Temporary Structure in Florida and California 1

[ThomasH]

Hello everybody
I asked this question some time ago but now the project has "woken up" again and now I know the locations.

It concerns a steel frame "box", four beams forming a rectangular floor (2.5 m x 6.0 m), four corner columns (total height 2.8 m) and four additional beams forming the roof. There are some additional secondary beams in the floor. Everything in RHS sections.

It's an exhibition monter with glass walls and there are a few of these "boxes" designed for the European market, using Eurocode etc.

The governing criteria has, so far, been overturning of the empty box for windload. One possible load situation is people and a few tables and chairs, so basically empty.

Now the client wants to do this in the US. Florida, high windloads, and California, possible earthquakes. I hope that somebody might be able to give me some ideas regarding requirements.

The duration for the exhibition is max two weeks. It's mobile but for each location we have max two weeks.

California, is earthquakes even valid? Especially for a one storey building. I am not worried about the forces but I don't think the current design will meet requirements regarding detailing, ductility and testing.

Florida, huge windloads. It is designed for high windloads by European standards, but Florida is worse. But a hurricane does not appear just like that. Would it be allowed to assume that we can ancor the "box" to the ground or even move it indoors if a hurricane warning should happen?

Is it completely out of the questing to have calculations according the Eurocode accepted for this situation? I have actually used FEM-analysis and modelled the sections with shell elements for certain reasons.

I have read in the IBC that for this type of sructure a permit from a building official is required. If anybody could perhaps share some info or experience regarding what that means for this type of structure? It would be very much appreciated.

Input is, as always, appreciated

Thomas

 

[r13]

pham,

Thanks for free me from that topic. Actually I think the OP's headache is how to make a design by Euro code compliant to the US code with least complications. I still think to consult with local licensed engineer is the better way to proceed, unless he wants to be able to practice in the states after all that investment.

 

[EDub24]

I’m not sure I’m on board with the use ASCE 37 in this case. I agree with the intent but this structure isn’t involved in a construction site. CBC section 3103 is for temporary structures in use less than 180 days. I think that’s what the OP should reference IMHO.

 

[Ron]

@EDub24....not every instance fits neatly into a "standards" mold. That's where engineering judgment comes in. The OP is dealing with issues in California and Florida, so CBC guidelines would not apply as closely as ASCE guidelines.

 

[ThomasH]

Hello,

I see that the discussion has continued while I was offline. Interesting, and I appreciate it.

Like I said before, it is a very simple structure in terms of geometry (figure of beam model below):

The walls and the roof (not in the picture) are made of glass and the floor (also not in the picture) is similar to the roof with some secondary beams supporting the exhibit.

Since the roof is lightweight the forces from an earthquake will be small. I also suspect that the windload in Florida for this structure will be worse that the earthquake in California.

The ULS utilization is < 50% due to other requirements.
An earthquake should not be an issue, partially due to probability but also due to low utilization. My concern is the detailing.
Wind, well an hurricane does not appear "just like that", there will be warning. So we can either anchor the structure to the ground, close it from people. Or simply move it indoors.

Regarding the use of Eurocode. I have actually to some extent used non-linear FEM-analysis as described in the Eurocode. One reason is the handle different national annexes by not using the standard design "formulas" but rather something more general. I don't know if that is possible in the US.

Thomas

 

[JLNJ]

If you want to be terrified of "temporary" structures, Google Indiana State Fair collapse.

 

[phamENG]

Yikes. I think that's a good illustration of why it's important to understand the source of the wind loading we use when looking at buildings - especially temporary structures with reduced statistical environmental loading.

In Indiana and other places more than 100 miles west of the Atlantic coast (or north of the Gulf coast), the wind loads are primarily based on straight line winds that can happen most any time (though there may be higher frequencies at certain times of the year). In hurricane country, though, the design winds are based on hurricanes. Since they are easier to see coming, it's much more reasonable to think that sense will prevail. A 7-14 day warning leaves a lot more chances for somebody to prepare and do the right thing than a 24 hour cautionary statement and a 6 hour warning.

I know ASCE 7-10 revamped its wind loads to try to make things more consistent in terms of relative importance, correct? (Varying MRI with more accurate wind speeds to obtain a more consistent "effective importance factor" while dropping I[sub]w[/sub] to 1.0.) And now 7-16 has even more accurate maps for middle and western America. Anyone know how ASCE 37 took these into account when considering statistical reductions? Or would it even matter?

 

[r13]

I can see ASCE7 is very busy in overriding itself, the papers/publications/standards must have a very difficult time to catch up, not to mention consistency in/through out the organization!

 

[EDub24]

@EDub24....not every instance fits neatly into a "standards" mold. That's where engineering judgment comes in. The OP is dealing with issues in California and Florida, so CBC guidelines would not apply as closely as ASCE guidelines.

I agree that engineering judgment is required but technically ASCE 37 isn't a referenced document in the building code (IBC, Florida or California) so it's not a guarantee that a bldg dept would accept it for a permit. If a permit isn't required then by all means go ahead and use it. The intent is still there to make sure the structure won't endanger the public.

The Florida Building Code and the California Building Code are both based off the IBC and so they both have the same section 3103 for Temporary Structures constructed for less than 180 days. Section 3103.2 says that 'a permit application and construction documents shall be submitted for each installation of a temporary structure.' My understanding of Section 3103 is that Temporary structures have to meet basic strength requirements and not the serviceability or ductility requirements so I think it boils down to the same thing everyone has been talking about already. It's just a matter of taking a different route to get there that I think would be more acceptable to a building dept.

 

[EDub24]

Anyone know how ASCE 37 took these into account when considering statistical reductions?

I haven't used ASCE 37 extensively but I know for wind they have a reduction factor based on the expected length of time the structure will be there. See section 6.2.1. The intent is to lower the return period from the ASCE 7 wind loads which I believe are based off a 3% chance of exceedance in 50-year event. I'm not sure if they've updated based on the revised wind maps but I doubt it matters much. For seismic they basically say to use engineering judgment to determine a design return period. ASCE 7 is based off a 2% chance of exceedance in 50 year event (2,475-year return period). You could argue for a higher return period like 2% in 1-year event or something like that which would equate to a smaller design acceleration.

 

[phamENG]

Thanks, EDub24. I should have dug out my copy to read the commentary again:

The quantitative method used to achieve this objective is that the wind load should have the same likelihood of being exceeded in the construction period as the permanent structure design wind does in a 50-year period.
..Factors for construction periods less than one year are developed based on judgement, because statistical analyses of seasonal wind variations have not been performed for all regions.

Two things jump out at me. One is that these factors are just shots in the dark (or perhaps a dimly lit room). They've worked before, so we'll keep using them. That's less encouraging, but not unique. The second is that these are based on the 50-year return period. 7-10 and 7-16 are based on variable MRIs from 300 years (RC-I) to 1700 years (RC-III/IV). Of course, the actual wind load applied to the structure hasn't changed much. Is that sufficient reason for this document, published 4 years after 7-10, to still reference the 50 year MRI?

 

[ThomasH]

Hello,

I get the impression that the answer to my question is less clear-cut then I expected. Let me explain how it works with Eurocode.

You have a design wind velocity (50 year return period). Then I use the Eurocode for loads during execution (EN 1991-1-6). If the structure is used less the 3 days, I can use use a 2-year return period. För > 3 days bur less than 3 months I can use a 5-year return period. That means 50 year velocity x 0.855. Since the pressure is proportional to velocity squared I have ~75% of the wind pressure.

Accidental loads are (as far as I know) not considered in this case. And since the structure is lightweight the forces from an earthquake will be small if I include them.

If I use this approach and can show low stresses in the structure, do you think that would be acceptable?

Edit: An additional question, does this type of structure require a stamp from a PE? Perhaps this is obvious but in Europe there are different approaches depending on the country.

Thomas

 

[EDub24]

If I use this approach and can show low stresses in the structure, do you think that would be acceptable?

I think that's acceptable. Sometimes what I do is just design for the full wind/seismic loading. If it's OK I'm happy, if not then I would argue the need for a reduced return period. It sounds like your structure is pretty stout and lightweight so it might be OK. If you design for the full wind pressure and it passes then you can rest assured that you *shouldn't* have any issue with a building department in getting a permit.

An additional question, does this type of structure require a stamp from a PE? Perhaps this is obvious but in Europe there are different approaches depending on the country.

Out of curiosity what part of Europe are you in? If this structure is going around to different locations within a state and required to get permits I would be safe and have the PE stamp. In some jurisdictions they might let it slide but in others they might require it. Better to get it now then have to scramble.

 

[ThomasH]

Hi everybody,

I just want to thank you for this discussion. As for how I will proceed with this I haven't yet decided. I just got information from my client that the project is stopped due to Corona. Exhibitions and things related is not a priority these days. Quite understandable with the current situation I would say.

@EDub24
The structure has traveled in Europe but I have no list of locations. The only location I know for certain is in Sweden where I saw it.

Thank you all

Thomas

 

[dold]

I wouldn't bother with trying to meet seismic detailing requirements. Which, for directly welded HSS would be fairly difficult. I'd be very surprised if any building official even brought that up.