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RE: seaint Digest for 28 Apr 2004

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Re: Cambered Slabs

Gail, as noted in other thread responses, it is not uncommon to camber 2 way
slabs. 

It is usually called out as a positive number on the plans or in a schedule.
The amount of camber required is, in my opinion, subjective because there
are so many variables (actual dead load vs. design load, Modulus of
Elasticity, time of removal of shores/reshores, early age loading, etc) that
determining the amount seems to be an art.

As David and Jim have noted, the formwork can usually be configured to make
this happen. In "stick" form systems (frame/stringer/joist) the joists must
be arranged to allow a change in elevation without "breaking their back" or
crippling them. In table systems, the same approach applies with either a 2
table bay with shims off the trusses to create the camber (so the camber is
actually planar not parabolic in the slab soffitt) although as David points
out there are cases where double framing is used. If the camber is not too
great, it is possible to "turn the screws up" on shore frame or post legs in
the center of the bay even if a random joist arrangement is used, as the
aluminum joists are pretty flexible and can accept a fair amount of
deformation and remain elastic in response. Finally, Titan and Peri have
post and panel systems that can do this also.

The deck should ALWAYS be checked for alignment (Usually done from below
with a laser and set targets) prior to a pour to verify that the deck is
where it is desired to be, and should ALWAYS be monitored during the pour
because parts of the shoring system always have some take-up as load is
applied. We task a 2 man crew to monitor form elevations throughout a pour
and they turn screws up or down as needed during the pour. The top of slab
is set "hot" at the slab edge and construction joint bulkheads, and screed
lines are set at the columns on the vertical steel with paint lines; the
rest of the floor is usually controlled for thickness using wet pads or by
"dipping" (a rod or bar painted to show the slab thickness) as it is struck
off. Random dipping and striking with a long straightedge mitigate the
planar aspect of the deck form and results in essentially parabolic top
profile.

As far as cost is concerned, there is some small cost incurred if the camber
required demands more form/shoring material or an arrangement of material
that is "extraordinary"; as far as additional survey to check final
elevations that is usually a 2 man crew per pour on the day after the pour.
Just like floor FF and Fl profiles, it must be checked directly after the
pour before shoring is removed. 

Finally, there is always the condition that results after the shores are
removed; will it come out? Is it affected by the maturity at loading with
self weight? Is the camber intended to offset creep? There are a lot of
variables and one way to control them is to set minimum maturities for form
removal, shore removal, reshoring duration, allowable superimposed loading
from reshores for floors above, etc....these are time related issues that
really end up as incurred costs because it means less efficient form, shore,
and reshore cycling and more form, shore, and reshore material and just
plain "going slow"-so to sum up, camber is not uncommon, but be aware of the
effects to schedule and cost.




Richard W. Stone, P.E.
Project Executive
Technical Services/Project Management/Estimating
Concept Design/Engineering/Quality Assurance
 
Madison Concrete Construction Company
130 Quaker lane
Malvern, PA 19355-2479
email: rstone(--nospam--at)madisonconcrete.com
Voice:       610.695.8800
Facsimile: 610.695.8678
 
Nextel:      610.496.5764
 
 
Visit our webpage at  www.madisonconcrete.com  !!!!
 
 



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