Title: /
Modification to Wind Overturning
Date: / May 15, 2011Contact: / Name: / Ty Hagen
Company: / HEI
Phone: / 651-686-5686
E-mail: /
Purpose: / Provide additional overturning formula to address all tank configurations
Source: / Email to John Lieb5/12/2011
Revision: / 0
Impact: / Correct un-conservative assumptions for open top tanks and large H/Dratio tanks and eliminate unnecessaryanchoring of structurally supported low cone roof tanks
Background: / 1) Open top tanks are subjected to no roof wind uplift. Under current rules, controlling formulas see 60% of the design horizontal wind in the empty condition.This results in overturning that is un-conservative compared to historical values.
A similar condition occurs on tanks that are much taller than the diameter. The roof uplift due to relative small area is insignificant compared to the horizontal wind on these tanks large H/D tanks. The 60% wind again is un-conservative compared to historical values. It is possible under current rules for an 80 ft tall x 40 ft dia tank subjected to typical wind loading to not require anchorage.
The historical 0.6605overturning factor limit for unanchored tanks was established for full wind with the tank empty. This value limited uplift of the tank resulting in high compression loads and distortion of the shell. The current limits allow significantly greater tank uplift for these H/D ratio tanks.
2) The API 650 addendum 3 changes failed to eliminate the roof weight from definition of MDL. Correct for addition of termMDLR. This is corrected below.
3) An option has been added to design lowcone roof overturning stability using the previous method of applying horizontal wind pressures only (no vertical uplift).
Proposal: /
- Add a third overturning condition which would check for full (100%) wind on the shell without consideration of roof uplift.
- Change definition of MDL
- Allow supported cone roofs to use the historical 100% horiz. wind loading with no wind uplift
Specific changes: / 5.11 WIND LOAD ON TANKS (OVERTURNING STABILITY)
5.1 1.1 Wind Pressure
Overturning stability shall be calculated using the wind pressures given in 5.2.1(k).
5.1 1.2 Unanchored Tanks
Unanchored tanks shall meet the requirements of 5.11.2.1 OR 5.11.2.2
5.11.2.1
Unanchored tanks shall satisfy bothallof the following uplift:
1. 0.6Mw + MPi MDL /1.5 + MDLR
2. Mw + Fp(MPi) < (MDL + MF)/2 + MDLR
3. Mws + Fp(MPi )MDL /1.5 + MDLR
where
FP = pressure combination factor, see R.2,
MPi = moment about the shell-to-bottom joint from design internal pressure,
Mw = overturning moment about the shell-to-bottom joint from horizontal plus vertical wind pressure,
MDL = moment about the shell-to-bottom joint from the nominal weight of the shell and roof structural supported by the shell that is not attached to roof plate,
MF = moment about the shell-to-bottom joint from liquid weight,
MDLR = moment about the shell-to-bottom joint from the nominal weight of the roof plate plus any attached structural.
MWS= overturning moment about the shell-to-bottom joint from horizontal wind pressure
5.11.2.2
Unanchored tanks with supported cone roofs meeting the requirements of 5.10.4 shall satisfy the following criteria
. Mws + Fp(MPi )MDL /1.5 + MDLR
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