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Analysis of Existing Structure

Analysis of Existing Structure

Analysis of Existing Structure

(OP)
The client wants to add a 24” flare line on top of existing piperack built in 1969. The structural model of the whole piperack were developed and wind loading was applied per client’s current design criteria (ASCE 7-10, V=150mph, Risk Category III, Exposure “C”). The additional loading on the bents did overstress some columns and therefore a couple of additional transverse X-bracings were proposed. After reviewing our detailed Calculation Report the client’s C/S engineer made the following surprised comments: “Analysis of the pipe rack should be performed using the code for which it was originally analyzed and designed. Minor modifications to the pipe rack structure do not alter the original design or intended use. Then provide separate structural analysis based on current site criteria for our use and comparison.”
I have talked to several very good engineers and they all agree that existing structure needs to be analyzed per the current design criteria. I would appreciate your comments on this subject.
iv

RE: Analysis of Existing Structure

Two new transverse X braces doesn't seem like much to argue about here for a non-building retrofit.

What is really going on here?

Retros costing 10 to 15% of the value of the structure, depending on the jurisdiction, can trip a total structural upgrade to current code.

Mike McCann, PE, SE (WA)


RE: Analysis of Existing Structure

When analyzing to the current code, what percentage of the column capacity is being utilized by the new 24" flare line? Anything over 10% I would probably argue isn't a minor modification.

I have never liked the idea of assessing to previous codes. Fair enough if you have a forensic engineering case where the lawyers need an assessment of the original design, but for this type of work I am a firm believer that our codes and standards have evolved with the purpose of achieving safer structures as a result of years of research. I would approach as follows:

1) Analyze existing loading scenario and assess members to current code
2) Analyze new loading scenario and assess members to current code
3) Determine how much capacity the critical members are using for the additional load only
4) If assessment does not pass case #2, perform sensitivity analysis on load factors to determine what reduction is required for members to comply with the relevant code.

I had a similar project to this. When analyzing the existing structure as per the clients design criteria I found that the maximum live load factor that could be considered in the load combinations was 1.3 (not 1.5 as per code requirements). At the end of the day the client wore the risk and implemented their own controls to restrict live loads on the structure despite my recommendations to strengthen various structural members. As a rule, I stick with load factors required by the code and make certain that I do not represent to the client that a reduced load factor of '1.3' is acceptable. I don't think that's my call as a consultant to make. This issue always sparks some discussion in the offices I have worked in as almost all engineers working with industrial structures will come across a scenario similar to this at some point.

I agree with msquared48. A handful of cross braces seems like a small price to pay for bringing a 45 year old structure up to the requirements of the clients current design criteria with the additional loading.




RE: Analysis of Existing Structure

Many structures on refineries and chemical plants, electrical substations, etc. are not subject to building codes, do not require approval of the code enforcement department. In such cases, your client's request is legitimate.

This is not to say that they don't design to code on original work, they do.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

RE: Analysis of Existing Structure

This sounds like an engineer trying to justify his position. Unless the new flare is located in an open area the cost of the braces is chump change compared to the other costs. I rarely have this kind of problem in oil and gas.

RE: Analysis of Existing Structure

(OP)
Thank you all for the comments. Next Tuesday we have meeting with his boss - I will keep you posted.
iv

RE: Analysis of Existing Structure

Is it going to make a difference if you use an older code? The types of provisions that apply to a pipe rack aren't the sort of thing that's become more conservative over the years. If it's a seismic or hurricane design governing, then maybe a new code would change things. Anything other than that shouldn't make much difference I wouldn't expect.

Pipe rack loading now is basically the same as pipe rack loading twenty five years ago. When generating loads and cases, most of the reference is going to be to a client's design standards, PIP standards, or common practice rather than to a building code.

What's the governing case?

But really, unless there's some complication involved, adding a little bit of cross bracing when you put in a 24" pipe isn't a big deal anywhere I've ever done this kind of thing. Is there something that makes this look unusual?

RE: Analysis of Existing Structure

I notice you are using Risk Cat III....is this a pipe rack on top of a critical structure? If so, you might need to consider current code regardless of renovation cost. Check with the building official.

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