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Re: Pipe Rack

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Joe,

	Since I do not fully understand your needs and since my own experience
deals mainly with petrochemical industry piping which can involve large
pipes and large temperature ranges, what I am about to write below may
be gross overkill.  If so please bear with me.

	There are two parts to the piperack problem: stresses within the pipe;
and stresses within the pipe rack.` Usually these are analyzed
separately and by different people (except, possibly in critical
facilities like atomic energy facilities, which have not been part of my
experience).

	The piping design group specify where the pipe is supported, where the
pipe is anchored or guided, and what the anchoring or guiding forces are
for various load cases.  The structural group design the pipe rack as
required.  NOTE: THE PIPING MAY NOT BE GUIDED OR ANCHORED AT LOCATIONS
OTHER THAN THOSE SPECIFIED BY THE PIPING GROUP FOR THE CONVENIENCE OF
THE STRUCTURAL ENGINEER.

	The pipe rack structure is designed for the following loads:
1. Dead weight of pipe, fittings, and piperack structure.
2. Weight of product in service, or weight of water for pressure
testing.
3. Anchor and guiding forces as specified or determined by the piping
analyses
4. Wind or seismic (on structure and piping).
5. Frictional forces of the pipe sliding where it is not guided or
anchored.
6. Temperature changes of the structural components of the pipe rack
(NOT THE PIPING that's included in the anchoring or guiding forces).
7. Appropriate combinations of the above.

	Note that temperature stresses within the pipe rack structural
components are internally generated and only apply while the structure
is constrained otherwise they reduce to zero.  If the piperack is to
remain in the elastic range analysis must be by working stress method
and temperature stresses must be included in all appropriate load
cases.  If full elastic constraint is not required (the usual case) the
structure may be designed by plastic analysis, strength methods (LRFD or
limit states or whatever you wish to call it) and the temperature
loading may be excluded from the STRENGTH portion (but not the
serviceability portion) of the analysis because of the larger
deformations inherent in these methods of analysis.

	To answer your question regarding the placement of expansion joints: it
depends on the configuration of members and whether there is any
anchorage in the 210'.  If you have anchor forces I expect you will
require vertical bracing; even a small (2,000 lb.) anchor force 18' up
will cause unacceptable horizontal deflection in reasonably sized
members.  If you require longitudinal members in the piperack for any
reason (to reduce kl/r, for example) you may also require vertical
bracing.  Do not connect the piperack to the piping to reduce kl/r or
for any other structural reason unless you are very sure of what you are
doing or have control or influence over the piping design.  With 50 - 55
degrees F temperature range you should easily be able to have expansion
joints 100 feet from your bracing point; this permits about 3/8" change
in length for any longitudinal members in your piperack over this
distance.  On this basis, if you have one bay of bracing in the center
you don't need any expansion joints; if you have multiple bays of
bracing (and I can't imagine why you would require more than two) you
need expansion joints between each pair of braced bays (about midway
between is ideal).

	I hope this isn't too much more than you wanted.

				Regards,

				H. Daryl Richardson

Jnapd(--nospam--at)aol.com wrote:
> 
> My question is concerning thermal expansion of steel.  The difference from
> day to night time temp. is 50-55 degrees F.  I have an elevated pipe rack, 18
> ft. To supports fully exposed all day to direct sun.  the sections between
> supports is 30' with a total straight run of  210ft.  The question is should
> I calc the expansion and induced stresses on the 30' or the 210'.  The beams
> are welded to the column caps 1/4" fillet 6" each side. I thought that 2 -
> 1"expansion joints would be necessary. Any thoughts ??
> 
> Joe Venuti
> Johnson & Nielsen Associates
> Palm Springs,  CA
> 
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