Any 2-way slab experts here? 1

[JPzBeenjamin]

I posted a topic at the formwork area but can't seem to get it to post here

http://www.eng-tips.com/viewthread.cfm?SQID=92851&SPID=582&page=1

I could really use some help before I start laying out the rebar grid and pouring the piles
Any/all help would be much appreciated...I know I am registered as an Automotive person but I do have skills in construction.
Thanks
Jay

 

[JAE]

Jay - I guess that if you don't know how to analyze a two-way concrete slab system, I wouldn't shoot from the hip and blow your toe off - or worse.

The proper reinforcing in the slab is key - do you know how much to put into the column strip? Do you know what a column strip is? How about checking punching shear at each column based on the loads -do you need a drop panel or capital at the columns?

If you don't know the answers to these questions, better go hire yourself a structural engineer who could quickly analyze this and get you the correct rebar layout - including column strip / middle strip layouts in each direction.

Sounds like you have lots of experience in construction and such - but not in structural analysis.

 

[JPzBeenjamin]

JAE
You are correct about my not being a structural engineer.
Thank you for the input and the safety pointers. I had investigated the column strips and the capitals and dismissed them as this will be a flat slab with 4 concrete/rock walls 9" thick supporting the slab between the columns, except for the overhang. I am most concerned about the slab not collapsing in the middle of the room. The slab is basically 12,000lbs and I was hoping for input on the integrity of a slab this size..can this slab maintain its integrity at 4" thickness using #4rebar and a remesh screen(I forgot to mention the remesh before)and 3500psi concrete..I have installed 4 post above ground lifts for diesel trucks which weigh more than this when loaded on a concrete wherehouse slab and it just seemed that the 24" piles plus the 4"x4' RC foundation I am pouring around the outside of the rockhouse with the columns sitting on the foundation/piles would be pretty rigid(the foundation and piles being poured together for a 28" thickness at the piles). Of course I would like to design the slab to be supported only by the columns....I decided to reinforce the concrete/rock wall existing structure right up to and attached to the bottom of the slab as further support so as not to punch through the slab at the columns and also give me added lateral stability. The floor of the rockhouse will also be repoured further stabilizing the wall integrity. The final outcome will be a concrete box.
I had thought that a slab of this size would have a basic structural formula for the grid spacing and rebar size and depth and proper concrete rating.
Thanks again JAE for taking the time to respond and looking out for my big toe.
Jay

 

[JAE]

Jay,
Well, short of doing a full blown finite element analysis, you cannot "dismiss" the column strips - that is the only ACI code methodology for designing two-way flat slabs.

4" thickness sounds thin - I would definitely worry about columns punching through the slab (shear failure) ...unless I'm not understanding your layout and you have no interior columns.

In any case, you would need to design the proper reinforcing to span the distance based on the required dead plus live loading of your floor. Also, you need to watch out for long term creep deflection in the floor - ACI has minimum slab thicknesses for set spans for two-ways slabs -but I'm away from my office right now and cannot get to it.

If you don't know how to do these things - again, I'd get a structural guy to help - shouldn't take a whole lot of fee to do this and you'd ensure your mother-in-law's safety.

 

[JPzBeenjamin]

Maybe I am not giving the slab the right name(2-way)
Maybe this will explain the building a little better but my main interest is the elevated flat slab 19'x16'x4" #5rb at a 12" grid + remesh overlay...3500psi concrete.
The 9"thk concrete walls "z" will come all the way to support the bottom of the slab leaving an unsupported slab area inside the rockhouse of 16'8"x8'4". Now I do have a steel I-beam that I could run between columns b and ba if I had to...but I would rather leave the ceiling clear inside. If that means going to a smaller rebar grid or increase to a 5.5" or 6" thk slab or both I would be happy to do that. Or go to a higher rated concrete?
I just want to know if this design will work and if not....just an explanation as to why?



|a b a|
zzzzzzzzzzzzzzzzzzz
z z
z z
z z
zzzzzzzzzzzzzzzzzzz
|aa ba aa|

|bb d d|
----------------------


z=the existing rockhouse structure
od=18'4"x10'
id=16'8"x8'4"

a-b-a=columns outside the rockhouse structure west side
a=12"sq x8'tall RC filled cinderblock columns #5rb 3500lb concrete
b=8"sq x8'tall RC filled cinderblock columns #3rb 3500lb concrete

aa-ba-aa=columns outside the rockhouse structure east side
Same specs as a-b-a

bb-d-d=4' Porch overlap supports
bb=same spec as b
d=4x4 pressure treated wood post

 

[JAE]

While the slab is supported on four walls/supports, it is elongated enough where you would design it as a one-way slab spanning the 8'-4". According to ACI, the minimum slab thickness for this span is 5" (based on L/20).

The general design would work - assuming the support walls and your surrounding columns are adequate - I can't do the engineering for you but the general concept appears OK. I would still (broken record here) get an engineer to review it all, though.

 

[JPzBeenjamin]

JAE
Thanks for your time, expertise and persistence "broken record" I realize that asking anyone here to do the engineering is above and beyond the scope of this forum and I expect no one to do this for nothing and you have already given me plenty of food for thought.
Thanks for all and much appreciation again for your input.
Jay

 

[JPzBeenjamin]

Well I decided I would come back and let you folx know how things are going. I rebuilt the foundation and filled the floor in and sunk my 5/8 rebar 24"+ into the piles. Wore my butt out and decided to build the roof/upper-floor out of wood. I give you guys credit for workin this concrete...definately not a one man job...That's for sure. Although working wood has its down side.....I don't think there was a single board that didn't try to stab me or try to give me some kind of grief...still pulling slivers. To level out the inside floor...the difference was 15" from the highest spot to the lowest inside the rock house. So I decided I would rather work with the nice slow pace of wood than the "get your butt in gear, It's starting to stiffen up" )). But I did pour about 8 yards of concrete just to level out the floor area and another 2 or so for the columns.
I just wanted to thank you folx for not heaving me out of here and giving me a few good thoughts. I haven't given up on concrete....I will just think twice about restoring concrete opposed to tearing down and just starting from scratch.

Thanx again...And a big thanx to JAE for even sparing me a few moments of his time...

and if you like a little older heavy rock
Come on by and enjoy some axe kickin rock

http://www.ic-musicmedia.com/artist_pages/artistpage.php?id=41128

Jay

http://www.anycities.com/beenjamin/