Applying NBC 2005 Seismic to towers/vessels 4

[s1111]

Does anyone have experience applying NBC 2005 Seismic Code to pressure vessels? I am looking through it right now but am unclear at the moment to its application to vessels and towers.
Any help is appeciated.
Thank you in advance.
s1111

 

[vesselguy]

I have not seen the 2005 NBC yet. But as in the past you use it to calculate the seismic and wind load. Then you apply the loads to vessel as you normally do. If you want detail method on how to calculate the loads then I suggest you read NBC directly. Am I missing your point?

 

[weeeds]

I saw on the Canadian NRC web site that the 2005 eddition has over 800 changes to the 1995 eddition. There is also some confusion in industry as to the Seismic calculations as you point out. My understanding is that the 2005 version is more stringent but I have no other details. I am trying to learn some more about the changes and resulting problems and will advise about my findings.

 

[TomBarsh]

I asked a similar question in thread771-195632 (NBC code issues). The new NBC seems to consider only limit states analysis (allowable stress design was dropped in this Edition), whereas ASME considers only allowable stress design. A problem comes up when combining stress due to wind and seismic loads with stress due to pressure.

 

[Tomtation]

According to the NBC 2005 Commentary, page J-60 item 226: "The design of free-standing tanks is outside the scope of the NBC 2005. Their design should be based on current industry-accpted practice and consensus design standards (API 650, API 620...). In applying these industry standards to Canadian locations, the designer should use ground motion parameters consistent with those used in NBC 2005". For the jobs I'm currently working on, I applied paragraph 4.1.8.17 which is for "Elements of Structures, Non-structural Components and Equipment".

 

[s1111]

everyone thank you for your comments.
vesselguy, I did read the NBC 05 seismic loads section under structural sec. However, it does not provide explicitly any application to towers or vessels.
As you can see from other users' comments.
My question again is how and if it is possible to apply the code. I think user Tomtation's response is what my answer is and I will look into it tomorrow, when I get back to work. The link pointed out in Tom Barsh's reply also throws some light.
Once again, thank you all. I look forward to dig a little more into this. I would appreciate any further comments on this.

Regards,
s1111

 

[weeeds]

Does anyone have contacts in the NRC who knows why the change was made to this design in this eddition?
It may be helpful if we understood the issue from the technical committee's side, and then perhaps a design approach can be developed.
Right now this seems to be a real design problem.

 

[vesselguy]

I'm sure you guys know that NBC is a "building" code. NBC is especially useful in that it has a good recommendation on vortex shedding criteria. The idea is to use it only to calculate the "load" from wind and seismic and then use the standard vessel design methods to evaluate the stress in a vessel; not a building/structure. Since I'm a novice on all this business, I would be very interested to see how you guys been applying the buidling codes to a vessel.

 

[TomBarsh]

Analysis for wind, seismic, etc, loads in the current US model building codes (IBC, NFPA, etc) are based on ASCE 7 "Minimum Design Loads for Buildings and Other Structures". There is a nod in ASCE 7 and the model building codes to pressure vessels but it seems they do leave some things to be desired. In any case, designers have been applying the wind and seismic provisions of model building codes to pressure vessels for many years (the NBC, and ASCE 7 and the model building codes also address rain loads, snow loads, flood loads, etc, that are not generally applied to pressure vessels).

The conceptual problem arises when the design standard or model building code (in this case, National Building Code of Canada, NBC 2005) specifies combinations of loads to be considered as acting concurrently. eg: combine dead load, live load, and wind load but not seismic load, etc. Usually there are a dozen or so combinations of loads listed. The loads are often "factored" by applying a knock-down factor (I leave this to the Code people to explain how and why this is done). The "factors" may be different depending upon if the structural design is performed using allowable stress design (ASD) or load and resistance factor design (LRFD) (the AISC nomenclature for steel construction) or other limit states analysis.

The problem with NBC 2005 is that the Code addresses only limit states analysis. But the ASME Code is based on allowable stress design. Consequently, the load combination factors stipulated in NBC 2005 cannot be used because they have different values than would be those for allowable stress design (comparing to the old 1995 Edition which treated both design methods).

Currently, there is little direction to the vessel designer on how to apply the NBC 2005 to design of ASME pressure vessels.

 

[gr2vessels]

As usually, Tom is very thorough and nothing much to add, except that software like PV Elite ignored valuable code like NBC 2005 all together, whilst listing all other building codes under the sun. However, a designer familiar with "other" codes would realise that the user input to the software allows a large design flexibility, thus making the design more realistic.
cheers,
gr2vessels

 

[s1111]

The only consideration at this time is Seismic, and is important to size the skirt thickness. We're not looking at wind currently, but we'll be looking into it sooner or later.
Given Tom's comments, a clear understanding is vital for developing per NBC. Thank you again for your valuable comments.

Regards,
s1111