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Re: Early form release for two-way slabs

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Keith,

As Scott mentions you wont find a whole lot in ACI 318 which is referenced by most codes but you may find in your project specifications or general notes a reference to ACI 301, "Specification for Structural Concrete", which has more details and requirements on formwork and removal of formwork.  As to a general guide you will want to look at ACI 347, "Guide to Formwork for Concrete".

Another thing to consider is that the modulus of elasticity increases at a slower rate than does the strength of concrete during the curing process.  Your structure may have the strength to resist its own dead load but it may develop undesirable deflections or sag if the forms are removed too early.

Thomas Hunt, S.E.
ABS Consulting




Scott Maxwell <smaxwell(--nospam--at)engin.umich.edu>

04/07/2004 06:30 AM

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Re: Early form release for two-way slabs





Keith:

First, be careful when using terms like "required" when talking about the
ACI MCP (what you likely are referring to when mentioning the "ACI
Manual").  There are only about 5 or so code documents in the ACI MCP that
may (or may not) be adopted as local building code requirements for
structural concrete.  There are a handful of specifications in the MCP
that _COULD_ be incorporated into the project specifications/contract,
which would then make such items included to be "required" by the contract
(but not code).  But, the vast majority of the documents in the MCP are
"guides" which contain recommendations.  They have little force other than
they can create a "standard of care" or if such recommendations/guidelines
get incorporated into contract documents (i.e. when writing the spec).

Beyond that, section 6.2 of ACI 318-02 deals with the "typical" concrete
code requirements for removal of forms and/or shores (but could be
modified by local amendments or "tweaks" in the model building codes).
Based up a quick review of that section, I would say that it is possible
that the proposed "quick release" system could be permitted by code _IF_
they prove using "...structural analysis and concrete strength data..."
that the structure will be sufficient to support the imposed dead and
construction loads.  Note that provision 6.2.2.1(c) does specifically
outline what is construed as "appropriate" concrete strength data...it
states: "Concrete strength data shall be based on tests of field-cured
cylinders or, when approved by the building official, on other procedures
to evaluate concrete strength."

So, if they can prove using a reasonable structural analysis method that
at an f'c of 500 psi, the slab can span two-ways with no reinforcement and
with out forms/shoring, then it would be permitted by code.  They would
also have to prove the 500 psi by using field-cured cylinder tests (or
another method that is approved by the building official).

That then raises the question of what is a "reasonable" structural
analysis (basically your second question).  Doing some sort of two-way
analysis with the moment capacity determined by using the modulus of
rupture would get to at least within the "ballpark" of "reasonable".  I
would think that "reasonable" would be to do a two-way analysis with the
slab capacity determined by Chapter 22 of ACI 318-02 (the plain concrete
provisions), which if based upon similar process as using the modulus of
rupture (but I don't think it directly uses the m.o.r....it uses
Mn=5*sqrt(f'c) where I seem to recall that m.o.r. is typically around
7.5*sqrt(f'c), but memory is a little hazy this morning...which makes
sense as if you reach the m.o.r., then in theory the concrete cracks at
that point, so you would want to design it such the m.o.r. is NOT
reached).

As to the 70% of the 28-day f'c, I am not aware of any code provision that
requires that.  That is the general "rule of thumb" recommendation that is
commonly used.  Usually, reaching that level of concrete strength is
enough for most loading situations...and it generally occurs at about the
1 week point, which is when most formwork/shoring "wants" (i.e. timing
wise for the contractor's perspective) to be removed...at least to my
knowledge.  So, the point is that the ACI 318-02 (and previous versions)
would allow removal of forms/shores prior to reaching 70% of 28-day f'c
_IF_ the provisions of 6.2 are met.

HTH,

Scott
Adrian, MI


On Wed, 7 Apr 2004, Keith Pabst wrote:

>
> I am working on a project which has a 12 " thick, two-way flat slab
> system. The contractor is proposing to use a forming system that offers
> an early release of the beam and girder forms. The way the system works
> is there are shores at 6'-0" on center each way with a beam and girder
> system supporting the plywood at 16 or 24" o.c.. The forming
> manufacturer has made recommendations for an early release of the beam
> and girder system but leaving the vertical supports in place until the
> concrete has reach the required strength. They have a one day release
> time specified for a 12" slab as long as the min. compressive strength
> is 500 psi. Basically they consider the slab to be a 2-way slab
> supported at 6'-0" o.c.e.w., unreinforced. They have analyzed the slab
> based on the modulus of rupture to determine the early release time. I
> have looked through the ACI Manual and have only found the section which
> requires forms for horizontal surfaces to remain in place until the
> concrete has reached 70% of the required compressive strength. I guess I
> have the following questions:
>
> 1. Is this type of early release system allowed per code.
> 2. Is the method of analysis described above acceptable per ACI
> utilizing the modulus of rupture.
> 3. Can anyone point me to a portion of the code which would allow such a
> system to be used without achieving the 70% f'c.
>
> I would appreciate any information anyone could provide me regarding
> this matter.
>
> Thank You,
> Keith E. Pabst, E.I.T.
> Highland Associates, Ltd.
> Architects Engineers Interior Designers
> 102 Highland Avenue
> Highland Center
> Clarks Summit, PA  18411
> (570)586-4334
> (570)587-8164 fax
> email: kpabst(--nospam--at)ha-pa.com
>
>


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