API 650 and ASCE 7-16 and 7-22 Wind Loading Updates

[jimmyhutmacher]

ASCE 7-16 and 7-22 have some pretty significant changes to wind loading on tanks, see below:

ASCE 7-16

• Updated wind speed maps
• Ground elevation factor
• Significantly updated section for bins, silos and tanks
  • Including large increases in the force coefficient (Cf) for tightly spaced groups of tanks
  • New Roof Pressure Coefficient

ASCE 7-22

• Update of Topographic and Velocity Pressure Factors
• Updated wind speed maps

Any idea why API 650 hasn't included any of these updates in the code? If they're planning on it?

If your tank is in a tightly spaced group, like many tank farms, the Force Coefficient Cf, and thus the overall wind pressure basically doubles. Are you including this significant increase in your tank design or just designing per ASCE 7-05 or 7-10 (which API 650 only references) which does not include this provision?

 

[JStephen]

The situation is sort of a mess that needs to be addressed more completely.
Short answer: I design per any contractual requirements (which is generally API-650 or AWWA D100) and also per any applicable building code (which may be ASCE 7-16 or ASCE 7-10, I don't think I've seen anything for ASCE 7-22 yet). Note that where ASCE 7-16 is used, you also need to use the load combinations from ASCE 7-16, which requires design check with an empty tank.
Note that there are areas where there is no prevailing building code, so this is not always an issue. Areas far inland are usually not that affected by the ASCE 7-16 wind loadings.

All three standards need to be updated and also need to be reconciled with each other. It seems to be one of those situations where people writing the code never actually need to design the tanks.
-API-650 neglects wind effects from grouped tanks, as you note.
-API-650 furnishes separate factors of safety for overturn that are different from, and fail to meet, ASCE 7-16.
-API-650 allows a portion of the tank contents to be used to resist uplift. Which I entirely agree with, but this does not meet ASCE 7-16 requirements.
-API-650 limits wind uplift on the roof to the failure pressure of the roof. Which, may or may not be a reasonable provision, you're basically assuming the roof is expendable. But regardless, here again, it doesn't meet ASCE 7-16.

ASCE 7-16 fails to recognize that an "empty" (ie, out-of-service and drained) tank condition is a probabilistic condition just like wind or seismic, and should be able to be factored in with reduced wind loading. There are thousands and thousands of tanks that that don't meet ASCE 7-16 but have set there for decades with no issue due to this. Especially in hurricane regions, it is unlikely a tank will be intentionally left 100% empty if windstorms are expected.
ASCE 7-16 fails to include internal pressure or corrosion allowance in their load combinations.
ASCE 7-16 requires roof uplift loading for tanks in the 0.25 <= H/D <=4 range, but a fair number of tanks fall outside of this range. What about them?
ASCE 7-16 requires increased wind loading for groups of 3 or more. What about groups of two? How about when the tanks are different sizes?
It appears that the intent of ASCE 7-16 is to calculate wind loads based on tank dimensions and roof shape alone, without regard to whatever handrails, stairs, ladders, piping, platforms, antennas, etc., are hung on the tank. This simplifies the design, but also makes the result somewhat more questionable.

I believe ASCE 7-22 has tornado requirements that would also enter into the design considerations, but haven't looked into them extensively.

 

[HTURKAK]

If your tank is in a tightly spaced group, like many tank farms, the Force Coefficient Cf, and thus the overall wind pressure basically doubles.

My points ;
-API 650 suggests the use of wind exposure Category C although a typical plant layout would be with closely spaced vessels, pipe racks, storage
tanks and equipment structures so Category B should be more representative . Just for comparison , the wind pressure for an exposure Category B is approximately 70 percent of the Category C ,
- API 650 uses working stress design , and ASCE 7 Allowable stress design combinations uses 0.6 coefficient for wind loads.
- Designer can adjust the wind pressure effect using the formula (V/120)**2 ,
- API 650 suggests Design wind pressure using design wind speed (V): The design wind pressure on shell (PWS) be 0.89 kPa (V/190)**2, [(18.6 lbf/ft2)(V/120)**2] on vertical projected areas of cylindrical surfaces for uplift , anchorage etc. But for girder design different the wind pressure is used ( e.g. 1.48 kPa (V/190)**2)
- API 650 became largely recognized international standard and probably 80 percent of existing API 650 tanks are outside the USA. I do not remember the no. of tanks that i have involved for design , construction and we always used local wind data.

I would like to know if there is comparison study API 650 vs ASCE 7-22 which shows API 650 less safe.