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Re: Slab on steel deck question

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To me, it is going to depend on how you want your slab to behave.  Most
slabs are designed as "continuous beams", which mean that you will have
maximum positive moments roughly in the middle of the span and maximum
negative moments right over the beams/joists.  Thus, the typical
recommendation that I have seen on the placement of construction joints it
to place them roughly 1/3 of the distance from one beam/joist to the next
beam/joist, which will correspond roughly with the location of zero
moment.  More than likely if you are placing the construction joint at
midspan of the slab, then you will have problems unless you design the
contruction joint to transfer moment across it or design the slab to
cantilever to that point.

So the way that I look at it, decide how you want your slab to behave
(i.e. continuous beam or simply supported beam, etc) and then place the
joint at the appropriate location.  Keep in mind that even though your
joint location may occur at about zero moment, it may still need to
transfer some significant shear across it, potentially requiring dowels,
etc.

HTH,

Scott
Ypsilanti, MI


On Wed, 26 Mar 2003, Bruce Holcomb wrote:

> I designed a 4" total thickness normal weight concrete slab on .6C28ga galvanized steel form deck with 6x6-W2.1xW2.1 WWF @ 1" to 1.5" below the T.O. slab.  This is on K-series joists at 2'-6" o.c.; Live Load is 100 psf.  The architect wants to pour integrally colored concrete in various different size pours to create an interesting pattern... this will involve several construction joints around each separate pour.  I told him we would need to lap the WWF and apply a bonding adhesive at the construction joints.
>
> The concrete contractor is concerned that the construction joint as noted above will cause each section of concrete to crack and wants to leave off the bonding agent and not extend the WWF across the construction joint.  I have never done this... usually, I provide dowels at the joint and I am careful in locating the joint.  I also limit the construction joints to a minimum.  But on this project, we will basically be creating several small slabs as opposed to one large slab.  I am concerned how this will behave structurally.  Lateral shear forces transmitted through this slab are minimal, so that doesn't concern me.  I am mostly worried about construction joints parallel to the joists located at the center of the joist space.
>
> If the concrete slab can cantilever from the joist to the construction joint with the WWF as the tension reinforcement, should I have any other concerns?  There would be no WWF or dowels across the construction joints and the joints are randomly located.
>
> I would like to know how other engineers handle construction joints on elevated slabs.  Thanks!
>
>
> Bruce D. Holcomb, PE
> Butler, Rosenbury & Partners
> 300 S. Jefferson, Suite 505
> Springfield, MO 65806-2217
> ph (417) 865-6100
> fax (417) 865-6102
>
>
>
>
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