Analyzing an Existing Pipe Rack 1

[Rabbit12]

I'm curious to how others approach the problem of analyzing an existing pipe rack for new piping.

I seem to run into a lot of instances where I have limited or no info on the existing structure. The popular answer I hear a lot is using Chapter 34 of the IBC and invoking the 5% gravity/10% lateral exceptions. That's all fine and good if you're the only thing that's been added over the life of the structure. From my experience in the industrial industry they are constantly running new piping so the odds of this new project being the first additional loading is slim to none. You can't keep adding 5% over and over. Eventually it's going to be too much. It's a hard argument I have often with a PM in my company that was in our structural group.

I know the industry standard tends to be to design pipe racks for 40 psf but without existing drawings I can't be comfortable that was what was originally designed for.

How do others approach this problem?

 

[TehMightyEngineer]

If I'm in that situation where I have no idea what the original design included then I always re-evaluate the pipe rack from scratch. If it's overloaded by less than 5% then fine; any more and it requires reinforcement. Usually the reinforcement is not terribly expensive.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[SlideRuleEra]

Agree with TME, evaluate from scratch. Many industrial structure designs are not code based, instead they are designed to carry the loads safely and often include provisions for future additions or changes.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[Rabbit12]

The clients we have all reference ASCE 7, IBC, AISC, ACI, etc in the Codes and Standards in their specifications. Company designs are all based on those codes.

We generally work in high seismic areas so if it's analyzed from scratch you get thrust into designing for the new(er) seismic design provisions. That tends to blow the structures up before we even get started.

 

[Leftwow]

PIP STC0105 Structural Design Criteria 4.1.2.5 Pipe Rack Piping Loads

The 40 psf is an Operating dead load and is equivalent to 8-inch diameter schedule 40 pipes full of water at 15inch spacing.
Empty Deadload for checking uplift and components controlled by minimum loading, 60% of the estimated piping operating loads shall be used if combined with wind or earthquake unless the actual conditions require a different percentage.

For any pipe larger than 12 inch nominal diameter, a concentrated load including the weight of piping, product, valves, fittings, and insulation shall be used in lieu of the 40 psf.

Pipe racks and their foundations shall be designed to support lads associated with full utilization of the available rack space and any specified future expansion.

There is also a deflection criteria. I believe it is pipespan/200.

We usually send our designers to go measure beams and sketch out existing rack if there is no drawings. I have not yet encountered a seismic controlled application. Also, I have never met a client that has turned a blind eye to re-evaluating a pipe rack. We use staad to evaluate pipe racks, and also designers make note of any noticeable deflection and current "health" of structure.

 

[TehMightyEngineer]

OP: This seems to be the intent of the reason we are required to update to match new codes; there was a large increase in the load and thus any historical performance is invalidated. I'm with leftwow that most of the industrial clients I've worked with have been understanding of this as they understand how expensive it can be for a piping system to fail, especially if it shuts down a critical process or includes hazardous chemicals. Short of building a new pipe rack or finding a clever way to reduce the seismic load, I think your best option is the most direct.

Leftwow: Interesting, never had that reference so I didn't know that was the origin of the 40 PSF load (assuming it is). If I still did industrial work I would have to pick that one up.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[Leftwow]

OP: I'd like to add 2 more things. Pipespan/200 would be in inches. So, (pipespaninft*12in/ft)/200. Another good book for you to have is "Windloading and Anchorbolt Design for Petrochemical Facilities"

TehMighty: I am honored sir.