API 620 App. L - additional requirements to API 650

[BrBalboa]

Dear community,

First post here...
Doing some reading on ASCE 7-16, API 650 and API 620 seismic design requirements. It seems that sliding resistance, for API 620 (§L3.3.3) tanks, shall be checked regardless of anchorage while API 650 and ASCE 7-16 only requires this check to be performed for 'self-anchored tanks'. This, it seems, is intended to ensure bottom stability. Question is why only for API 620 and not API 650 tanks?

What would be typical solutions when this verification cannot be met, D/H adjustment, seismic isolators, other?

Thanks in advance for taking the time.
Balboa

 

[IFRs]

In API 650, you check for seismic and wind stability. If the tank does not require anchors (it is self-anchored) then you check for sliding stability. If the tank is anchored then you don't have to check for sliding stability because the anchors are there. If you think the anchors are not sufficient to resist sliding forces then by all means run a quick check.

 

[BrBalboa]

Thank you IFRs.
Indeed that is the procedure according to API 650, also ASCE 7-16 § part 15. However, if you read API 620 appendix L you will find the following:

"The tank system, whether self-anchored or mechanically-anchored, shall be configured such that the overall horizontal shear force at the base of the tank does not exceed the friction capacity"

You will find this requirement in § L.3.3.3, L.4.3.10 and L.4.4.4.

 

[HTURKAK]

This is not true..
Pls look API 650 Clause E.7.6 . The Sliding Resistance is checked for both self-anchored tanks and for mechanically-anchored tanks.
For anchored tanks , the clause ;
( No additional lateral anchorage is required for mechanically-anchored steel tanks designed in accordance with this
Annex even though small movements of approximately 25 mm (1 in.) are possible.)
I would like to remind that ; mechanically-anchored tanks are equipped with anchor bolts and chairs having ht around 450 mm high and are not prevented from sliding laterally at least 25 mm .

[quote
"The tank system, whether self-anchored or mechanically-anchored, shall be configured such that the overall horizontal shear force at the base of the tank does not exceed the friction capacity"][/quote]

My understanding is ; the tank shall be proportioned such that the seismic base shear, V, does not exceed friciton resistance. ( The options are , increase the dia and reduce height, try to justify the max. friction value of 0.4)

I would like to remind the last statement of clause E.7.6
( The lateral shear transfer behavior for special tank configurations (e.g., shovel bottoms, highly crowned tank bottoms,
tanks on grillage) can be unique and are beyond the scope of this Annex. )
....




He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[IFRs]

API 620 does not allow using the tank anchorage to resist sliding, as API 650 does. Therefore you must check the tank for both self- and mechanically- anchored tanks. I'm not sure why the committee chose this but I don't think the analysis is too time consuming. If you need to protect the tank from sliding the standard does not offer any real help.

 

[BrBalboa]

Thank you both for your feedback.
HTURKAK, not sure what you mean by this is not true. API 650 allows V to exceed Vs for anchored tanks, of course anchorage shall be checked to be sufficient. API 620 does not allow V to exceed Vs, regardless of anchorage...

IFRs,
Indeed that is my question, why API 620 differs from API 650 on this respect?

Rgrds,
Balboa

 

[IFRs]

Balboa - There is an API conference coming up, you could visit the design committee meeting and perhaps ask the question...

 

[HTURKAK]

You said ( It seems that sliding resistance, for API 620 (§L3.3.3) tanks, shall be checked regardless of anchorage while API 650 and ASCE 7-16 only requires this check to be performed for 'self-anchored tanks'. )

API 650 requires that the Sliding Resistance is checked for both self-anchored tanks and for mechanically-anchored tanks.
My points
- API 620 scope covers also Appendix Q and R tanks ,
- API 620 explicitly does not allow using the tank anchorage to resist sliding, ( I have 11th ed. L.3.2.3 Resistance to Sliding ; The tank system, whether self-anchored or mechanically-anchored, shall be configured such that the overall
horizontal shear force at the base of the tank does not exceed the friction capacity as defined in API 650, Section
E.7.6. Mechanical anchorage shall not be used to resist sliding. )
- API 650 implicitly allows the use of tank anchorage to resist sliding . Then states ,you should not need additional anchorage for sliding resistance and if mechanically-anchorage necessary for ( OT UPLIFT ), this type of anchorage having movements of approximately 25 mm . Literally , the contribution of anchors to sliding resistance negligible.
- So all the ways to ROME!!

My points are;

- If the tank is large enough and needs anchorage the found. should be ring type , flat slab , not only the tank but the overall system may be prone to sliding.
- If the tank is small ,( probably indoor ) supported on raft mat , grids , the sliding resistance could be increased with shovel bottoms, tanks on grillage, or using anchor to resist shear .( Anchors which will not allow limited sliding )

If this is a real case , i will suggest you to provide more info. to get better responds.
...


He is like a man building a house, who dug deep and laid the foundation on the rock. And when the flood arose, the stream beat vehemently against that house, and could not shake it, for it was founded on the rock..

Luke 6:48

 

[BrBalboa]

HTURKAK,
I believe my questions are clear (we are engineers not lawyers).
- why requirements in API 620 differ from API 650 (in this specific point regarding sliding resistance)?
- When this requirement cannot be met, V > Vs, any solutions other than adjusting H/D and/or adding seismic isolators?

If you know great, your input is appreciated otherwise lets wait and see if others can help.