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RE: Joints in concrete floor slab on bar joists

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Bill,
Short span elevated slabs have been designed for many years as "plain concrete" in the commercial building world. The SDI recommended reinforcing steel used in elevated slabs on metal decks is more for crack control and may not meet the ACI requirments for crack control. Many of these buildings have been in service since the 1950's. Almost all of the 1950's and 1960's shopping malls used form deck with temperature reinforcing only. I was involved many years ago in shopping malls even in your neck of the woods (Denver). You have done a lot of your Christmas shopping on unreinforced concrete floors.

Early composite metal deck had WWF welded to metal deck. The shipping costs were too high to gain wide acceptance. Only in the early 1980's did the trend shift to composite concrete metal floor deck when deck manufacturers used rolls to deform the sides of the metal deck to create a composite deck that could be stacked and shipped the same as regular form deck.

The problem comes about when NO crack control steel is used or if the contractor tries to substitute polypropylene fibers. The aggregate interlock is lost and then the model can become a cantilever as opposed to a simple span. There has been a lot of research at SDI and SDI members to substantiate the use of concrete slabs on form deck with only temperature steel. The safety factor is also fairly generous

With all of that, today I would be very judicious about where I would use a form deck and plain concrete slab today. But I have used them in my early career.

Regards,
Harold Sprague


From: "Sherman, William" <ShermanWC(--nospam--at)cdm.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: seaint(--nospam--at)seaint.org
Subject: RE: Joints in concrete floor slab on bar joists
Date: Tue, 20 Jul 2004 09:49:28 -0400

Several responses note to check the slab as an unreinforced section, as
"plain concrete". Is this permissible for an elevated slab? Admittedly the
weight of the slab is supported by the metal deck but I'm not sure that is
equivalent to the support given a slab on grade. If the metal deck can only
support the concrete dead weight but not  the live load and if the plain
concrete cracks, won't the deck then need to support both the dead load and
live load? Is that a safe design?

I generally don't use "plain" concrete, so I am not clear on what the code
permits in such circumstances.


William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

 ________________________________

	From: Stuart, Matthew [mailto:mstuart(--nospam--at)schoordepalma.com]
	Sent: Thursday, July 08, 2004 2:27 PM
	To: seaint(--nospam--at)seaint.org
	Subject: RE: Joints in concrete floor slab on bar joists

	I don't know what your live load criteria is but my hunch is that
when you run the numbers the slab section will check as an unreinforced
section under the provisions of ACI 318 Structural Plain Concrete.

	D. Matthew Stuart, P.E., S.E.
	MASCE/SEI/BPAD
	Senior Project Manager
	Schoor DePalma Engineers and Consultants
	200 State Highway Nine
	Manalapan, NJ 07726
	732-577-9000 (Ext. 1283)
	732-431-9428 (Fax)
	908-309-8657 (Cell)
	mstuart(--nospam--at)schoordepalma.com

	-----Original Message-----
	From: DSmith7367(--nospam--at)aol.com [mailto:DSmith7367(--nospam--at)aol.com]
	Sent: Thursday, July 08, 2004 4:17 PM
	To: seaint(--nospam--at)seaint.org
	Subject: Re: Joints in concrete floor slab on bar joists

	Matthew:

	Thank you for the reply regarding the slab problem.

	The slab is on light metal slabform spanning two feet between bar
joists.  How would you add sub framing?  It would be tough.
	The same with external reinforcement.

	Regards,

	Dave Smith

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