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Re: Precast Panel Cracking

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It would make sense to me that you get a very small crack down the reveal -
you have effectively restrained the panel perpendicular to the span, so any
shrinkage strain at all would translate to a crack. This is not helped by
the lack of transverse rebar, and the reason it occured at the reveal is
that it is the weak point of a section in tension.

It should be a very small crack though. If you have 0.02% shrinkage strain,
and the panel is 48" wide, the crack would be a maximum of about 10
thousands of an inch. Do you know how big the crack is? That might be
enough to cause a leak, I don't know.

If they are talking about structural failure and panels falling off, I can
only imagine the anxiety levels around your friend's place. Must be
unbelievable.

Michael



                                                                           
             "Chad Van Kampen"                                             
             <Chad@GeraceConst                                             
             ruction.com>                                               To 
                                       <seaint(--nospam--at)seaint.org>                 
             05/26/2004 09:02                                           cc 
             AM                                                            
                                                                   Subject 
                                       Precast Panel Cracking              
             Please respond to                                             
             <seaint(--nospam--at)seaint.or                                             
                    g>                                                     
                                                                           
                                                                           
                                                                           




Michael, Jim, and others,











The panels were supposed to be Spancrete but were changes after crushing
became and issue.  The panels are horizontal in direction stacked 110’ up
and bearing on each other.  The lower few panels have insulation, and the
upper ones (above the lower roof) are solid un-insulated panels.  There is
significant bow difference between these two panels, about 2” so there is a
ledge for the water to sit on here.  I am no longer working at this
precaster but I am good friends with the project manager and he is
struggling to figure out what is going on, with no help from the current
engineer.  He has told me that the mix for this particular panel was the
“Spancrete” mix wetted down, and it was not extruded, but formed and poured
on the Spancrete beds.  The structural is only cracked at the embeds.  This
building is welded everywhere in every corner to everything else, so
cracking at the embeds is not a big surprise.  My guess is these welds
cracked during erection while dead load was still being applied.  The face
cracks however, are a bit more puzzling.  They are all cracked 2’-0” in
from the end and down the center at a reveal.  The cracks at the ends run
perpendicular to the stand, the reveal crack runs with the strand, there is
no other mild steel in the panel, and this was the first time they have
used this procedure.  As for the truss system it is running perpendicular
to the strand, and span.  It is the Meadow Burke Single Girder Truss system
at 4-’0” oc in every panel, but is running in the short direction, not with
the span.





I appreciate your help because the P.M. is afraid that the face will fall
off punching the lower roof, the only thing holding on the face is these
trusses. This is not a composite panel the face is not acting to resist any
loads.





I was on SMA (The Spancrete Manufacturing Association) and issues like this
were discussed but not in great detail.  Have any of you ever “designed”
the face?  The P.M. is asking how we engineers take into account issues
like this.  I typically ignored the face since it was just for looks.








Thanks again,








Chad Van Kampen P.E.

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