Torsion on vessel

[PVGuy]

The Seismic codes talk about inherent torsion and accidental torsion. I cannot find an exclusion for non-building structures not similar to building. Is there an example somewhere of how to determine the resultant loads on the vessel supports from torsion on the vessel? I am especially interested in lug and leg supported vessels.

 

[SnTMan]

PVGuy, I have found the last couple of issues of ASCE-7 just about incomprehensible on Seismic loadings. Very difficult for us "blacksmiths" to apply with any confidence.

Good luck,

Mike

 

[JStephen]

To put it kindly, the building codes are a mess. But if it's an oddball design requirement, I'd look for something that specifically required it, rather than looking for something to exclude it.

Do you have a specific reference?

One issue that came up on the structural forum a while back was the applicable code years. For example, a city will specify what year of IBC or UBC is its building code. That issue of IBC or UBC specifies certain specific years of other standards- including ASCE 7, ASME B&PV Code or API-650. Meanwhile, the ASME and API codes themselves specify that latest edition is always used- and are updated in such a way that you don't normally have the previous year's code. So right off the bat, you get a glaring discrepancy in trying to satisfy the building codes, which the code writers seem oblivious to.

 

[PVGuy]

This is the pickle, the requirement for the non-building structures not similar to buildings is that the Effective Laterl Force (ELF) method be used as for buildings with some alterations that are then listed. It allows exceptions for certian requirements. It stops there for the last 2 ASCE codes (7-02 and 7-05) and IBC 2003 and IBC 2006 simply defer to them. This is not a design requirement now; but I want to understand the requirements prior to that case.

 

[SnTMan]

This has been my experience: The Customer, a big E&C who presumably has Civil E's sitting on top of each other, puts a vaque requirement in their spec.

The fabricator is then required to perform the calculation in the absence of any hard data.

He puts a page in the mechanical calculations with some equations, answers and perhaps a sketch, and with the words "SEISMIC LOADING" at the top.

Customer sees the page and notes the requitement has been met.

Everybody's happy, and the fabricator gets approval to ship.

Mike

 

[JStephen]

PVguy, is there a specific paragraph you're looking at that requires the torsion?

 

[PVGuy]

As an example:

ASCE 7-05 15.4.1 (pg. 162) states that these structures must be designed to resist minimum seismic lateral forces that are not less than the requirements from 12.8.

In 12.8 there is a subparagraph on inherent torsion (from unevenly distributed masses) and a subparagraph on accidental torsion (out of plumb, etc. - as best as I can tell).

The requirement in 15.4.1 specifically states lateral forces, so I thought it meant only the lateral forces from 12.8; however, the title of 12.8 is Equivalent Lateral Force Procedure. So at this point I am thinking the torsion is required.

 

[JStephen]

After some study, my conclusion that ASCE-7 is a mess is confirmed.

First off, is the vessel covered under ANY of the reference documents listed? If so, the item in 15.4.1 requiring the forces per 12.8 is not applicable.

Secondly, when a paragraph is plainly meaningless when applied to the situation at hand, are you constrained to somehow twist the meaning to make it apply, or can you just ignore the paragraph and assume it is not applicable? I think that's the situation in this case, in dealing with the rigid/flexible diaphragms. A similar case is the calculation of the period using the equation for buildings- this is obviously pure coincidence if it matches your structure's period, but the code allows you to do it.

 

[PVGuy]

Thank you all for your comments.

 

[JoeTank]

In my early tank design days, we just took the seismic shear and assumed it was offset from the centerline of tank by 10% of tank diameter. It added a marginal addition to shear load in sway rod due to "accidental" torsion on the tower.

Steve Braune
Tank Industry Consultants

http://www.tankindustry.com