Temporary Structure - Wind and Seismic Load Reductions?

[CWIGGPE]

I have been asked to provide an engineering certification of an existing (already in-place) temporary concrete batch plant. The plant will be in place for 6 months. It is located in a hurricane-prone region and also a Seismic Design Category D. Given the height and weight of the structure, both wind and seismic are major factors. The weight of the plant helps me with wind stability, but the seismic forces are a problem. Batch plants have large silos 60' tall and the overturning at the base is large. The company I am working with uses these plants all over the U.S., and not once have they been required to anchor them or provide engineering given their temporary nature, until now.

My questions are -

Do you think ASCE 37-14 applies to this situation? I can rationalize it this way - the plant is technically a construction-related entity used to facilitate the construction of a major road. However, while it is a temporary structure itself, I don't feel it is temporary in the way that ASCE37 states mean it to be temporary, as in bracing or shoring. Of course, ASCE37 would exempt the plant from seismic design altogether given S1 is less than 0.40, assuming the local building authority allows it. If not exempt, it could at least be reduce substantially per 6.5.2.

If ASCE37 cannot be applied, are there any other options for decreasing the seismic loading to avoid massive footings and anchorage? I tend to lean towards requiring them to meet all the provisions of ASCE7, but that will mean shutdown, massive expenditure, and huge waste for a statistically unlikely seismic event.

Thanks for your help

Cwigg951

 

[dhengr]

CWIGGPE:
These types of problems have much to do with considerable engineering judgement and experience. They are really not something that can be codified. Flip a coin, and if you guess right you’re a saint, you saved the client a bunch of money, if you guess wrong your are and s.o.b. It it your job to do a basic analysis, and lay out the alternatives and costs of each, and then let management make a decision. They can spend the money for bigger footings, etc. or they can spend some lesser money on a gamble, on an insurance premium that an EQ never show up. And, there is no code answer to this type of issue, it is a business and economic decision, it is their decision with your best judgement guiding them. This approach is not intended to show any disrespect for life safety, and the like, it is just a business fact that we can’t design everything for a nuclear detonation 100' above the top of the silo.

 

[SlideRuleEra]

CWIGGPE - I agree with dhengr's advice. However, IMHO, there are some reasonable issues that may considered when making the decision.

1. The batch plant is not a "temporary" structure in an engineering sense, it is more of a "short term" structure. Just words... but words can be important.

2. The important Paragraph for the decision is NOT the technical details of 6.5.2, but the "Applicability" of 6.5.1. The ASCE commentary states:

C6.5.1 Applicability
It is not reasonable to require seismic resistance for temporary works where large earthquakes are infrequent or not considered probable.

3. If steps are taken to meet the wind requirements, those actions will provide some (possibly small) level of improvement in meeting seismic requirements.

Try to "interpret" and frame the technical requirements so that an informed decision can be made, possibly by a non-technical person.

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[msquared48]

My arm could be twisted to ignore the Seismic Load considering the duration 6 month mentioned, but, considering the frequency of hurricanes - every year - not so for this part.

Remember too, that there still exist Army barracks today, constructed in the 1940's as "temporary structures", that are still used as such.

So, how well do you trust the use projection here? Plans change, so limit your analysis to a 6 month duration, after which, all bets are off.

Mike McCann, PE, SE (WA)

 

[JStephen]

A couple of years ago, I took a short course in seismic design of equipment, etc. One of the topics covered was the effect of a piece of equipment rocking from side to side. Imagine, say, a refrigerator rocking back and forth. Anyway, with that kind of analysis, the period and possibly seismic forces may be a lot different from what a building code would specify. Ditto if the equipment can slide. So you may have some design options that would be helpful.

Update- this is a Pushover Analysis, referenced to ASCE 43-05.

 

[Lomarandil]

Short answer, yes, ASCE 37 would apply in this case -- We commonly use it for temporary bridge supports and construction access structures(which sometimes stay up for 6mo-2yr).

Mike offers some good advice about considering the actual duration of use. And absolutely make sure this is clear in whatever your deliverable is (and clear again to the client verbally). If the client is sophisticated enough, they may be able to make the decision as dhengr said. Or they may kick that back on you as the engineer.

If that's the case, dhengr is right that the decision of whether or not to use reduced load does still rely on good engineering judgement. If you and the client are willing to accept the risk that an infrequent event may exceed the design load per the accepted standard of care, ASCE 37 does make the analysis itself (nearly) codified.

I recall that ASCE 37 had some language about reduction in wind even in hurricane regions if it isn't hurricane season -- or based on wind studies if you have those available (hah!).

 

[SlideRuleEra]

I have been asked to provide an engineering certification of an existing (already in-place) temporary concrete batch plant.

What are the wind rating and seismic resistance of the as-built batch plant and how do these values compare with the requirements of ASCE 37-14 for the project location?

That is the first question a sophisticated client will ask you. That's where the conversation to do anything... or not, should begin.

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