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Re[2]: Reverse Camber (was: RE: Long cracks in

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In trying to get a handle on these thermal effects, including thermal moments 
and corresponding stresses, the information in ACI 349.1R-91 (Reapproved 
1996) may be useful. However, this was prepared way back in 1981, and there 
probably are better analysis tools out there today.


______________________________ Reply Separator _________________________________
Subject: RE: Reverse Camber (was: RE: Long cracks in
Author:  seaint(--nospam--at)seaint.org at Internet
Date:    01/10/2001 9:41 AM


Harold,

This effect was noticed in many single span (with integral end columns) and 
multi-span parking garages in Australia about 20 years ago. Once we 
determined the cause, we then started designing for and detailing for 
temperature differential effects in all prestressed roof members.
As you said, it is necessary for the bottom reinforcement to be fully 
developed into the supports at both end and interior supports and it is 
quite often necessary to add extra reinforcement in the bottom to contain 
the cracks to reasonable limits as the bottom reinforcement required for 
flexural strength is often very nominal in these members, especially with 
bonded prestress as is used in Australia and Asia. For wide flat beams 
(bands) we have found that a minimum area of reinforcement of #5 @ 8" 
(16@200) is necessary in the bottom.
It is also necessary to check for this effect at midspan as the tensile 
stress will add to any tensile stresses due to loading and will result in 
cracked sections at mid-span while the designer may think he has uncracked 
sections in these areas due to normal loading. The temperature stresses in 
a previously uncracked member due to temperature differential alone could 
be as high as 500 to 700psi thus cracking the concrete by itself. It is not 
logical or economical to try to resist these stresses with more prestress 
so a partial prestress design is needed allowing the concrete to crack and 
providing sufficient reinforcement to contain the cracks to reasonable 
levels. Fortunately, once the concrete cracks the temperature differential 
stresses reduce (they are stiffness dependent) and less reinforcement is 
required than would be for the full temperature differential effect on an 
uncracked member.

Interestingly, in some of your colder climates, the reverse is true and 
continuous reinforcement would be required at the top at mid span to 
counter a reverse temperature differential (colder on top) for concrete 
roofs of enclosed buildings. In fact, it may be necessary to allow for both 
effects in areas with relatively warm summers and very cold winters.

Even though similar temperature differential effects (but reduced due to 
the effects of the higher levels of normal cracking) occur in reinforced 
concrete members, the cracking does not occur or is not noticed in 
reinforced buildings as
-       the compression stress due to self weight of the member tends to 
counter the temperature effect in the areas of the members where we do not 
normally expect tensile stresses (at the bottom at a support or top at 
mid-span)  thus resulting in net compression in these areas and
-       at areas where both effects cause tension (bottom at mid-span or top 
at supports) we have bonded reinforcement which has been added for flexure 
to contain the cracking. In this case, extra reinforcement may still be 
needed for crack control so if you have a reinforced concrete roof or 
spandrel member which is cracking more severely then expected in the bottom 
at mid span in hot weather or at the top at supports in very cold weather, 
the cracking could be caused by temperature differential effects adding to 
normal loading effects.



At 09:14  9/01/01 -0600, you wrote:
>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)
>
>Regards,
>Harold O. Sprague
>
> > -----Original Message-----
> > From: Roger Turk [SMTP:73527.1356(--nospam--at)compuserve.com] 
> > Sent: Monday, January 08, 2001 5:47 PM
> > To:   seaint(--nospam--at)seaint.org
> > 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
> >
>



Regards  Gil Brock
Prestressed Concrete Design Consultants Pty. Ltd. 
5 Cameron Street Beenleigh Qld 4207 Australia
Ph +61 7 3807 8022              Fax +61 7 3807 8422 
email:  gil(--nospam--at)raptsoftware.com
webpage:        http://www.raptsoftware.com/