Concrete Slab Construction - 1968 1

[PEinVA]

Our office is currently in the middle of a hotel renovation in DC Metro area, and the concrete slab is defined on structural drawings as 6.5" typically reinforced 2-way concrete slab. The spans are typically 12'-14', nothing really out of the ordinary.
The hotel is completely stripped at this point, and this when our firm typically walks the site to see if there is any other structural issue pressing that should get addressed while still early in the job. While walking the site I noticed a number of joints in the midspan of the slab in the east-west direction of the slab and I found this rather odd, as I could not see any rebar crossing the joint, and as we all know, this is where bottom steel wants to be.

After reviewing some cores and other openings in the slab, it looked as if the bottom 1.5"-2" was precast with the bottom mat and then the "topping" upper slab was poured over this on-site. Our only thought is that the 2" precast layer was utilized as "stay in place" formwork. The area around columns looks like the column tops were poured with the upper slab, but I'm worried we don't have the shear depth we thought had. As with any renovation there will be a number of new cores, and I'm trying to determine how to work with shear around columns.

Has anyone seen this type of construction? My boss had heard of it, but never actually saw it in practice. There is nothing on the construction documents referencing this option, I'm sure it was a VE/Contractor substitution.


I'm also worried the slab won't act compositely, how would this have been achieved?

Thanks guy!



RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[jike]

Sounds like it could be the Filigree system.

http://www.filigreeinc.com/

 

[JAE]

Nice link jike.

I would say - as far as shear goes, that you have to get a good composite action in flexure to be able to develop shear capacity at the column with the full depth used for "d". i.e. including the stay-in-place forms.

Otherwise, you'd ignore the 1.5" to 2" of forms in determining the [φ]Vc of the section around the columns for punching shear checks.

 

[hokie66]

We use a similar system in Australia, called Transfloor. But it is a one way slab with band beams in the other direction. The picture of a "flat plate" in jike's link is a one way system.

The precast depth cannot be included in your punching shear calculations. We sometimes use conventional formwork around the columns where punching shear controls.

If the structural engineering firm is still in business, you might try contacting them to see if they can give you any information about the system substitution.

 

[PEinVA]

hokie66,
Unfortunately I do not believe they are still in business, but the system listed by jike seems to be correct. The flat plate example shown is actually not a one-way slab. I found some documentation that states just splice the joints with bars at 2-1/4" at hooked bars at the end. Also, the areas around the columns seems to have been poured with the "topping" slab, but its a 6.5" slab. Could I count the entire 6.5" - 2.25" (precast) slab? for a 'd' of 4.25"?

Also, all the documents I found for this particular system state that the system should be designed based on ACI 318 Ch. 17 - Composite Concrete Flexural Members.

17.4 - Vertical Shear Strength states the following -

" Where an entire composite member is assumed to resist vertical shear, design shall be in accordance with requirements of chapter 11 as for a monolithically cast member of th same cross-sectional shape. "


Doesn't that seem to say the opposite of what hokie says?

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[hokie66]

If the precast extends to the face of the column, I think d for punching shear would be 4.25" minus the depth to the top bars. If it stops short and the area around the column is cast monolithically, then you can use 6.5" minus depth to top bars.

For the composite member to act as monolithic, the precast would have to extend over the column. I believe you may have the same situation which occurs where columns are cast too high and left that way. The depth for punching shear is reduced by the amount of penetration of the column into the slab.

The bottom steel perpendicular to the precast panels would have to have a reduced effective depth. May contribute some two way action, but it is not a true flat plate, in my opinion.

What I have said is only based on my experience in Australia with a similar system, so may not be directly applicable.

 

[PanamaStrEng]

I would assume that the precast pannels stop a certain distance short of the column. Is it noticeable from the bottom of the slab? If so, perform 2 punching shear calculations:
1. at the required distance by code (typically d/2 by ACI) from the face of the column using the full d;
2. at the section where the pannel starts, discounting the depth of the pannel.

 

[hokie66]

Yes, good advice, Panama.

 

[PEinVA]

I've attached a picture which should help you see the connection around a column. I think I'm better off using 4.25" - d". I just don't see that working. We'll see though.
Thanks for all your help and suggestions.

RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[jike]

What is the 2" on the photo indicating?

Are we looking at the underside of the precst form or poured in place. It is hard for me to tell by the photo.

 

[PEinVA]

jike
Yes, sorry. The picture is of the underside of the slab, and the 2" is where you can see the line of the topping slab which was poured monolithically with the column, and the precast panels meeting. This picture is from the underside.

Sorry for leaving that very important piece of information out!


RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke

 

[hokie66]

Yes, I think you have to use 4.25" as the depth of your slab for punching shear. If the 2" is consistent, I would use that as your column size for punching shear calculations, so it increases the shear perimeter applicable.

Back to your question about the ACI 17.4 clause talking about composite action, this is in reference to horizontal shear between the layers. The vertical joint you are looking at cannot be considered monolithic.