×
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

Control joints in slab on grade
2

Control joints in slab on grade

Control joints in slab on grade

(OP)
I have a slab on grade in an industrial structure, and I am trying to figure out where to put my control joints.

The slab is 9" thick, and has two layers of reinforcing, each with #5 bars on a 12in grid. The slab is continuous with the strip footing around the perimeter of the building for overturning reasons, so no control joint can be placed around the perimeter. The building is long, and ~40ft wide.

In order to avoid putting control joints underneath machinery, I would like to run a single control joint down the middle of my building making the joint 20ft away from the strip footing. The slab can not shrink away from the strip footing, so the effective length of slab doubles to become 40ft.

Is this design feasible? Do I need to do something special to make the single control joint system work?

RE: Control joints in slab on grade

The recommendation for unreinforced panels is generally 4.5m and yours is almost three times this. However with appropriate reinforcement you could make it work. Smaller reinforcement at smaller spacings is more effective in distributing shrinkage cracking. However you only speak of a joint in one direction. You will also need them in the other direction and at the edges in both directions you will have to cut through your footing which is going to be a tricky detail. You will need some kind of dowel detail to prevent differential settlement across the joint.

A useful formula from PCI for reinforcement required to control cracking is

p(percentage reinforcement)=1.77L(length between edges)/S(allowable steel stress).

The most important thing in a concrete floor slab and the hardest to control is the quality of workmanship including mixing, placing, vibrating, curing, floating.

Carl Bauer
www.bauerconsultbotswana.com

RE: Control joints in slab on grade

ludvik:

The current May-June 2002 issue of the ACI Structural Journa has a technical paper, No. 99-S37,titled "Shrinkage-Based Analysis for Control-Joint Spacing in Slabs-on-Ground".  The article covers the subject very well and has a list of 13 references.  It does not carry the weight of a code provision yet but, it does give a way of computing the necessary crack control.  Table 1 lists the PCA recommended joint spacing for a 9" thick unreinforced slab with a maximum course aggregate size less that 3/4" as 18'.  You should also note carlbauer's comment, joints are required in BOTH directions.

RE: Control joints in slab on grade

(OP)
Thank you carl and jheidt for your advice.

Just to clarify, I do have joints in both directions. The joints in the short direction are easy so I have not mentioned them.

Carl, you said that I need joints along the edges. I would like to be able to mobilise the moment connection between the slab and foundation - in fact I have seen this detail in other projects - but placing a joint around the perimeter would kill the idea. Why do I need perimeter joints? Also, what are the units of your slab reinforcement formula?

Jheidt, it is interesting to know that the slab will go 18' with no rebar...my firm is mostly a bridge design firm and I don't have immediate access to the ACI Structural Journal. Does the article mention how much extra distance you get by introducing rebar?

Another question: I may have to put my slab joints under the machinery. The machinery in question is for an electrical substation. It is switching boards and transformers mostly, and is quite heavy. I am keen to ensure that the joint does not cause any problems for the machinery...Will a normal dowel joint be acceptable?

Thanks


PS: carl: I noted from your little CV that you worked with Arup in Botswana. Did you ever meet Eugene De Souza? He was quite senior in the Arup Zimbabwe office before all the violence errupted there, and he moved to America. I worked with him briefly in the Arup Detroit office in 2000 (I'm now with another firm in New York).

RE: Control joints in slab on grade

Sorry of course you need the units. L is in metres and S is in MPa. The formula assumes a coefficient of friction of 1.5 under the slab.

What I meant about the joints was that they must go through the edge (perpendicular to the edge) not along the edge. Because your slab is joined to your footing this means the joint must go through the footing too (also perpendicular to it). My concern with that is that you then have to dowel the footing.

I heard of him but never met him.

Carl Bauer
www.bauerconsultbotswana.com

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


Resources

White Paper - PLM and ERP: Their Respective Roles in Modern Manufacturing
Leading manufacturers are aligning their people, processes, and tools from initial product ideation through to field service. They do so by providing access to product and enterprise data in the context of each person’s domain expertise. However, it can be complicated and costly to unite engineering with the factory and supply chain. Download Now
White Paper - Medical Device Design Control
Medical device product development is a highly integrated and regulated process. Implementation of a requirements tracking solution requires attention to a variety of nuanced topics. When presented with the task of tracking the many concept relationships in a project of this type, the initial software solution of choice tends to be a two-dimensional text systems. Download Now

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