Concrete structural slab - Constructability issues 2

[hotmailbox]

Hi all,

I was hired as a structural observer for a residential project in LA 5-story wood over 2-story garage basement) since the EOR refused to work with the new owner of the project. The contractor just poured half of the foundation. When I reviewed the structural slab I found some area in the podium slab are extremely congested and I don't think it's feasible and safe to do so. Please see pictures below for additional top bars in a 12foot strip each way on top of #7 @ 12" o.c. each way. That means you have to place 60 #7 bars each way within a 12foot strip leaving 1.5" clear gap between bars (if bars are perfectly placed evenly). Below these two reinforcing mat is a concrete beam with #3 ties @ 4" o.c. and longitudinal bars. I sent an email to the EOR for his opinion but I don't think he would ever respond.
So there are few options for the owner:
1. Keep building per plan and hope nothing happens.
2. Hire me to redesign the entire slab for better bar replacement (this would cost a lot and project gets delayed for revision approval).
3. Me suggesting using bigger bars for better spacing between bars but keeping the same steel area per foot but I really don't want to make such change without EOR blessings. This is the first time I am structural observer for someone else's project, I don't know all the pitfall I need to avoid other than making sure everything is built per plan.

Do you think my concern is valid? Is this still feasible? Code wise, it is acceptable because clear spacing is at least equal to the greatest of 1 inch, db, or (4dagg/3). When I design structural slab I always try to make sure clear spacing between bars to be 3" minimum.

 

[Celt83]

How thick is the slab? That is a lot of steel in the strip to the point that you might question if the sections violate ductility requirements.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Enable]

What are the loads on the drawings? I don't do a great deal amount of concrete design but this seems rather unusual.

Just checked my latest project file for a commercial space where I'm a shoring engineer (not the EOR). It has the following loads:

DL: 250mm (10") concrete slab
LL: 4.8 kPa (100psf)
Superimposed DL: 3.6 kPa (75psf)

2-way slab, 5-6m spans (16-20ft), no drops, and bars are roughly as follows:

Bottom bars: 15M @ 400mm each way +/- (16" c/c)
Top bars at columns: 20M @ 300 each way +/- (12" c/c)

What you have sounds like it's rated for a freaking tank.

As far as Celt83's concern (very valid), I think you're probably past the point of resistance > 1.2 * demand so are probably okay (at least in Canada our code gives us an out on this one). But....why do this? Also your concerns are valid in terms of congestion. No way a vibrator does well here. You're talking SCC for that kind of bar spacing.

 

[hotmailbox]

How thick is the slab? That is a lot of steel in the strip to the point that you might question if the sections violate ductility requirements.

12" thick. You're probably right.

 

[OldDawgNewTricks]

Great point, Celt83. Depending upon the concrete cover dimension, the concrete compressive strength might need to be 6500 or 7000 psi just to get the minimum reinforcement strain (i.e. maximum reinforcement ratio) requirement to work out.

 

[hotmailbox]

What are the loads on the drawings? I don't do a great deal amount of concrete design but this seems rather unusual.

This is 5-story wood structure over the podium so:
Dead Load = 35psf (Roof, including all exterior & interior walls) + 4 * 50psf (Wood floors, including all exterior & interior walls) + 10psf (finishes on concrete floor) = 245psf
Live Load = 5 residential floors * 40psf * 0.5 (live load reduction) = 100psf (or can be 120psf to be safe).
Based on my experience, this is way overdesigned slab. I typically have #5 @ 12 top and bottom with 14 to 18 #7 additional top bars and 8-10 #6 additional bottom bars at critical column strip(12ft strip).

 

[hokie66]

There is definitely a constructability issue, but what some may have missed with the loading, is that this is a transfer slab supporting 5 stories above.

Hotmailbox, you have been placed in an invidious situation. Your observations have merit, but without the EOR to fall back on, be very careful.

 

[hotmailbox]

There is definitely a constructability issue, but what some may have missed with the loading, is that this is a transfer slab supporting 5 stories above.

Yes, typical construction in LA. 5 stories of wood on top of concrete slab podium.

Hotmailbox, you have been placed in an invidious situation. Your observations have merit, but without the EOR to fall back on, be very careful.

Thanks. I am also scared of the retaining wall design in this project. Some portion of the wall is only 10" thick but retaining 16 to 22 feet of soils with 6-7-story neighboring building right next to the property. Never seen anything like that. I have no idea how the guy could make the design work on numbers.

 

[human909]

Run!

Is this job really worth the risk? You might not be the EOR but if things are as bad as you suggest is it worth being associated with this project? It sounds like you are too close to this not to avoid some responsibility if things go south.

If you have no confidence in this EOR's worth then you surely you must politely tell your client that and either extricate yourself from the contract or get it redesigned.

 

[Lomarandil]

Agreed with human909, I'd be looking to extricate from this project.

I haven't designed podium slabs, but I've designed retaining walls on a similar scale in seismic zones... and 10" isn't going to cut it. Especially without going active.

----
just call me Lo.

 

[hotmailbox]

Unfortunately, this is one of my biggest clients. They just bought this project with permit. I already informed the client that without the EOR engagement, the project is going to stop because I can't make changes to the project and there are tons of other things that don't match between architectural and structural. I am trying to minimize exposed risks here but who knows what I will get into if things go south. Below is the retaining wall in question (blue) with slab and ramp and return wall supporting laterally. There's 5-story building 7 feet away from this wall.

 

[Celt83]

Those spans seem suspect for a 12” transfer slab with 5-stories of wood above, long term deflections are probably pushing 3+ inches. Being in LA I’d expect the shear wall chord forces at the transfer level to be pretty large especially once the over-strength factors are applied.

Smear loading doesn’t look like it would be appropriate for this column layout either.

What supports that split column in your last post?

EDIT: Those drop panel dimensions don't appear to satisfy L/6 requirements which would negate their use for negative flexure. Perhaps the job was left unfinished by the previous engineer.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[Settingsun]

Is there professional duty of care in the US? I'd imagine so. If it looks like there's a problem, you've got to do enough to convince yourself there isn't, otherwise you've got to inform your client.

 

[dianium500]

If you make recommendations and change his design, this becomes your building not his. Why isn't the EOR responding? Did the client fail to pay a bill?

 

[hotmailbox]

Being in LA I’d expect the shear wall chord forces at the transfer level to be pretty large especially once the over-strength factors are applied.

Most of the design firms (small or big) in LA ignore all seismic forces and omega on podium slab. Only averaging dead and live on the podium and call it a day.

 

[hotmailbox]

Why isn't the EOR responding? Did the client fail to pay a bill?

Client's side of the story: When they acquired the project, they made sure all consultants fully paid and had them sign the paper. Later on after escrow being closed, the architect told the client that since you are not in the original contract, you need to pay me 30 to 50k more for a new contract plus fee for CA. Client refused. Architect went on soliciting all other consultants and told them not to work on the project.

 

[Celt83]

Most of the design firms (small or big) in LA ignore all seismic forces and omega on podium slab. Only averaging dead and live on the podium and call it a day.

I really hope this is just your perception and not actually the case, the overturning chord reactions even over here on east coast can really mess with the punching shear ratios. Averaging (smearing) the Dead/Live loads can work for regular column grids and trusses framing to unit demising walls as the line loads tend to get spread out equally. For irregular column grids and/or trusses framing exterior to corridor load averaging is not appropriate as the load tends to get concentrated along the slab edge and within a span with no counterbalancing action in the adjacent slab bay within the units, this condition gets worse for the lateral chord forces as the resisting gravity load isn't lumped along the same line of action.

I'm making a thing:

http://www.thestructuraltoolbox.com

(It's no Kootware and it will probably break but it's alive!)

 

[dik]

I think you're at an impasse... too much reinforcing. If you change the EOR's design, you bought it. If you cannot get the EOR to respond, try to find out why. You might be walking into the same 'pasture'. Does the existing EOR have a professional duty to make good? A redesign may be in the works; careful you don't end up in the same boat as the EOR.

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

-Dik

 

[hotmailbox]

I really hope this is just your perception and not actually the case

This is from my experience. This is how it has been done in my previous office (which designed hundreds of podium projects in the last 20 years) I haven't seen anyone design the podium using line loads and point loads from wood structure above but me (I did few jobs when the bearing walls were simple and framing was easy to track loads down). I also have reviewed quite a few designs from big design firms and all were designed the same way without tie-down forces being mentioned. And I haven't seen any problems so far (doesn't mean it's correct of course but gravity wise it kinda work. For seismic, a big one will tell). I myself design the podium for tie-down forces with overstrength combinations.

I am testing some old projects with 2 design methods (averaging loads vs actual loads from walls and posts) to see the differences. Will report back once I have something to share.
PS: I like your structural toolbox. I'll reach out to you for some ideas of calcs for multi-story stud walls with different joist span configurations at each level plus wind load where occurs.

 

[hotmailbox]

A quick re-design for the retaining wall mentioned above in SAFE and it seems to be ok. Restrained soils pressure = 62pcf, seismic soils pressure = 28pcf inverted triangle. Rebar shown below is in2/ft. Cracked long-term wall deflection is 0.5" at the middle. Current design shows #7 @ 4" o.c. vertical inside and #7 @ 6" o.c. vertical earth side.