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RE: Two-way flat plate hairline cracks radiating from columns

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1951.  The plans say to use deformed bars. 

-----Original Message-----
From: Stuart, Matthew [mailto:mstuart(--nospam--at)schoordepalma.com] 
Sent: Friday, July 13, 2007 1:43 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Two-way flat plate hairline cracks radiating from columns

Sounds like an old SMI slab.  Are there concentric smooth round bars also?
How old is the structure?
 
Matthew Stuart

________________________________

From: Haan, Scott M POA [mailto:Scott.M.Haan(--nospam--at)poa02.usace.army.mil]
Sent: Fri 7/13/2007 4:46 PM
To: seaint(--nospam--at)seaint.org
Subject: Two-way flat plate hairline cracks radiating from columns



I had to go look at an old building where they are removing asbestos floor
tiles.  They had sand blasted the floor on one level. 

On each level there were cracks in the top of the floor slabs radiating
outward a few feet from columns.  The building was constructed with only top
steel over the columns and with five dog leg shear rebars over the columns
each way. The slab was 12" thick.

Most of the cracks were hair line on the levels without sand blasting, none
were more than 1/16". On the sandblasted level the cracks were more
pronounced because the edges got rounded. There was no cracking visible from
the bottom of the slabs.  The inspectors were alarmed by these sandblasted
cracks.

It appeared to me the cracking was from bending over the columns but that the
reinforcement was limiting the size of the cracks. It is not what I would
expect from a punching shear failure from looking at text books, but I was
thinking about how one of my college professors told me thin ice behaves when
you fall through and this looks like the picture he drew - cracks going every
direction from where your standing.

Any thoughts?

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