Bolt loads on Seismically stable tanks as per 5-21

[BlakeThomson]

Hi, I'm looking at minimizing my bolt loads on my tank.

Currently my tank is stable for seismic loads, but the tank has sliding/overturning issues formed by wind which require mechanical anchorage.

This requires I go to Section 5.12 / table 5.21 to calculate my uplift loads. However 5.12.1 seems to be exceptionally vague. it states

"When a tank is required to be anchored per 5.11, Appendix E, Appendix F, or when a tank is anchored for any other reason the following minimum requirements shall be met."

This can be interpreted as, if only one is applicable, then the other three are applicable and must be checked. Or, if only one load case is applicable, then the other three cases need not be checked if they do not uplift.

The second seems to make the most sense as why would I design for a tensile load that is not present, but if the first is followed there is uplift in the seismic formula in table 5-21 does not take into account the resisting contents load to prevent uplift.


I would appreciate some input (and conjecture if necessary)

Thankyou

 

[fabtank]

Hi Blake,

I interpret 5.12.1 as if a tank is anchored for any reason it must meet all of the requirements of section 5.12.

Your anchorage should be designed for all of the load cases in table 5-21.
Note that none of these cases account for the product weight to resist uplift.
It is mentioned in section E.6.2.1.2 that: "When tanks are anchored, the resisting weight of the product shall not be used to reduce the calculated uplift load on the anchors."

I would also think that if the wind overturning moment was the driving reason for anchorage that the seismic load wouldn't be much higher than the wind load if at all.

 

[JStephen]

The assumption is that for a tank to develop uplift resistance from the floor plate for seismic loading, the shell must move up to some extent. If the tank is anchored, it is limited in movement and can't develop that uplift resistance, so the uplift will need to be resisted by the anchor bolts instead. IE, if the tank requires anchorage for wind but not seismic, the anchor bolts still have to be adequate for the calculated seismic bolt loads, which are quite often higher than the wind loads.

 

[BlakeThomson]

Fabtank, in my experience, the uplift load in the bolts from are usually significantly higher than loads required for wind. This is due to the fact that the more critical wind loads are applied while it is empty and with seismic with overturning checks the most critical loads are applied with contents resisting to calculate overturning, but as you said, only the shell (and applicable sections of roofing) are used when calculating the bolt loads for seismic and as such the bolt loads in my usual design cases are significantly higher.


JStephen, wouldn't that only be when the overturning ratio is stated as between 0.785 and 1.54? As the phrase uplifting would imply that the tank itself moving up. Whereas if it is below 0.785 it is stated as stable.