What's the best explanation? Just stuff like that. See above

 

[redeyedfly]

Thanks. Found this on Quora

What is the minimum percentage of steel in slabs, beams and columns?
Solid slabs and stairs:Minimum reinforcement:

high yield bars - 0.13% of gross cross-section

mild steel bars - 0.24% of gross cross-section

Beam:Minimum reinforcement:

Longitudinal steel:

high yield - 0.25% bh

mild steel - 0.50% bh

where b is the width of beam and h is the overall depth of beam

Links:

mild steel: 0.25% of a horizontal section through the web

high yield steel: 0.12% of horizontal section through web

Columns: Minimum reinforcement:

Longitudinal steel:

1% of the necessary concrete area

Links: make the choice to satisfy the following:

size at least one-quarter of the biggest longitudinal bar

spacing: 12 x size of smallest longitudinal bar but not more than 300mm every corner and each alternate longitudinal bar should be restrained by a link in each direction

Walls: Minimum reinforcement:

Vertically 0.4% of cross-sectional area

Horizontally 0.2% of cross-sectional area

https://www.quora.com/What-is-the-minimum-percentage-of-steel-in-slabs-beams-and-columns?top_ans=95591525

Quora is wrong and vastly oversimplifies the equations.

That said, there is a negligible about of flexure in a typical strip footing. The rebar is nearly always min steel required. #4 bars will probably work but a lot of engineers start with #5 bars and consider #3 & #4 only for stirrups. I would push back on the engineer scheduling #5 bars in a single family wood frame house.

As to the cost, #5 bars are 50% more expensive than #4 bars. More importantly, #4 bars can be field bent fairly easily, #5s must be shop bent.

 

[redeyedfly]

negligible amount...

 

[bill1952]

Admittedly California is different but I think of the single family framed houses I've lived in and the many neighboring houses all without an ounce of reinforcing, all still plumb and level.

I am bemused on how easy it is to spend OPM.

 

[BN4537]

2018 IRC, not California version.

R403.1.3.3 - Slabs-on-ground with turned-down footings.

In Seismic Design Catagories D0, D1, and D2, slabs -on-ground cas monolithically... shall have not fewer than one No. 4 bar at the top and bottom of the footing, or...

Where the slab is not cast monolithically... No. 3 or larger vertical dowels with standard hooks on each end shall be installed not more than 4' OC in accordance with Figure R403.1.3, Detail 2...

Perhaps a California guy can verify if this section is the same in their code book?

 

[BN4537]

This is the figure referenced by the code above.

 

[Mark K]

Almost without exception engineers will base their design on the IBC and not the IRC. For them this is faster and cheaper.

The IRC provisions are based on specific details but if the design is based on other details the IRC provisions are inappropriate. The IBC provides the engineer with the tools to accommodate the preferred detail.

I have seen situations where in order to justify less material it cost more in engineering time than the savings in material. Thus if the engineer is expected to never use an #5 bar when a #4 bar will work the engineer will have to spend more time refining the design and as a result his fee should be higher. But is seems that some individuals will only focus on the cost of the engineers fee and not the overall cost of construction.. Engineers need to make a modest profit so there is a tradeoff.

Another thing that should be considered is that while contractors and developers can use corporations to shield their personal liability, professional engineers cannot shield their personal liability behind a corporation.

While the cost of a #5 bar is more than a #4 bar the other costs such as forming and placing do not change. Thus the cost impact is minor.

In addition even if you do not have to deal with earthquakes there are a lot of reasons why the ground under the building will settle differentially. Thus to provide some minimum amount of reinforcement makes sense.

 

[bill1952]

While the cost of a #5 bar is more than a #4 bar the other costs such as forming and placing do not change. Thus the cost impact is minor.

Since it can't be bent on site, doesn't that make it more expensive? It sure would go or me.

In addition even if you do not have to deal with earthquakes there are a lot of reasons why the ground under the building will settle differentially. Thus to provide some minimum amount of reinforcement makes sense.

Any guess why IRC, the committee and the ICC members who go vote disagree?

 

[BN4537]

Re-reading the OP, I guess he did not say it was a single family residence - that was just an assumption on my end.

 

[MtnArch]

Here in California only licensed architects or engineers are allowed to design and sign on commercial buildings. Since Christopher mentions he is a building designer, he likely is not licensed and can only provide drawings for single-family residences (and less than a single 4-plex on a single lot) unless another design professional over-stamps his work (which could violate our licensing laws).

 

[bill1952]

Here in California only licensed architects or engineers are allowed to design and sign on commercial buildings. Since Christopher mentions he is a building designer, he likely is not licensed and can only provide drawings for single-family residences (and less than a single 4-plex on a single lot) unless another design professional over-stamps his work (which could violate our licensing laws).

Any square ft limit on single family dwelling before RDP is required in CA? I believe in New York State " Residential buildings with a gross habitable floor area of 1,500 square feet or less" are exempt from having sealed plans. Agricultural buildings also exempt.

 

[Mark K]

"Any guess why IRC, the committee and the ICC members who go vote disagree?"

I am not sure that they have explicitly said that you do not need reinforcement. In any case I suspect that many members of ICC are not trained as engineers and thus have a different perspective. Could it be that an IRC compliant building does not provide the level of robustness that most people feel is appropriate.

We should also be sensitive to the impact that special interests have upon the ICC standard adoption process.

 

[steveray]

Any square ft limit on single family dwelling before RDP is required in CA? I believe in New York State " Residential buildings with a gross habitable floor area of 1,500 square feet or less" are exempt from having sealed plans. Agricultural buildings also exempt.

You seal a 1501 ft house in NY?...Sure is easy to spend OPM.....

 

[tmurray]

"Any guess why IRC, the committee and the ICC members who go vote disagree?"

I am not sure that they have explicitly said that you do not need reinforcement. In any case I suspect that many members of ICC are not trained as engineers and thus have a different perspective. Could it be that an IRC compliant building does not provide the level of robustness that most people feel is appropriate.

We should also be sensitive to the impact that special interests have upon the ICC standard adoption process.

I think you hit the nail on the head here. The level of damage we will accept often changes based on several factors. Take buildings that we need operationally post-disaster. Man, sure would be nice if we didn't have to re-build the water treatment plant for the whole city after a major earthquake. How do we do that? you design it to a higher safety factor. Can we build a house to the same level? Sure. It's more expensive though.

 

[Paul Sweet]

(3) #4 top & bottom would give you the same steel area as (2) #5 T&B.

 

[Mark K]

for 3-#4 As=0.6 in sq and for 2-#5 As=0.62 in sq thus essentially the same for material. But if you consider the labor to install the reinforcement since there are fewer bars to be handled #5 bars would be cheaper.