×
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!

*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

Control Joint in Structural Slab on Voidform

Control Joint in Structural Slab on Voidform

Control Joint in Structural Slab on Voidform

(OP)
A pump house is designed sitting on concrete slab supported on piles (kind of common in the north area when the building is not considered heated and no uplift movement is allowed). This kind of slab is little bit special, from structural design viewpoint.

The slab is designed as the typical structural slab (like the flat-plate concrete floor), on the other hand it is a slab on “grade” - the undegradable void form. The slab concrete is reinforced to support the load on floor (with neglecting the voidform) and the frost heave pressure from the voidform. The pressure (somewhere around 600psf based on the locality and voidform property) is usually more critical.

Is control joint necessary for this slab?  The house is about 30ftx20ft footprint with 9-24”dia piles. The slab is 18" thick with double reinf. The cracks (because of the restraint from the piles)on the slab may be a concern.

Thanks very much for shedding some light on this!

BTW, the punching shear from the middle pile (when frost heave occurs) seems special as well, since the effective area against the shear is much smaller than those in the typical punching shear check.

RE: Control Joint in Structural Slab on Voidform

Control joints are never required for structural slabs.  The control joints in slabs on grade are used to control the locations of cracks in slabs which usually have low quantities of reinforcing and are subjected to subgrade drag forces as the concrete shrinks upon curing.

For a structural slab..on forms or on void forms, the drag forces are less and the reinforcing is higher.  

For an 18" thickness - a control joint wouldn't do you much good unless it was very deep and that would work counter to the structural behavior.

RE: Control Joint in Structural Slab on Voidform

J1D:

See the thread by cdi12 on 'Concrete Mat Expansion Joint' that was started last Friday. Many of the same ideas apply.

If you use .5% reinforcing (.005 times gross cross sectional area of concrete) cracks will stay closed, and no joints are needed.

RE: Control Joint in Structural Slab on Voidform

(OP)
Thanks, JAE, for mentioning the drag force. Good point! Do you think the piles may serve as the source of drag force?

lkjh345, I'm not sure if I have to go up to 0.5% reinf for better crack control. From stregth point of view, the minimum reinf ratio (0.2%) is good enough. Some cracks are okay for the slab. I just don't know whether control joint is necessary to locate the cracks.

RE: Control Joint in Structural Slab on Voidform

J1D - now that I read your first post more closely (I admit I am guilty of reading posts way too quickly) - The 24" diameter piles (piers?) would certainly participate in some level of resistance to overall shrinkage of the slab.  

I suppose you could get some sort of spring coefficient and model the thing, but for a house that's probably overkill.  I guess I'd go about it by making sure the concrete mix is chosen to minimize shrinkage (i.e. larger coarse aggregates, low water/cement ratio, water reducer, etc.) and just be careful how you detail the slab top reinforcing to keep cracks closed in likely crack areas.

Even though where you think the cracks will appear, they think differently.

RE: Control Joint in Structural Slab on Voidform

For a slab of 30ftx20ft I wouldn't consider control joints. If using 0.2% reinf top & bottom you will have quite effective crack control. If using 0.2% total the control obviously won't be as good, but probably adequate for a basically dry environment.

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! Already a Member? Login



News


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close