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Re: Are Roof Top Units considered Dead or Live Loads?

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> From: "Daniel Boltz" <dboltz(--nospam--at)1st.net>

> I've had different answers from various Senior Engineers.  Some say that
> since the RTU weight does not vary in magnitude and location, it is a DEAD
> LOAD.  Others say that if the RTU is changed or moved in the future, it's
> weight and location would be considered a LIVE LOAD.

This has been discussed on the list in the past, as well. RTUs are a
little schizophrenic, if that's an appropriate term. They have two
natures:
1) they are "static" in that they don't move around without direct
intent by the owners. The operating loads and locations, at the TIME OF
INSTALLATION, are 100% known or knowable and this does not change. The
RTU is dead load.

2) at DESIGN TIME we rarely have good numbers for the actual weights or
operating effects of the final units (e.g. Christopher mentioned dynamic
issues). Additionally, they can be removed or relocated with some
structural detriment. So, for design purposes, we are inclined to think
of them as live loads.

The real issue comes down to how we, as designers, handle the effects of
these RTUs, not whether they should be classified as dead or live loads.
The building codes typically place RTUs in the category of Dead loads
which, in my opinion, they are.

"Dead" loads are treated in a different manner than "Live" loads for the
purpose of REGULATORY compliance and for rational DESIGN CONVENIENCE.
Dead loads are ALWAYS in a fixed place (e.g. they don't move around on
the roof when the wind blows, they don't mulitply like mechanical
rabbits, they don't over-eat at Christmas, etc.). The drawings submitted
for permit show where that RTU will be - end of story. Relocated dead
loads are REVISIONS, not live loads.

What are your design and contractual objectives? If you are concerned
about the possibility that a different unit will be installed and you
want to cover yourself and avoid working on design changes, assume an
altered load or different load factors (same net effect). Do not be
concerned about whether you use 1.2 or 1.6 because you want to fit it
into a nicely defined category - heck, use 2 and forget about it! It's
STILL a DEAD load even if you treat it like a live load to be
conservative in design.

>From experience, we know that things change. Now, we start to see the
opposite nature of RTUs. Maybe they aren't installed where we designed
the structure to support them. Maybe, they are heavier or lighter than
we anticipated, in a detrimental way. Try to cover reasonable
probabilities: 

Heavier - design the support for more load than expected (careful not to
be extreme or you can be black-mailed for that as well). Anything else
is a redesign issue.

Lighter or not installed - uplift may be a problem. Unless your customer
advises you that these MAY NOT be installed until a future date, this is
a redesign issue.

Location - allow a tolerance of installed location based on the plan
that you are given. Anything else is a redesign issue (OWSJ supplier
will quote/design with loads/locations EXACTLY as defined in the
drawings/specs).

Future Renovations - A redesign issue. Unless your mandate is to produce
a design that would permit defined future conditions, you are done.

RTUs are a gold mine for extras and future work. If the RTU specs change
between design and installation, it is NOT your fault and NOT your
nickel to review and reinforce for the change. The owner should have
given you appropriate specifications up-front if they wanted to avoid
delays or additional expenses. Their mistake = their cost.    

-- 
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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