Minimum Uplift Anchorage Forces

[anthony gravagne]

Hello:

I am designing cast-in-place steel anchor bolts (ASTM F1554 Gr 36) to be used to anchor vertical steel tank support legs to concrete. Due to the relatively low seismicity I frequently calculate that the legs do not go into tension. Seismic shear and wind shear are present of course. Is there a minimum uplift force required to be used despite the fact that there is no tension at the base plate?

 

[JAE]

If the steel tank is totally empty and you have full code wind or seismic is there still no uplift (i.e. using 0.6D)?

 

[JStephen]

For municipal water towers, AWWA D100 specifies minimum loads. See 3.8.5.3 and 3.8.7.
ACI 318 has additional requirements.

 

[TLHS]

In a perfect world, design the anchor rod to develop the yield capacity of the rod (i.e. make it so the failure mode is ductile). Some tank standards ask for this specifically.
You get a numerical benefit out of this with seismic design, but for the types of loads you get with a tank (generally transient loads) it's going to give you a way better overall strength even if you don't get to account for it in the math. It's also an easy design basis to point to.

Given the same anchorage depth, I'd likely rather have a smaller rod size if it takes me to yielding governed rather than a larger rod if I'm concrete governed. Depends on the actual numbers, obviously.

For really small tanks, this may not be realistic. Also, for leg supported tanks the anchorage may not be the governing issue.

 

[anthony gravagne]

These steel tanks are typically 9-12' in diameter and are supported by 4 or 6 equally spaced vertical steel posts with base plates. I am checking the uplift forces due to seismic full, seismic empty and wind empty. I am also designing the anchorage in accordance with API 650 Section 5.12.13 wherein it states "When mechanical anchorage is required for seismic, the anchor embedment or attachment to the foundation, the anchor attachment assembly shall be designed for the anchor attachment load. The anchor attachment design load shall be the lesser of the load equal to the minimum specified yield strength multiplied by the nominal root area of the anchor or three times the seismic design uplift force per anchor, Tb."

I interpret this to mean that if there is zero seismic uplift, the lesser is zero. Am I interpreting this correctly?

There will always be seismic shear or wind shear to deal with. In some cases there is wind uplift.

Comment please. Thank you

Anthony Gravagne, PE, SE
Tucson AZ

 

[JStephen]

API-650 doesn't address leg-supported tanks, only flat-bottom tanks, so that's one reason that question is not addressed by the code.
Typically, if there is no design loading on the anchor bolts on a flat-bottom tank, then anchor bolts are omitted.
See requirements in ASCE 7 and ACI 318. I believe those requirements are more stringent in Seismic Design Category C and above.

Long ago, I was involved on a project where a multi-leg tank was supported on separate foundations. There was differential movement in the those foundation elements. All was fine when the tank was loaded, but when load was removed, it would lift selected legs off the foundation and pop the anchor bolts out of the foundation at those points. I would think that was less likely with a 4-leg tank, but reason not to skimp on embedment, still.