Maximum plan dimensions for slab on grade 2

[mar2805]

Hi folks!
What are maximum plan dimensions of a 4" thick slab on grade that will not crack due to shrinking problem?

I can use a "drag formula" for 1-layer reinforced SOG, but what aboth unreinforced slab?
Is it safe to put an very very small value of reinforcement area in the drag formula and calculate joint spacing?
Thank you.

 

[mar2805]

I agree partialy (there is contact of slab and the soil where there is no heaving) with you on that BUT show me just one egsample calculated using this procedure that reinforces slab in 2 layers!
Few egsample I found only reinforced slab in top layer, no bottom reinforcement for postive moments!
Beams are reinforced in top and bottom layer.
Slab reinforcement that was calculated and provided only in the top layer was usualy less then 0,5% wich in turn mean that there will be shrinkage cracks---can anyone correct this last statement couse I cannot understand this value of 0,5% that controls shrinkage when suspended floorslabs (1st, 2nd,3rd floor) have less then 0,5% reinforcement for bending and there where never any shrinkage cracks!

 

[mar2805]

I found this great article for stiffened slabs on grade:

http://www.wirereinforcementinstitute.org/pages/pubs/pdf/TF 700-R-07_Slab_on_Ground.pdf

On the last pages theres an solved egsample.
You can see clearly that the slab is reinforced ONLY in the middle of the slab!?
SInce this is in the middle I can only assume that this is for shrinking purposes.
If it would be for flexure it had to be positioned in the upper or bottom part to adress tennsion forces in section.
Can someone plase tell me whats the amount of slab reinforcement provided in the egsample ( in percentage )
min. reinforcement needed for beams and slabs in bending is 0,15% for EUROCODE.

Thanx and Merry Christmas everyone!

 

[mar2805]

Hi guys!
I found this instruction manual for software called SlabWOrks (made by mr. Eric Green) wich is used for designing stiffened slabs on grade on expansive clays.
Manual is located here:

http://www.scribd.com/doc/161912886/SlabWorks-Manual-1-07Beta#page=27

I will quote some sections here and then hopefully discuss with you.

page 22/95
"Overall structural performance of stiffened slabs is generally independent of the
performance of the thin slab in the areas between beams. This portion of the
foundation slab is generally intended only to acts as a separator between the
building and the soil below. However, if thermal or shrinkage cracking is noted
in these areas, many owners will perceive the foundation is in a failed state. This
is particularly important if the owner anticipates the use of tile or stone finishes.
Therefore, performance expectations with respect to slab cracking should be
discussed with the owner and architect prior to design.
In the past, many engineers have relied on the minimum temperature and
shrinkage steel requirements from ACI-318 (0.18% steel). These guidelines are
intended for elevated structural slab and are not applicable to slabs-on-grade.
This is discussed in the commentary to the latest version of ACI-318.
The engineer should instead refer to ACI 224 “Control of Cracking” for guidance
on controlling cracking of slabs-on-grade. Generally, cracking in stiffened slabs
is controlled with bonded reinforcement, and control joints are not used.
According to ACI 224, 0.50%-0.60% steel is required to control cracking with
steel alone. Control joints can be installed, with the control joints located midway
between the stiffening beams. Control joints near beams will not be effective because the
beams restrain the concrete from movement."

I found an interesting thread here on the forum on MINIMUM reinforcement requirements for members in bending.
It seems that ACI codes made this a bit confusing but generaly it was agreed that a ratio of 0,18% is the minimum reinforcement that should be provided anywhere in the member where tension occurs.
If you have moment appearing in the top and in the bottom of a slab you should provide 0,18% ratio in the top portion of the slab and also 0,18% in the bottom of the slab.
This wll ensure ductile behaviour of a member because of a slightly higher moment capacity o the cracked section vs. uncracked section.

The sentense "These guidelines are intended for elevated structural slab and are not applicable to slabs-on-grade" is where I get confused and if you read further in the manual, page 89/95, Quote:
"As discussed in the commentary, ACI 318-05 minimum steel requirements
(0.18%) for elevated slabs are not applicable to slabs-on-grade. Instead, refer to
ACI 224 “Control of Cracking” for directions on controlling cracking of slabs-ongrade. Generally, cracking is
controlled with a combination of steel and control joints. According to ACI 224,0.50%-0.60% steel is required
to control cracking with steel alone."

0,5-0,6% ratio is VERY HIGH reinforcement amount and I cannot understand with this.
So why doesnt rule apply also to elevated slabs wich are not resting on the ground and wich usualy have higher bending moments?
Would this rule also then apply to raft structures since they are also constructed as jointless, let say for residential house a 12inch raft, plan dim. 50ft x 35ft would need a 0,5% reinforcement ratio if poured all at once (no joints that is).
Please correct me couse I dont wanna belive that this is rational design and Ive designed raft with 0,18% both sides many times.
Was I wrong?

 

[dik]

The reason for placing the steel in the top is to minimise cracking on the top surface which is subject to 'wear and tear'. Cracking of the top surface is not aesthetically pleasing because it is visible. Cracking of the underside is not so noticeable <G>.

Dik

 

[AELLC]

If this is becoming an intractable problem, have you considered using a "California" - style post-tensioned slab?

5-1/2" to 7-1/2" thick

12" deep perimeter "beam"

 

[dik]

As a result of this thread, I've put together some items for Slab-on-Grade design and have attached these. If anyone has any comments, please add them (*.pdf files can be annotated). I'll combine the result into something a little more coherent and post it in E-T Journal.

Dik

 

[cvg]

suggest the following is a very good reference for both flexible and rigid pavements

http://www.pavementinteractive.org/category/pavement/

 

[mar2805]

Hi guys.
I found here an lecture on stiffened SOG on swell soils.

https://ceprofs.tamu.edu/briaud/Stiffened Slabs on Grade on Shrink Swell Soils Lecture.pdf

It explains design procedure using Abaqus model wich simulates edge up and drop lift.
I was thinking of maybe using winkler soil model wich would be applied depending on the scenario.
edge drop - no surface support (k modulus) on perimetar of the slab
edge lift - no surface support (k modulus) in the center part of the slab
Design members and required reinforcement but ensuring that maximum permissable deflections for downstand beams are within code limits wich will ensure that there are no cracking in walls and partitions above.
What do you think?

One thing I noticed in the lecture above, is that they only reinforce 4inch slab with ONLY ONE layer of reinforcement.
I cannot understand this.
Is this for flexure or shrinkage ?

 

[AELLC]

Sounds a bit bogus. If the rebar is in the center, then you get equal, but very small, capacities in both bending moments, positive and negative.

 

[mar2805]

But if your bending moments in slab are very small it could work....?

 

[cvg]

you cannot possibly put two layers of reinforcement in a 4 inch thick slab.

and why would you design a slab that thin anyway?

 

[mar2805]

Seems that guys have done it succsesfuly, so I dont see any problems?!
Slab is thiner becasue of the downstand beams wich greatly enlarge the ovearll stiffnes of the structure as a whole.

 

[AELLC]

But after all the expense of providing the beams, why not use a "California" - style post tension slab on grade? If the soil is classified non-active, it can be only 5-1/2" thick, and there you have more reliable precedence.

 

[mar2805]

Post tensioning is something that hasnt been much used in small residential project in my country and it only used in highway bridge design.
So Im not going to be considering it at all.

 

[mar2805]

People again, I need you advice.
Plase look at the picture attach.
For edge heave situation there will be lost of contact in the midle portion of the entire slab wich will couse a load transfer from structure to parts that remain in contact with the ground. Since perimetar beams are usualy place under load bearing slabs they will produce the bigest pressure on the ground. When edges are lifted up and due to lost of contact the loads will be transfered to edge beams that in contact with ground but NOW the pressure will be increased substentialy!
I havent seen this be controled in any of the design methods PTI, WRI, BRAB...
How safe is to ignore this effect?

For "edge heave state" you can clearly see that due to lost of contact between slab and ground you will get positive bending moments in the slab from self-weight and area loads on the ground floor, yet in previous egsamples and case studeis no bottome reinforcement was ever used in the slab.
Why?

 

[cvg]

Seems that guys have done it succsesfuly, so I dont see any problems?!

concrete cover on the bottom of slab on grade should be at least 1 1/2 inches and top could be as thin as 1 inch of cover. Assuming two layers of #4 bars in a 4 inch thick slab, then you have just a half an inch of space between the two layers of steel. That is just too tight for adequate concrete placement, can't be done.

If you want to design a structural slab on grade with double mats of steel bar reinforcement, than you will just have to buck up and make it a bit thicker than 4 inches.

 

[mar2805]

Agree with you on that, but if you look at my previous post I dont undersatnd why they never provide bottom rinforcement in slab for edge heave situation?

 

[hokie66]

The system illustrated, with a 4" slab and edge beams only, is not appropriate for sites subject to high shrink and swell conditions. A "raft slab" is one system used for such sites, and a raft incorporates a grid of stiffening ribs.

 

[mar2805]

Sorry the sketch is wrong.
I was thinking of stiffened SOG with a system of perimetar and internal beams (waffle raft).
The question on reinforcing 4inch slab in waffle raft with ONLY ONE layer of top reinforcement, is still not clear.

 

[mar2805]

Im I BLIND??!!
Flanged beam effect, effective width of the beam for positive moments!!!!!