Structural height on ASCE-7 seismic calculation 1

[ftoran]

Hello forum,
Can anyone help me with this issue?

I have performed calculations for a vessel that requires to consider seismic loads as per ASCE-7 2016. Some questions have arised concerning the structural height.

Let me describe you briefly the vessel: it is a cylindrical shell 4000 mm TL-TL, with elliptical upper and lower heads, and supported on skirt 2500 mm height.

When I ran the calculation with my software AutoPIPE Vessel, it took as structural height (hn) the value of 6500 (corresponding to the height of the skirt plus the shell height)
My client’s engineering thinks I must consider the overall height (7740 overall height, from base to upper nozzle gasket face)

When I check the definition of structural height on ASCE I get this:
“The vertical distance from the base to the highest level of the seismic force-resisting system of the structure. For pitched or slooped roofs, the structural height is from the base to the average height of the roof”

So now I think that my client is not right, but neither I do.
According to ASCE definition, I must add to the 6500 mm value that has considered my software a half of upper head height, am I right?

Thanks in advance

 

[dig1]

ftoran,

You have taken the definition from Chapter 11. You are likely in Chapter 15 but possibly Chapter 13.

In Chapter 15 Table 15.4-2 the structural heights are no limit (NL) for skirt supported vessels. Is the question over the mass distribution? I've been designing vessels for over 30 years and have never seen this question raised or perhaps phrased in this manner. I would take the seismic height, normally, as the top of elliptical head unless there is something large/heavy sitting on top of the upper flange or the flange/nozzle large in diameter compared to the shell diameter. I'm not saying that is "right" but that is the way I was taught by people involved 30 years before I started. This is a pretty small vessel so I'm not completely understanding the concern.

Chapter 13 may be a different discussion.

 

[ftoran]

Thank you very much dig1,

I will read chapters 15 and 13 tomorrow when I get back to office. I admit that I am not familiar with ASCE so I overlooked that.

I am not really concerned about design nor safety. Anchor and skirt are pretty oversized. Seismic loads for this project are not a big deal.
The difference on the values of structural heights that I exposed above would not have further impact on skirt/anchor dimensions

However I wanted to know if my client's comment was appropiate (for take into account on next calculation revision) Also to verify if my software calculates properly the ASCE structural height value,

Many thanks for your time

 

[Trestala]

Refer to ASCE Guidelines for Seismic Evaluation and Design of Petrochemical Facilities.

Is your vessel directly support on ground with a foundation/pedestal, is it on flexible piers, or is it installed on top a steel or concrete structure?
The seismic activity the vessel will experience on its base will vary depending on where it is supported and calculation is different for each case.

The ASCE Guideline should point you on the correct considerations.