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RE: Construction/Isolation Joints for Composite Slab on Steel Beams

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Most of the slabs on metal deck have no crack control joints in the 
concrete.  The only isolation joints are at expansion joints, and most 
owners do not like isolation joints.  I try to eliminate joints as much as 
possible in slabs on metal deck.  They are a pain and are not necessary in 

The concept is to control the cracks using WWR in most of the areas.  Use 
the ACI recommendation for temperature reinforcing for the WWR as opposed 
to the SDI.  At girders, cracks tend to open up more.  What is generally 
most effective to provide #4's x 8'-0" at 18" o.c.  and stagger the rebar 
by about 1' so that you have a 2' net offset at the ends of the bars.  An 
abrupt change of area in the reinforcing steel tends to focus the cracks 
along that plane.  Offsetting the rebar tends to mitigate that.  This is 
the general practice that is most effective.

Back in the day, I designed acres of shopping malls all over the US.  This 
was the general method that proved to be the most effective.  You will get 
developers that will compare your work to others, and you have to be 
willing to defend the ACI temperature steel as opposed to the SDI.  Never 
ever use fibers in lieu of temperature reinforcing.  It will cost about $7 
per lineal foot for the crack repair because the cracks will be fewer, but 
bigger to where aggregate interlock is lost and the slabs will curl and 
create a tripping hazard.  I have seen that in every area of the US.

Concrete slabs on metal deck behave differently.  The steel deck is rigidly 
 connected to the supporting structural steel.  Concrete will still shrink. 
 The steel deck is bonded to the concrete so that there will be distributed 
 shrinkage cracks.  Trying to create a plane of cracking can't happen 
because there is no defined location to form a defined plane of crack 

Put in the temperature steel, have them use a laser screed, use a good mix 
design for limiting shrinkage, try to get a mini-conveyor as opposed to an 
overhead line pump for placement.  I imagine that you are using a light 
weight concrete which creates other issues.

Harold Sprague

> From: wpolhemus(--nospam--at)
> Date: Sat, 21 Dec 2013 18:42:14 +0000
> Subject: [SEAINT-SEAOSC] Construction/Isolation Joints for Composite Slab 
> on Steel Beams To: seaint-seaosc(--nospam--at)
> Having a bit of trouble deciphering what I'm reading in various sources
> including ACI 302.
> If I have a composite slab on steel beams (steel form deck plus stud 
> shear connectors), what is the proper way to "isolate" each adjacent 
> floor slab panel? For instance, I really don't want the negative moments 
> over the girder between panels in the slab. I THINK ACI 302 is 
> recommending an isolation joint "offset" from the centerline of the 
> girder about four feet or so (rule of thumb I guess).
> Is this correct? How should it look? Use the "dowel basket" detail shown 
> in ACI 302?
> William L. Polhemus, Jr. P.E.
> Lead Civil/Structural Engineer

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