API Tanks - seismic base shear and friction

[FOX89]

Hello,

I have a group of tanks which shall be verified under seismic actions. My question is that API clearly doesn't count on anchors to resist seismic loads based on clause E.7.6 "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", so what happens when the seismic base shear exceeds the frication resistance at the base ? can we allow the tank to slide during seismic events and count on anchors axial ductility ?

On the other hand, since anchor chairs are recommended, it will be very hard to use anchors for shear resistance since the bending moment on anchors will be significantly large.

 

[Rabbit12]

Not sure the size of the tanks you are dealing with here, but my experience has been that the compressive force caused by overturning has been enough to resist sliding through friction. One of the ASCE Guides to Seismic of Anchor Design (can't remember which one exactly) have a good design example.

 

[HTURKAK]

Pls read again Clause E.7.6 ,
- The first two paragraphs for SELF ANCHORED tanks and the sliding resistance will be calculated with expression (E.7.6-1),
- The third para. is fore MECHANICALLY ANCHORED TANKS ,
'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.'

Literally saying, the tank is already mechanically anchored, the anchors provided for overturning , uplift; has a flexibility 25 mm then will be effective for shear .
So , if the sliding resistance is less than the overall horizontal seismic shear force, the anchors SHALL resist to the remaining portion.

- Pls look E.4.6.2 Site-Specific Response Spectra,Exception:
-—( mechanically-anchored tanks that are equipped with traditional anchor bolt and chairs at least 450 mm (18 in.)
high and are not otherwise prevented from sliding laterally at least 25 mm )

I will suggest you, next time , write your post to the ( STORAGE TANK ENG. FORUM )to get better responds and pls look a new thread ( Estimating Displacements thread1452-519031 )
...

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

 

[FOX89]

Thanks all, but i have one more question; wouldn't the assumption of the anchor bolts resisting the remaining of the shear force means that we already have considerable slip ? would we still have static friction in this case ? in other words can we have shear friction/anchor bolt bearing interaction ?

 

[TLHS]

This is not really a stress problem. You have significant frictional restraint plus some amount of shear restraint from the bolts. You'll have ground movement that alternated directions. The friction dissipates a bunch of energy. They're saying that if it's bolted to something that stops the overturning, the expected movement of the system as a whole due to shear effects is in the order of 1". Large shear movements in tank foundations don't seem to have been the concerning failure mechanism as long as they're bolted to prevent overturning unless there's a geotechnical failure involved.