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Re: 75 PSF Equipment on 50 PSF Floor

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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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