×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Uniformly Loaded Slab on Grade

Uniformly Loaded Slab on Grade

Uniformly Loaded Slab on Grade

(OP)
I came across a situation which seemed simple but apparantley I have no references on the proper way to do this. I have a slab on grade that needs to be designed for a 460psf uniform load. Im specifying 6" porous fill under on controlled fill or original soil. I will spec contraction joints 20' max. It is a small area. It is a pit for a prefab "pool treadmill". It will not be exposed or walked on so minor cracking is not a huge concern.

I need advice or references on thickness and reinforcement.

Thanks

RE: Uniformly Loaded Slab on Grade

One of the best books for slab-on-grade design is "Designing Floor Slabs on Grade" by Ringo and Anderson.  One of their major concerns, in addition to the applied loading, is the subgrade soil conditions.  Is your subgrade a granular soil, a plastic clay or a compressible clay?  

A loading of 460 psf on a dense, granular subgrade is not too bad, but on a compressible clay it could lead to continual settlement problems.

So, first find out from your geotechnical engineer the subsoil charactoristics and capacities, then design your slab.

Differential settlement under your pool treadmill could ruin your pool!

RE: Uniformly Loaded Slab on Grade

I recommend that your contraction joints be placed at 2.5 times the slab thickness in feet (i.e 2.5X6 = 15.0 feet). This is a good rule of thumb for joint spacing.

I ditto Jheidt's input regarding the reference.

RE: Uniformly Loaded Slab on Grade

I'm not a structural so correct me if I'm wrong, isn't the spacing of contraction joints dependand on your mesh strength, material the slab is placed on and the tolerance for displacment at the crack?

Ken

RE: Uniformly Loaded Slab on Grade

Slowzuki - For unreinforced concrete (which this seems to be), I concur with Lutfi. If joints are spaced too far apart curling becomes excessive. This causes unpredictable cracking.

jjeng2 - As always,be sure to cure the slab thoroughly (preferably wet curing to minimize curling).

RE: Uniformly Loaded Slab on Grade

You might want to consider having thicker edge footings (turned down footings).  This will help the curling issue because if you have a slab of uniform thickness, the edges dry first and begin to curl.  If you thicken the edge, then it dries more uniformly and it won't (or shouldn't) curl.

RE: Uniformly Loaded Slab on Grade

I'm not sure I subscribe to mjflorio's theory regarding a thicker edge footing (turned down footing) at a slab edge as a "cure" for slab curling.

First of all, if this were a cure, then each construction joint in a large floor slab would need a thickened edge.  This would add a significant cost to floor construction.

Secondly, not all floors curl.  From my experiance, floors that are wet cured and allowed to slowly loose their excess moisture, curl less.  The key seems to be a more even drying of the slab through it's thickness.  If a vapor barrier is used under the slab (which I am in favor of) the process takes longer, but it can be done.  One thing that seems to help even out the rate of moisture loss is the use of a high solids curing agent on the floor slab.

RE: Uniformly Loaded Slab on Grade

It also depends on the pour size.  I reread the original post, and I believe my comment doesn't apply because of the relatively close spacing of joints.  But for large pours like that at Home Depot or a warehouse, this does present a solution.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources