Seismic and External Loading

[PipingEquipment]

Reading from "Chapter 11-Seismic Design Criteria" in the book "Minimum Design Loads for Buildings and Other Structures":

The first paragraph states: "The specified earthquake loads are based on post elastic energy dissipation in the structure, and because of this fact, the requirements for design, detailing and construction shall be satisfied even for structures and members for which load combinations that do not contain earthquake loads indicate larger demands than combinations that include earthquake loads."

Whew...So does this mean if I have a structure that is subject to external forces and moments, in design I should only be focused on the earthquake loads?

I have done WRC-107 calcs to ensure the nozzle to shell connection is secure on the loaded nozzles, but I don't have any way of analyzing the base structures (skirt) integrity for these external loads. Can I skip this analysis based on the what the seismic code says?

 

[PipingEquipment]

In case it wasn't clear above, I have done the seismic calcs, I am wondering if I have to do them with these external loads and moments being applied or can they be done without the loads being applied and still be code compliant?

 

[jte]

I always figured the main reason we attach piping to vessels is for the pipes to act as restraints/supports for the vessel when the next earthquake hits.

More seriously, though, I've never seen piping loads included in seismic evaluations of vessels. Maybe if you have unusually large external loads, it may have an impact. I've only seen once or twice where anchor bolt sizing changed due to pipe loads.

jt

 

[PipingEquipment]

There are two nozzles with external loads, the first (on the side of the horizontal vessel) has a resultant force of around 2000lbs and moments in x,y and z ranging from 24,000in-lbs to 110,000in-lbs, the second nozzle (on top) has a resultant force of around 350lbs and moments ranging from 3,000in-lbs to 14,000in-lbs.

My computer program cannot calculate the seismic with external loads being applied, and it seems to me that the above sentence in ASCE 7-05 states that this coupling of the loads and seismic is not required. So to me it seems we should be ok just doing the seismic and over designing the skirt and base ring a bit.

What do you think? Are these loads large enough to significantly influence the support structure of an 8ft tall 3/4" thick 3ft diameter vessel?

 

[PipingEquipment]

*vertical vessel

 

[gr2vessels]

The external loads on the nozzle will eventually translate into shear forces and overturning moments actind on the base ring and bolts. The same is valid for the horizontal vessel's saddle.
In addition, you have the wind and seismic loads on the vessel, generating additional shear forces and overturning moments. It is your engineering ability (plus Client specification) to compound all these loads in the right proportion, to size the skirt for the maximum expected loads. For example, you might need to build into your design a hydrotest on site, in fully corroded condition;- you can choose to do the hydro test in a calm day or in the middle of a storm, but a seismic movement can strike without notice, hence your loads will include full seismic load, some reduced (30%) wind load, vessel full of water, excluding the nozzles external loads (operating loads).
gr3vessels

 

[bernoullies123]

In regards to PV's to ASME VIII Div. 1, suggest having a look at Appendix G-5 (a)(3).

Some times specific requirements may be covered in the client specification, but generally the client is looking to the designer to provide an appropriately designed vessel. Designs shouldn't be limited to the capabilities of the default coding in software.