Justification for not using loads per 15.7 for retrofitting existing clarifier with new internals

[EMWWW]

Hi,
Can anyone tell me the grounds under which the engineer would take exception to verification of loads per 15.7 (ASCE 7.16). These are for calculations for a clarifier internals replacement project.

Thanks,

LK

 

[TLHS]

I haven't looked up what that clause says, but that's a really open ended question.

Are you planning on taking exception? What's your reasoning? Is someone else trying to justify it to you? What is their reasoning?

 

[r13]

I think, in general, if the replacement does not add to seismic weight; alter the structural system and/or seismic properties of the clarifier, then you may consider to take exception.

 

[EMWWW]

As a reply to TLHS:
The Structural Engineer listed this as an exclusion, and we were trying to figure out if there was any clause in the Code that stated that. (This is for replacement of Clarifier Internals, for a Project in California). The Clarifier itself is designed to 2016 (or earlier). We are mandated to use the latest codes for the internals. Our Engineering Consultant has stated that a loading of 72kips (due to sloshing) is to be assumed for the retrofit.

 

[GC_Hopi]

Appendix 11B has language for exceptions when dealing with existing building provisions. You will need to clarify your question if you are looking for a more detailed reply. Its not clear if you were expecting calcs and they were not ran or if you dont think calcs are required. Also not clear if the entire clarifier mechanism is being replaced and there is a question about the existing anchorage/tank or if its a electrical/mechanical upgrade and the question is more about the clarifier mechanism.

 

[EMWWW]

Thank you GC Hopi.
To clarify:
Yes we got the Structural Calcs, where the Registered Engineer stated that loads per 15.7 were not considered.
The existing mechanisms, scum troughs, drives, V-notch wiers, walkways and center columns are being replaced.
The existing clarifier drawings are stamped 1975.

 

[Rabbit12]

So really, what is your question?

 

[JedClampett]

Connecting the dots, if the existing clarifier was built in 1975, it is unlikely that the anchorage was designed for the seismic sloshing. So designing the clarifier mechanism for sloshing loads would likely just throw the sloshing loads into the inadequate anchor bolts and fail them. It would make a bad matter (a failed mechanism) worse (failed mechanism and anchor bolts).
It seems logical to me.
But the more he would explain this, the more questions (How good are the anchor bolts?; Could they be reinforced? etc.) he would have to answer.

 

[EMWWW]

Thank you JedClampett,

That was exactly how I analyzed it.
So short of seismically retrofitting the entire unit, there appear to be no good options for selective replacement. The system is likely to fail regardless during a seismic event, and not just due to misaligned code requirements due to the current retrofit. Having said that, is there any provision in the Code that allows an exemption to use the updated loads per ASCE 7.16, 15.7? If so where would I find them?

 

[EMWWW]

Along the same lines JedClampett, would an in-kind replacement of internals qualify for any exemptions? It is a replacement and not a redesign.

 

[JedClampett]

If you drill into the Existing Building Code (I don't have a copy with me), I believe that this could be called an alteration, not impacting the original load carrying system, and not be subject to following the newest code.
Of course, this isn't a building and there's not an Existing Clarifier Code. It's JedClampett's Hippocratic Oath: "First Do No Harm." Trying to fix a clarifier mechanism anchorage will be expensive and probably make it worse.

 

[HTURKAK]

I am looking to the thread and posts for a while.. I looked to the ASCE 7-16 and try to see the applicable items and copied and pasted below;

-15.7.6 Ground-Supported Storage Tanks for Liquids
-15.7.6.1.3 Equipment and Attached Piping
-15.7.6.1.4 Internal Elements.
-15.7.7 Water Storage and Water Treatment Tanks and
Vessels
-15.7.7.3 Reinforced and Prestressed Concrete. Reinforced
and prestressed concrete tanks shall be designed in accordance
with the seismic requirements of AWWA D110, AWWA D115.........

The conservative approach will be, to check the performance of walkways, bridge mechanism , central column .... Apparently, your concern is The system is likely to fail regardless during a seismic event..Is this concern based on a performance calculation ?

The code item 15.7.7 is for Water Storage and Water Treatment Tanks .. and IMO, Water treatment is for potable water treatment rather than Sewage treatment plant.
I will suggest you to look ;

https://www.americanlifelinesalliance.com/Products_new3.htm#WaterWastewater

..and apparently , the sewage treatment plant and related units are not a lifeline item....

IMO, the owner / operator should decide for the exemption .

 

[JedClampett]

The conservative approach will be, to check the performance of walkways, bridge mechanism , central column .... Apparently, your concern is The system is likely to fail regardless during a seismic event..Is this concern based on a performance calculation ?

I've been in this situation many times. It might be conservative to check the walkways, bridge mechanism , central column for current loading requirements, but if you:
[ul]
[li]Don't know the anchorage due to poor or non existent records.[/li]
[li]Or know it's not going to be adequate.[/li]
[/ul]

Beefing up the mechanism just to push the mode of failure to the anchor bolts is not doing your client any favors. The mechanism is going to be less expensive to replace than both the mechanism and the anchorage. And if you tear out the anchorage due to a seismic event, you're probably lost the contents of the tank, causing an environmental issue.
I think that involving the owner is correct, but making sure they understand that trying to improve the anchorage is likely to cost in the range of a new clarifier.

 

[EMWWW]

JeddClampett:
Improving the anchorage was a recommendation by the Design Professional we engaged. His specifications are detailed to a fault-fantastic for greenfield projects, but impractical for messy retrofit work.
It is a long and sad story. At any rate, the package is going for a rebid, and I am rewriting the scope as we speak. I am specifically requesting verification of applicability of the existing buildings code with its exemptions. It has certainly been an extremely interesting project, driven by a "simple" question of why the SE engaged by the equipment vendor excluded considering loads per 15.7.

 

[JedClampett]

One approach that is very seldom used but might be productive is to test the existing anchors with pull tests. I'd contend that any ACI approach is very conservative and that there's hidden capacity in those bolts.
The risk is that you might find a lot of capacity or you might have them pull out when you look at them.

 

[HTURKAK]

Dear JedClampett, I could not get how loss of content due tear out of the anchorage .. will you pls explain?

Although the OP did not define , I assume the clarifier is peripheral driven and is buried or half buried . During a major seismic event, the loss of content is more probable due to sloshing and scum boards , bridge may also experience dynamic water loading .
More over, the central column, travelling bridge will experience seismic loads..

My region is located at high seismic zone and i did not hear the rupture of buried tank but witnessed almost emptying of a pool due to sloshing...