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Re: Crack in Wall Connected to Post-tensioned Slab

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Initial thoughts are this sounds like restraint cracking.  I do not know the
geometry of the problem at hand, but if you picture two walls 100 feet apart
at opposite ends of a slab pull without a construction joint the walls are
trying to restrain the slab from shortening during the stressing operation.
This can generate large shear and bending stresses in the walls.  If the
expected shortening of the slab at the location of the wall is .25", the
corresponding force developed in the wall will be large.  I would look at
the pattern of cracking from the viewpoint of the center of the slab being
stressed.  If the direction of the cracks are opposite in opposing walls,
and increasing as you move further from the center for a series of walls,
the problem is probably restraint.

The layout of joints and sequencing of construction for restraint and
stability during construction are design considerations the same as overall
lateral and vertical systems.  We typically will "leave the walls behind"
during construction or specifically isolate elements of the structure with
pour strips to allow slab shortening during construction.

Paul Feather

----- Original Message -----
From: "Shahrouri Issam" <ishahrouri(--nospam--at)lagunabeachcity.net>
To: <seaint(--nospam--at)seaint.org>
Sent: Monday, July 02, 2001 8:17 AM
Subject: Crack in Wall Connected to Post-tensioned Slab


> The project is a 5 story hotel in Laguna Beach, under construction. As the
> tendons were being tensioned in the slab, cracks started appearing in some
> of the supporting walls. The direction of the tensioning was parallel to
the
> wall. The cracks are diagonal starting at the point where the
> post-tensioning begins downs to the ground. The width of the crack varies
> from about 1/8 inch at the top down to almost nothing at the ground. The
> crack occurs on both sides of the wall. The walls are 8 inches thick and
> designed to act as shear walls as well.
>
> I would appreciate any comments from a similar experience.
>
>
> Sincerely,
>
>
> Issam Shahrouri, PE, CBO
> Senior Plan Check Engineer
> City of Laguna Beach
>
>
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