Dear Friends,

Designing supporting legs using pvelite 2006 for a 3.6m dia and 6.0m HT tank i get a warning which is stated as:

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Warning: There appears to be a problem with this leg design.
Please check the input carefully and review these
calculations carefully. [Sma > sa or Sc < 0 or Sma > Spex]

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Structural shape used for legs is: Channel C6X10.5

Now if i go to more heavier structure like Channel C6X13 then the ' AISC Unity Check 'fails. And if i continue with Channel C6X10.5; the above warning appears.

I tried more 2-3 different sizes of c channels but pvelite replay same scenario.

So based on your personal experience what u think should i use Channel C6X10.5 or shift to other steel section like I beam. In I beam case there is no such warnings however the section is expensive and heavy (W8 x 35)

Thanks

How many legs are you using in the calc? A CH6 is a very. very small piece of steel for a tower that size.

How high is the leg (base to top of leg) and what is the length of each leg's weld to the tank wall?

Under modest eccentric loads, which a tank leg welded to the side of the tank always will have, a channel that small would fail by buckling (not compression) if it is too tall.

Did you account for the tank's liquid weight, or is it for non-pressurized gas service and will never be hydrostatically tested?

Dear Robert,

No. of legs = 4
Height of leg (from base to top) = 6.577 ft
Length of each leg's weld = 1.656 ft.

Tank = Tank is basically atmospheric storage tank designed on api 650.

I analyzed legs on basis that tank if fully filled with additive oil (SG = 1.02) and at the same time both wind and seismic loads are acting.

Thanks

You have a logical starting point then: You have identified the dry load, wet load, and chosen a design wind speed.

Couple of cases to consider:

Assume an easterly wind at your max wind speed.

1) For failure of the legs, the worst case (your design case) will be with a completely full tank (heaviest weight) with the wind force from the east adding to weight on the two west-side legs. You must show that this heaviest strain being resisted by only 2 legs is NOT going to fail the leg. My opinion, without knowing your wind condition, is that buckling of a light duty leg is the most likely way for a leg to fail: you want to use a very little channel member on a relatively long distance with a shear load (from the wind) combining with two bending forces from an eccentric tank load and the sideways wind force. Be sure to check the weld between the baseplate and leg of these two legs as well. Verify you've chosen a big enough weld that will be long enough: This is another reason to use a heavier member. A heavier vertical member means more length for the fillet weld between pad and leg without ribs or stiffeners.

Also, check the effect of a NE or SE wind: with only 4 legs, you could end up with the 45 degree wind causing the heighest load.

2) For pullout and prying of the anchor bolts and anchor pad, the greatest lifting force will be on the two east legs with an empty tank. On an empty tank with high enough winds, that lifting force could be enough to put a tension load on your pads and anchor bolts on the east side. Don't think that the anchor bolts will always be under compression.

3) On the west side pad, the worst down force is with a full-loaded tank and an east wind: a compression force that is low enough that a little C6 leg won't fail isn't likely to be big enough to break any reasonable pad and weld, but you have to show you checked every weld you sign for, and every piece of metal you choose, under its worst case condition.

Bottom line?

Four legs, each less than 7 feet? Compare the expense of your tank failing (and killing someone, causing a polution shutdown/cleanup/fines and penalties, fire and explosion of flamable oils destroying adjacent eqpt) to buying four WF 8 members at an increase of 4.00 to 6.00 dollars per foot. You need to consider very carefully the "wisdom" of a person who wants you to buy four cheap legs - with a computer model already telling you that the C^ channels are failing! - rather than get 4 heavier legs on a oil tank.

Robert,

Yes we now decide that instead of legs we should analyze a skirt support.

but what i actually meant by my post earlier is "That legs are not failing it just giving a warning message"..

====================================================================
Warning: There appears to be a problem with this leg design.
Please check the input carefully and review these
calculations carefully. [Sma > sa or Sc < 0 or Sma > Spex]

====================================================================

So is it so serious to take care of this warning message. As it shows legs will not fail but they will be very near to failure conditions... BUT AGAIN LEGS NOT FAIL...

Thanks for your help

Yes, a warning is not a "promise" or graphic picture of immediate failure: Don't expect too much from any software: As good as they are, the only thing a software package can do is mimic (faster and with exact repetition every time) the paper-copy-and-equation-and-assumptions analysis that are themselves an approximation of the fundamental equations that come from assumed conditions of theoretical loads and ideal metals that have no flaws.

But the software warning shows that you have exceeded limits: so look very carefully. (I don't recognize the warning itself, which is why I recommended a rigorous hand-calc to check several conditions.)

The actual metal under actual loads may not "fail" completely: It might yield or bend, yield at a weld, or simply be "past spec" but still under real-worlds failure limits. For example, we take the maximum strength of a metal as 70 percent of its yield strength. But does that mean that the metal will abruptly fail at 71% of the yield strength?

No, of course not.

But it means that if you are designing to "69 and 1/2 percent" of yield strength you're getting very, very close to (or past) any real reserve for uncertainly. It's not a smart way to work.

Not a smart way to live for those people who will abuse the plant and the pipes connecting to it, while walking under it and living around the oil and engines.

Hi Bajwa,

This message appears when the normal stress on the leg itself is greater than the allowable (Sma>Sa) or when the unity check (Sc) is negative (not good) or when the stress is greater than the allowable combined stress (Sma>Spex). If you excessively load a member, it will more than likely evetually buckle. It is good that you are paying attention to the message, taking appropriate action and looking into alternatives.