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RE: Reverse Camber (was: RE: Long cracks in

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This was an interesting one that I read about several years ago.  I believe
that it was in Concrete International.

A long span PT girder was cast for a parking garage top deck.  The project
was in an area of low seismicity, in the sun belt, and the girder was cast
integrally with the columns, but was not designed as a lateral frame.  As is
generally the case with PT, the bottom steel was minimal, and just embedded
into the column, and not fully developed.

Cracks developed at the bottom of the girder at the column.  They were
epoxied periodically for several years, and they continued to open up.  Some
studies were performed where they measured the elevation of the girder at
several points along the girder.  As they were doing the measurements, it
was noticed that the elevation varied in a cycle during the course of the
day.  They then measured the temperature of the girder across the profile,
and the deflections throughout the day.  

As the sun heated the top deck the concrete expanded relative to the bottom
of the girder.  The result was an increase of camber, reverse rotation at
the columns, and cracks as the girder cycled.  As the thermal gradient
equalized the girder flattened out.

The solution was to cut joints where the cracks could appear that they were
planned, and epoxy inject the random cracks.  The cut (control) joints were
filled with an elastomeric sealer.  The problem was solved.

My own practice is to develop the bottom bar at PT girder to column
interface, and actually try to calculate the amount of anticipated cracking
due to column rotation for creep, shrinkage, and thermal cycling.  This
involves intense calculations, the killing of a chicken, a full moon, and a
dart board.  (humor intended, no animals were harmed in the preparation of
this post)

Harold O. Sprague

> -----Original Message-----
> From:	Roger Turk [SMTP:73527.1356(--nospam--at)]
> Sent:	Monday, January 08, 2001 5:47 PM
> To:	seaint(--nospam--at)
> Subject:	Reverse Camber (was: RE: Long cracks in
> Harold Sprague wrote:
> . > There have been cases of the sun causing reverse camber in precast
> . > members.
> Harold,
> Could you expand on this?
> A. Roger Turk, P.E.(Structural)
> Tucson, Arizona