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RE: Joints in concrete floor slab on bar joists

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Concrete slabs on deck are rarely jointed. The suggested reinforcement in the SDI manual is less than crack control steel per ACI. And the reinforcing steel is placed solely for crack control. I went through all of the 20 years ago with Walt Schultz (R&D Vulcraft), and it has not changed.

The concrete cracks where it will, primarily due to drying shrinkage. If the crack control reinforcing steel is not present, the slab cracks will be unconstrained and will open up sufficiently to allow differential curling. This only makes a bad problem worse.

Contractors do things like this all the time. I like "periodic" on-site observations to head off these types of problems before they get this far. This problem has no good and easy solution unless you have an incredible over-design on the joists. Barring a very high over-design of the joists, the contractor will have to remove the slab and put down the system as originally designed.

You may be able to allow the 0.6" x 28 ga slabform to remain in place to help keep dropping stuff through to the occupied offices, but it will have to be flattened out so that the 1.5" composite deck can be welded through to the structural steel / bar joists.

If the crack control steel is placed according to the SDI, cracking will still be there, but the crack widths will be minimal because they will be constrained by the crack control reinforcing steel.

My preference is to drape the steel as indicated in the SDI manual, but I like to use the ACI crack control steel ratios.

Joints in the concrete slab decks on structural steel will not do any good unless they are at an expansion joint that goes through the structural steel. The structural steel is continuous and will constrain the slab in spite of a sawn or tooled joint. It is best to add a lot of reinforcing steel to just hold together the cracks that will form, and forget about joints in the concrete slab.

Another thing to keep in mind is the present system will not meet UL fire rating requirements. The 1.5" slab with 3" of concrete has a UL fire rating. The goofy system the contractor installed will not have the UL fire rating.

Harold Sprague

From: DSmith7367(--nospam--at)
Reply-To: <seaint(--nospam--at)>
To: seaint(--nospam--at)
Subject: Joints in concrete floor slab on bar joists
Date: Thu, 8 Jul 2004 15:35:08 EDT

I have run into an interesting problem. Hopefully, one of our list members
will have a comment.

I am investigating a cracked concrete mezzanine floor.  The floor is  to be
used for parts storage but is presently very lightly loaded. The floor is 44' x 51' and has cracked badly throughout into sections approximately 16' x 18'.

It turns out that the architect specified 1.5" composite deck and a total
depth of 3 inches. No reinforcement was specified. The contractor, without
notifying anyone, substituted 0.6" x 28 ga slabform.  No  reinforcement was
provided.  No joints were provided in the floor.

As I started looking into the problem, I realized that I could not find a
specific reference recommending the use or nonuse of joints in such a floor
system.  I would appreciate some help on this.

Since there is essentially no theoretical floor strength, it appears that
the entire floor may have to be removed and replaced with reinforcing. There
are occupied offices directly under this floor.

Any comments?

Dave Smith

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