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Re: 75 PSF Equipment on 50 PSF Floor

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I agree with Rick.  The first thing to consider before you even start to
think about back calculating the actual capacity (with can certainly be a
good thing regardless) is to consider that the 50 psf applies over the
ENTIRE floor.  As Rick points out, the loading due to the equipment
changes very quickly depending on what surface area it applied over.  If
you were to strictly require that equipment or furnature have the load
they create be determined strictly by the actual foot print of the piece
of equipment or furnature, then three or four drawer file cabinets would
likely not be permitted in office buildings (i.e. 50 psf by code).  The
point is that it is common to associate some little amount of room around
the equipment that is not part of the actual footprint to become part of
the surface area in the calculation of the surface load.  Now, it is
necessary it do some minimal "evaluation" of how likely it is to have some
other heavy equipment in the future placed near the equipment that you are
dealing with, considering my example of a file cabinet by its nature
prevents anything from being place immediately in front of it.  If it is
considered somewhat likely that something will be placed immediately next
to the equipment on ALL sides (frankly not to likely...kind makes it
difficult to work with the equipment), then only the actual foot print
should be considered.

To me, the real issue to check is to see if you have a local failure due
to a concentrated load, either due to it changing the bending moment
pattern (i.e. from bending moments due to a nice uniform load to bending
moments due to a lumped smaller uniform load that approaches a
concentrated load) or due to punching shear.  One question to consider
is...Is the support of the equipment really uniform in nature or does
really apply that load through specific local points (i.e. all load
through "feet")?  If so, then punching shear becomes a potentially much
bigger issue.

I have dealt with similar issues in the past...from medical equipment in
medical facilities (hospitals and medical office buildings) to pizza ovens
in a restaurant space in a casino.  I have even dealt with similar issues
when determining how high the casino could stack coins at various location
(it was a casino built in an existing building).

The ultimate point is that a uniform load assumes that load is applied
with the same intensity at ALL locations at the exact same time, when the
truth is that the odds of that happening are rather remote.  This is the
same idealology that allows the reduction of live loads per the code.
What you really need to do is determine how that piece of equipment will
really load the structure, see how that differs from the original
assumptions (pure uniform load), and determine if that difference can be
accommidated without any modifications or if modifications are required.

HTH,

Scott
Ypsilanti, MI


On Thu, 20 Mar 2003, Rick Burch wrote:

> Maybe I'm missing something here, but it seems to me like this case
> should be one of those "OK by inspection", after only a brief look at a
> beam or two and the slab.
>
> First, It is extremely conservative to look at the psf loading on the
> footprint of a piece of equipment and just compare that to the design
> floor load. That reminds me of the question of why a floor isn't
> overloaded if a 200 pound person stands with his feet close together so
> that he exerts a 200 psf load on the floor.
>
> In your case, if you look at a mere 6" extra width around all sides of
> the equipment, your "footprint" load is already less than 50 psf.
>
> Second, building codes require that floors be designed for a
> concentrated load as well as uniform live loads. According to ASCE 7 for
> example, offices are supposed to be designed for 2000 pounds and
> hospitals for 1000 pounds. These loads are applied to a 2.5' x 2.5'
> area, so these are worse than your case.
>
> I'm not necessarily saying that whoever designed your building
> explicitly considered the concentrated load case, however, when I have
> designed floors and checked this case for the first few beams, it became
> obvious pretty quickly that it never controlled anything.
>
> As far as the slab goes, since you mention floor beams (as opposed to
> bar joists), I suppose maybe you have a composite deck slab. If so, the
> SDI "Composite Deck Design Handbook" shows how to check concentrated
> loads and discusses the concentrated load design case (1000 or 2000
> pounds on a 30" x 30" area). Their summary: "This code requirement will
> probably never be the controlling factor for a steel deck composite
> slab."
>
> Just another perspective.
>
> Rick Burch
> Columbia, SC
>
>
> > Chris Banbury wrote:
> >
> > I've been asked to determine if a 1200 lb medical intrument can be
> > placed on the third floor of a hospital office building.  The
> > instrument is 30"D x 74"W.  The building was constructed in 1989 and
> > I've obtained the original plans from county records.  The design live
> > loads listed on the plans include 50 psf for office and higher loads
> > for corridors etc.  However, the entire third floor was originally
> > designed, permitted, and constructed as an empty shell with no
> > corridors, partitions, or offices so it seems I'm forced to use the
> > lowest uniform load given.  Non-bearing walls, hallways, offices,
> > laboratories, etc. were added later as the hospital found doctors who
> > wanted custom rental space.  Since the equipment exceeds the
> > minimum uniform live load I am considering the following:
> > 1. frame a platform to span several floor beams
> > 2. place a steel bearing plate to distribute load over a larger area
> > 3. paint or demarcate an area on the floor around the instrument where
> > no additional equipment can be located such that the painted area
> > satisfies the allowable superimposed live load
> > 4. modify the floor beams in order to increase the load rating of the
> > floor
> > 5. calculate the actual allowable superimposed live load of the
> > concrete slab and steel beams from the structural drawings.  Since the
> > corridor locations were unknown at design stage, it is possible that
> > 100 psf was used for the entire floor.
> >
> > Obviously there are concerns with all these solutions.  I would
> > appreciate any additional thoughts or advice on this problem.
> >
> > Christopher A. Banbury, PE
> > Vice President
> > Nicholson Engineering Associates, Inc.
> > PO Box 12230, Brooksville, FL 34603
> > 7468 Horse Lake RD, Brooksville, FL 34601
> > (352) 799-0170 (o)
> > (352) 754-9167 (f)
> > www.nicholson-engineering.com
>
>
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