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RE: Fixed File Cabinet - Dead Load or Live Load?

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> -----Original Message-----
> From: Eric Green
>
> A client wants to install a rolling file deck in an area designed
> for 50 psf live load. The file deck, which will be fixed to the
> floor, covers a 13x17 foot area and weighs 34 psf for the file
> system and 122 psf (max capacity) for the contents (files). How
> would you treat this under ASCE 7? Is it all live load? Could the
> file cabinet be considered fixed equipment and called dead load?
> Could it all be considered fixed equipment and hence be
> considered dead load?
>

The following is written from a background of limit states design, using
Canadian definitions  where applicable or relevant. IMO there are two issues
that create the distinction between dead load and live load:

1:	Dead loads are always there, live loads aren't
2:	Dead loads typically have smaller load factors than live load

WRT #2, the reason for the smaller load factor is that the actual value of a
dead load is known with a greater degree of accuracy than it is for a live
load. Indeed it is arguable that the typical 1.25 dead load factor (Canadian
practice) is possibly too high for a material like steel where it's really
unlikely that either the density and/or the volume of a steel beam is going
to be anywhere like 25% in excess of the tabulated values. Even for concrete
structures it's unlikely that the material will be more than 5% or 10%
denser than the standard density, and again the contractor is unlikely to
donate another 15% of free volume.

Much earlier in my career I was involved in the assessment of High Alumina
Cement Concrete structures in the UK. Typically these structures were
constructed of standard precast members, and the dimensions were known with
considerable certainty. As a result we were permitted to use a load factor
of (IIRC) 1.10 for the structural dead load.

Water in a tank is another example. There's only so much water one can put
in a given tank, and we know the density of water to a high degree of
accuracy. The load factor should reflect this level of certainty.

By contrast, live load is more an allowance than the load of an actual
"thing", and it is only an estimate, so a higher load factor is appropriate
to accommodate the greater degree of uncertainty, and the reduced or
non-existent control, regarding the loads that will be imposed on the
structure at some time in the future. In our code the live load factor also
provides for the effects of load patterns - although we are also required to
consider the effect of partial live loads.

Logically, therefore, anything for which the weight and location is
"precisely" known should be entitled to the lower load factor. Machinery,
equipment, filing cabinets, etc, should fall into this category.


WRT #1, dead loads may be used counteractively (with a load factor of 0.85
to allow for the density or volume being a bit short) to balance a live load
(eg a cantilever or an overturning load). But one has to be certain that it
is in fact there if it used for this - which I think is the reason for the
very tight definition often applied to dead loads as being only the
immoveable elements of the structure.


IMO there's room for a third category of load - known loads - a load which
we "know" what it is and where it is, but which we can't guarantee will
_always_ be there to act counteractively. It should have typical dead load
factors, but should be applied in a manner that it cannot inadvertently be
used counteractively.


Peter James


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