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RE: Deflection of steel beams when placing concrete slabs

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Cliff

If the construction documents " instruct contractors to
place additional concrete as needed when pouring slabs on metal deck so
that the finished slab elevation is constructed flat and level to the
top of concrete elevations specified" then they will have to do this at
edge beams regardless of the fact that the pour stop may not be at the
correct elevation.  If your detail shows the metal at the perimeter at the
same level as the finish elevation of the slab  it may create a conflict
with your specifications if you have explicitly dimensioned the height of
this permieter member.

I am not sure that ACI 117 addresses the reason for a level concrete slab. 
I believe you will find that tolerances for flatness do not apply to
deflected slabs. The reason you want a level finish slab is because of
movable partitions and furniture.  

Movable partitions that divide a big room into smaller rooms have a seal at
the bottom that when activated prevents noise in one room into the adjacent
room.  These seals are limited in how big of a gap they can accomodate. 
Thus if the deflection is too large then the seal dosent work.  By casting
a level floor you no longer have to deal with the dead load deflection.

Modular furniture systems and long assemblies of custom built furniture
often cannot be properly installed if the varation in the floor elevation
is  too large at time of installation.

As a result this criteria should apply to edge beams as well as interior
beams.  You could indicate in your de tail that the top of the "pour stop"
does not necessarily correspond to the top of the slab.  Part of your
problem maybe  due to the desire to show this pour stop when its only
purpose is to act as a concrete form.

If concrete fill is used at the roof level there is no need to adjust slab
thickness to maintain a planar top surface (given an appropriate roof
slope) since there is neither furniture nor movable partitions resting on
the  slab.


Mark Gilligan

**********************************************
The contract documents produced by our office instruct contractors to
place additional concrete as needed when pouring slabs on metal deck so
that the finished slab elevation is constructed flat and level to the
top of concrete elevations specified.  Pouring level slabs on beams that
are deflecting downward results in slabs with varying thicknesses (which
is not a problem as long as the weight of the additional ponded concrete
is considered in the design). When the beam deflection under the wet
weight of concrete gets to be 1" or more we normally camber the beams.

A question was asked of me the other day that I had no answer to - so I
would like see if anyone on the list server might be able to answer it. 

What construction load (wet concrete load) deflection limit should be
specified for steel beams parallel to slab edges?  

Slab edges always have either a gage metal pour stop or a bent plate
pour stop. The distance between the top flange of the beam and the top
of the pour stop equals the specified slab thickness.  When the concrete
is placed, pour stops are used as screeds.  Since the pour stops are
used as screeds, the floor slabs near slab edges will be constructed at
a constant thickness along the span of the deflecting edge beams.
Additional concrete may be placed to compensate for beam deflections
elsewhere in the bay, but there won't be any additional concrete near
slab edges because they are using the pour stop as a screed.  The result
will be that the top of slab elevation near slab edges will follow the
slope of the beam below the slab edge.  This practice may result in an
unacceptably large variation from the required top of concrete elevation
near the midspan of beams parallel to slab edges unless the construction
load deflection of these edge beams is limited to about 3/4".  Even a
3/4" deflection limit will result in the top of concrete elevation being
3/4" too low at the midspan. Is a 3/4" "dip" below the required top of
concrete elevation considered acceptable? That would be L/480 for a 30
foot span. (I don't have my copy of ACI 117 with me as I write this -
but I will check that publication to see what ACI says regarding the
permissible tolerances from specified top slab elevations.)  Some in our
office are suggesting that a construction load deflection limit of 1/2"
should be placed on edge beams (including beams adjacent to any opening
occurring within the middle third of a span. I am of the opinion that a
1/2" construction load deflection limit may be a bid too restrictive,
but I would like to get some other opinions on this matter.

In the past this never seemed to be an issue, but with the use of 50 ksi
steel and LRFD design it becomes a variable that we have to think about.

Thanks for any advice, suggestions, etc.


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