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

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>From my 2008 Lehigh lecture notes, which agrees with Harold's comments 
below:

"Because of the restraint caused by metal decking and headed-stud shear 
connectors, dry shrinkage and thermal contraction of the concrete can cause 
 cracking, just as restraint from the subgrade base can cause cracking in 
slabs-on-grade. Sawn contraction joints that help control random plastic 
shrinkage and restraint cracking in slabs-on-grades, however, aren’t as 
likely to be helpful in suspended slabs. For elevated slabs, flexural and 
shrinkage/temperature reinforcement control cracking while at the same time 
 tying the slab together to assure that it behaves as assumed by the 
structural design. Because this reinforcement can restrict movement at a 
control joint, reducing the slab cross section by sawing a joint isn’t 
likely to force the crack to appear at the joint. This condition makes 
random cracks in framed slabs likely to occur even if sawn joints are 
provided. Therefore it is recommended for all framed slabs that no 
contraction joints and as few construction joints as possible be used. As a 
 result of this approach, random cracks will form, however, any cracks wide 
 enough to affect floor finish serviceability can be easily patched and 
repaired.

Slab cracking in a composite beam and slab system also has a tendency to 
occur directly over the main girders as the supported bays on either side 
deflect causing tensile stresses in the concrete perpendicular to the 
direction of the slab and deck span. In order to help control this type of 
cracking it is recommended that additional slab reinforcement perpendicular 
 to the girder span be placed in the slab. For a typical combined 6 inch 
deep deck and concrete slab, #5 bars at 18 inches on center are 
recommended. The bars should extend beyond the girder, on each side, at 
least one third of the supported bay span."

D. Matthew Stuart, P.E., S.E., F.ASCE, F.SEI, SECB, MgtEng
Structural Division Manager
Pennoni Associates Inc.
Office 215-222-3000 x7895
Mobile 908-309-8657
http://www.pennoni.com | mstuart(--nospam--at)pennoni.com

-----Original Message-----
From: seaint-seaosc(--nospam--at)mail-list.com [mailto:seaint-seaosc(--nospam--at)mail-list.com] On 
Behalf Of Harold Sprague Sent: Saturday, December 21, 2013 2:09 PM
To: seaint-seaosc(--nospam--at)mail-list.com
Subject: RE: [SEAINT-SEAOSC] Construction/Isolation Joints for Composite 
Slab on Steel Beams

Bill,

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 
general.

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

Truncated 1976 characters in the previous message to save energy.

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