Base Shear Resistance for Ground Supported Tank

[T_Bat]

Hey Everyone,

I have a ground supported steel tank that I am designing foundations for. The tank will be welded down to embed plates (of my design) placed into a large mat slab. For anchorage I planned on designing the embed for any uplift resulting from overturning from seismic or wind. For shear my plan was to count on the friction between the embed and the concrete. I've seen discussion on on this topic as it relates to columns baseplates and it seems there were a fair amount of opinion to NOT use friction for shear resistance.

Considering all load combinations and including the vertical shear force (reducing the gravity load) it seems reasonable to assume the down loads on my embeds could be counted for friction. When all is said and done I get a unity check of around U=0.4 for base shear/friction resistance.

Given the case would you have any issues counting on friction for shear resistance?

Thanks in advance!

 

[T_Bat]

I'd also add that I've seen the comment, "codes don't allow friction resistance to be used for seismic" but I cannot find, for the life of me, the code provision that states this.

 

[gravityandinertia]

You would have to specify the finish/treatment to get the desired coefficient of friction you want, and it would have to be maintained over the life of your structure to ensure it still exists should the design event happen. That's the case against it.

 

[T_Bat]

Interesting points - however the surface would be embedded in the concrete and somewhat protected from corrosion. Also AISC DG gives a coefficient on friction without much "special" prep of the surfaces. I imagine I might as well add some lugs for belt and suspenders?

 

[JStephen]

If the tank is an API or AWWA tank, I would recommend using the normal anchor chairs, details, and sliding considerations from those standards.

Not sure of your detail intention, but note that the chime of a normal tank isn't designed for uplift.

 

[T_Bat]

This is not a API or AWWA tank. The manufacturer has recommended the welds to embed plates that are placed in the mat. The tank is not our design we are just supposed to provide the proper emebeds and foundations.

 

[haynewp]

Regarding using friction and seismic; I believe unanchored API tanks are fairly common. So unless it is a zero seismic acceleration area they would all be relying on friction.

 

[bones206]

Check ASCE 7-10 section 15.7.6.1.5:

15.7.6.1.5 Sliding Resistance The transfer of the total
lateral shear force between the tank or vessel and the
subgrade shall be considered:

a. For unanchored flat bottom steel tanks, the overall
horizontal seismic shear force is permitted to be
resisted by friction between the tank bottom and
the foundation or subgrade. Unanchored storage
tanks shall be designed such that sliding will not
occur where the tank is full of stored product. The
maximum calculated seismic base shear, V, shall
not exceed

V < W tan 30° (15.7-14)

W shall be determined using the effective seismic
weight of the tank, roof, and contents after reduc-tion
for coincident vertical earthquake. Lower
values of the friction factor shall be used if the
design of the tank bottom to supporting foundation
does not justify the friction value above (e.g., leak
detection membrane beneath the bottom with a
lower friction factor, smooth bottoms, etc.).
Alternatively, the friction factor is permitted to
be determined by testing in accordance with
Section 11.1.4.

b. No additional lateral anchorage is required for
anchored steel tanks designed in accordance with
reference documents.

c. 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
standard.

 

[T_Bat]

Thanks for the responses. I had read that section in CH 15 but there appeared to be some hesitation by some to use friction to resist base shear.

 

[bones206]

I recently designed a foundation for a Convault fuel tank, where I specified their patented steel seismic restraints at the base. The checker argued that the restraints were unnecessary and friction alone was sufficient to resist seismic forces. My counter argument was that friction is not allowed per ASCE 7-10 section 13.4. In that case, I was classifying the tank as a non-structural component governed by Ch 13, rather than by Ch 15. That may be where others are coming from when they argue that "codes don't allow friction resistance to be used for seismic". I imagine the allowance for frictional resistance in 15.7.6.1.5 is based on the performance experience of that specific type of tank structure.