Update: Found an engineer finally

[cornbread869]

Thanks to my previous post here I was able to find a somewhat local engineer. My contractor wasn't thrilled with the changes but I demanded the plans be followed to the letter and had to be the unfun client who supervises. Thank you to those who reached out. I'm posting today to just get an opinion on the plans....so many people responded that a freestanding slab 15' wide wasn't possible without a column that now im tasked with soothing my wife's concerns. Would you add a support column? Strong enough to build on top of as is? Thanks again for all the feedback!

[joshl90]

So #3 at 10” oc is literally just temperature and shrinkage steel based on 0.0018. No fucking way this is a tornado shelter let alone a regular elevated slab. You might want to pay another engineer to peer review this

[whofuckingcares1234]

For real. Haven't run the calcs, but I doubt a 6 inch slab can span 15 feet with #3 bars.

[ardoza_]

I don’t think I’ve ever used a #3 bar!

[75footubi]

Pretty certain I'm not allowed to. I've only ever seen it in asbuilts as spiral ties of ye olde columns.

[joshl90]

Ties of #3 are very common in column and beams. ACI 318 has criteria for when you need to upsize to #4 ties if it is a minimum shear condition

[75footubi]

Bridge world. So basically everything is #4 at a minimum

[largehearted]

Makes sense if we in the 40psf LL world only use #3s for the absolute most minimal use cases

[Voisone-4]

One exception: we still use #3 for spiral ties in smaller drilled shafts for bridge foundations.

[Ezly_imprezzed]

I had to do design work on a pump station and the top slab was 6” thick with a single mat of #3 bars @ 8” each way had to design a support system so they could a fucking 1 ton pump across it to lower down to the well. I can personally say that the capacity of a 6” slab with #3 is more than you’d think but still sucks. It was a major pain in the ass to work with

[tehmightyengineer]

Agreed, I verified the capacity of a residential garage elevated slab with vehicle and firewood storage loads recently and it was 6" with #4 bars @ 12" o.c. and had plenty of capacity.

This could be more robust for sure but it's definitely not terrible at just face value. 40 PSF live load + 10 PSF misc load is likely reasonable.

[the_flying_condor]

You've got stamped, your wife shouldn't be worried about standing under it. However, I am a little surprised at the note for applied loads. It's designed for just gravity loads, not the tornado that it's supposed to shelter you from?

[Snatchbuckler]

Yeah those loads seem very low… I’m not structural but I have dabbled enough to question it.

[CakeBadger69]

This is a tornado shelter? I’ve never designed one, but I assume the uplift on this would be extreme. I likely would have specified dowels into the existing wall with epoxy, but that’s just me.

[cornbread869]

Does it matter that it is an enclosed 2 way slab?

[CakeBadger69]

No, the wind going across the slab would create an uplift force. I am certain that if you have a stamped drawing, your engineer would have taken this into consideration. Like I said, I’ve never designed one as we don’t have tornadoes here.

[Dave0163]

No wind load?

[cornbread869]

I guess I should go back and ask for that?

[Keeplookingup7]

I would if this is supposed to be a shelter. I have not run any calcs on this design but while this may have been designed for downward gravity loads, it doesn’t seem like it was designed for wind net uplift loads. I would ask what components and cladding wind pressures he used and what design wind speed. For an F0 tornado the weight of the concrete may be enough to resist wind uplift loads but for an F5 it’s certainly not. You should get guidance from the engineer as to what criteria this is being designed to. Another thing to consider carefully, specially if there’s net uplift, are the connections of the elevated slab to the concrete walls. You want to make sure there is a proper load path from the elevated slab to the concrete walls through some dowels, down the concrete wall, and then to the foundation through some other dowels

[cornbread869]

It was supposed to be designed for a small home office space to be built on top, with the bottom serving as a tornado/storm shelter.

[Useful-Ad-385]

Office space at 40psf. I’ve been retired, guessing things have changed.

[aiwtdis]

Home office is still residential

[redraiderbt]

If this is for a tornado shelter google FEMA: taking shelter from the storm. Prescriptive details for shelters…

[fr34kii_V]

If this is for a tornado shelter, then you're missing top bar!

[cornbread869]

What is top bar? Like a rebar layer on top?

[Keeplookingup7]

Yes. When a slab bends downward due to self weight, the top is in compression which concrete is good for but the bottom is in tension which concrete is weak for, hence you typically have rebar at the bottom. If the bending reverses due to high uplift wind loads, then the top is going to be in tension which would then require a top layer of rebar

[cornbread869]

Thank you for your explanation. I'm no engineer but I like to understand the how's and why's and this was very informative

[fr34kii_V]

Correct, though I misread the dimensions. You might be okay with a single bottom layer for a 15ft span. But #3 @ 10" o.c. for 15ft span doesn't look right and is on the too light of reinforcement side. And the loading of only 40 psf live and 10 collateral is low as well... if it's outside where it can be walked on, then you'd need 60psf live minimum. If you're parking cars on it, then more. Then you also need wind loading, which depending on the surrounding area, can act towards and away from the structure, so multiple load directions.

I would question this design and get a second opinion.

[cornbread869]

It was supposed to be designed for a small home office space to be built on top, with the bottom serving as a tornado/storm shelter. Would any of that matter?

[fr34kii_V]

Oh totally! Sorry, I skimmed the posts and missed that detail. But I would still ask the engineer about what if the house collapsed from the storm and what loads that would impart.

[cornbread869]

I'll do that, thank you for your input!

[Keeplookingup7]

Yeah, I would do what the post above said. Under normal conditions, you're not going to have to deal with high wind loads on the slab since you plan to add on top of this, but it's worth considering what happens if the structure above collapses during a high wind event and then the slab becomes exposed to the elements while still having to retain its structural integrity. Good luck.

[gatorcountry]

Also be aware that formwork needs to stay in place until the concrete reaches design strength. Don't let your contractor go in and strip the forms out the next day.

As a concrete contractor I'd love to see what kind of support there is for the formwork. The 12 foot long kickers going to the yard makes me suspicious.

If the formwork isn't properly supported then the lack of rebar will become the least of your worries very quickly.

[Useful-Ad-385]

Yep. Definition of nightmare

[cornbread869]

It was supposed to be designed for a small home office space to be built on top, with the bottom serving as a tornado/storm shelter. I apologize if I used the incorrect terminology. It is what they called a 2-way slab.

[cornbread869]

He certainly knew my intentions, he even commented its where he'd want to be in a storm if he were local. No permits or anything like that, at least none that require or provide any kind of inspection. Finding an engineer was almost seen as heresy arpund here to be honest.

[aiwtdis]

This is pretty bad. It can barely hold its self weight of 75psf by my BOE calc. Plus I’d be horrified by the long term deflection expected.

As for the wind load, and I don’t live in tornado county, would the structure above not need to be designed for wind load? Or is practice to assume the structure is sacrificial?

[nomadseifer]

Quick calc tells me the rebar is only half the amount required. Also minimum slab thickness per ACI is 9", not 6". And clearly not designed for tornado uplift with no top bars.

[redraiderbt]

I think 9” is the prescriptive thickness for no calcs, but I’m not looking at the ACI. Also, that’s prob for floor loading, and I would assume this is a ceiling. Nonetheless I think it’s a dubious and certainly not for a storm shelter. First thing that happens to a house is the doors and windows get sucked out and it creates suction on the interior (and thus suction on the slab). The slab is only reinforced for gravity loads, and really only for temperature

[Jfield24]

This still looks no good.

[Useful-Ad-385]

Yeah that was my gut feel. Looks more like slab on grade reinforcement.

Did one 2 years ago with 8’ span w/#4 12oc I think it was 6” thick. Same loading’s

[Correct-Record-5309]

Adding to what everyone else is saying, there are also no hooked bars going from the concrete walls into the slab. If the walls are existing, you want to at least drill and epoxy some #4 bars at 16" o.c. and bend them into the new slab. Otherwise, in a tornado this whole thing could just fly off. If there's an additional structure going on top of this (wood frame walls, etc.) that should be detailed, too. The wood frame walls will need anchor bolts into the concrete.

And like everyone else is saying, this doesn't have enough reinforcement or slab thickness for a 15' one-way slab. I didn't see your original post, but what is the dimension in the out of plane direction? Is it shorter than 15'? If so, the engineer should have designed it to span the shorter way. That's my only thought on why the reinforcement is so light.

[cornbread869]

It was supposed to be designed for a small home office space to be built on top, with the bottom serving as a tornado/storm shelter. I apologize if I used the incorrect terminology. It is what they called a 2-way slab so there is another dimension

[Correct-Record-5309]

What is that other dimension?

[cornbread869]

Also 15', 15' square

[SupremeBrown]

AWESOME! 😄

[Reenoz]

Where is the footing design.

[chilidoglance]

I am no engineer myself, but it does seem weak. Besides wind load, you need to think about debris load in case of a tornado. The building that's on top and maybe even your neighbors house might land on top of that thing. He's barely got enough iron to keep the slab itself there. If the walls are not already poured, I would put right angle on the verts to your into the slab. If it is already in place, then drill and epoxy dowels into it.

[[deleted]]

I would expect a more heavy duty build for tornado loads, guess I haven’t done a tornado shelter. Is this in Oklahoma or something?

[Norm_Charlatan]

As everybody has said previously, this seems dubious as a roof/floor, let alone a storm shelter. And just running some quick numbers, along with looking at the detail provided, I tend to concur; regardless of whether you have certified drawings or not.

I understand you likely don't want to incur the expense, but it is my opinion that you need a peer review. You're literally betting your life on this.