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RE: Control joints in Elevated slabs

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A few years ago there was a series of AISC lectures that I attended.
Lecture 3 was "Control of Floor Elevation and Levelness", was presented by
Bob Lorenz.  In this lecture and in the handout there was a section on
concrete slab construction placement methodology specifically for slabs on
metal deck and steel beams.  I would highly recommend the lecture and the
handout.

Maybe Charlie Carter could comment on the availability of the notes.

Basically, what you can require in construction sequencing on a slab on
grade, is significantly complicated by placing an elevated slab supported by
steel deck and beams.  If you place the concrete in a "checkerboard"
pattern, your deflections will be a moving target with concrete that may
have already taken a set.  The problem that you solve for shrinkage will
create a more difficult problem for floor levelness.  I would suggest using
what works for sequencing and CONTROL the cracks (especially at the girders)
with additional rebar.  Cracking over the joists or beams is easily
controlled by elevating the WWF with chairs.  It is best to have the mesh
with 3/4" cover at the joists or beams, but bear in mind where the finish
floor elevation will be and what the camber will do.  Again, I would
recommend the lecture and the notes.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Nigel Blair [SMTP:nwblair(--nospam--at)sunbeach.net]
> Sent:	Wednesday, November 28, 2001 11:50 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Re: Control joints in Elevated slabs
> 
> Harold,
> 
> I agree that some amount of rebars should be placed at girders locations 
> to minimise cracking, especially if concrete pour are likely to be 
> large.  Recently on a job I saw some cracking between columns but only 
> at girder locations.  There was no cracking at beam locations.  Mesh was 
> used throughout.
> 
> I  think good construction practice would be to avoid pouring large 
> areas in one pour e.g. pour floor in smaller but alternate bays, and a 
> little extra reinforcement would minimise cracking.
> 
> The contractor probably inserted control joint due to his experience 
> with cracking is the past.  I would not recommended it.
>  
> Regards,
> 
> Nigel.
> 
> 
> 
> Sprague, Harold O. wrote:
> 
> >Hyndman,
> >
> >No.  But cracking in concrete slabs on metal deck is an issue.  
> >
> >Number one, never use polypropylene fibers.  Use mesh or rebar.  This is
> >another topic.
> >
> >If the slab is covered by carpet, use the minimum SDI recommended steel
> and
> >forget about it.  
> >
> >If the slab is going to be exposed or covered with a material that could
> >reflect cracks, CONTROL the cracks and consider the following.
> >*	Add TOP rebars about 5' long perpendicular to beams and girders.
> >This is not required for strength.  It is just to CONTROL the cracks that
> >WILL form due to negative bending along the center line of steel girders,
> >beams, and joists.  The only exception is if the joists or beams are
> spaced
> >at about 6 ft. or less or if you drape the mesh.  You will still need the
> >rebars at the girders.  The crack widths can be calculated, and when they
> >reach the point of ugly (architectural term), add rebar.  
> >*	Use the ACI minimum reinforcing steel (as opposed to SDI).
> >
> 

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