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RE: Deflection of steel beams when placing concrete slabs

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Cliff,

Don't expect concise answers out of either the ACI 117 or the AISC Code of
Standard Practice.  ACI 117 is often mis-applied to composite construction.
Look carefully at ACI 117, and how the particular tolerance is applied.

Also look closely at the AISC Code of Standard Practice.  The tolerance for
camber is minus 0 and plus 1/2" for members less than 50 ft long.

There are many issues that you need to explore before settling in on a
solution.  First call AISC and see about getting a copy of the lecture,
"Designing Steel for Serviceability, Lecture 3, Control of Floor Elevation
and Levelness".  (Another AISC classic piece of literature that rivals the
works of Homer.)

If your tolerances are tight, use a leveling course on top of your
structural slab.  A leveling course is expensive so you will have to assist
the owner in finding out if a tight tolerance is worth the expense.  It is
also good to keep in mind that the slab will be placed with screeds that
will be set relative to the deck.  You may want to set your elevations
relative to a theoretical horizontal plane, but you have to work within the
limits of standard construction practice.

Regards,
Harold O. Sprague

> -----Original Message-----
> From:	Cliff Schwinger [SMTP:clifford234(--nospam--at)comcast.net]
> Sent:	Saturday, September 28, 2002 6:58 PM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Deflection of steel beams when placing concrete slabs
> 
> 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.
> 
> Cliff Schwinger
> 
> 

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