API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
(OP)
Hi,
I have a steel tank with diameter of 16' and height of 32'. The base is flat and anchored to the slab with 16 anchors (1.25" dia = 0.104 ft). The weight of the tank without any content is 5000 lb.
I'm not sure how to use the equation in API-650 (Section 3.11.3) Where the tension per anchor is given by: tB=4M/dN-W/N
if I have 20 psf of wind pressure, the total effective surface (ft^2) of the tank is 2*pi*(16/2)*32=1608.5 ft^2
The moment is M=(20*1608.5)*32/2=25.74 kips*ft
tB=(4*25736)/(0.104*16)-5000/16=61.55 kips per anchor
The tension force in the anchor is excessive, I really don't think the value is O.K. What I'm doing wrong ?
Thank you,
Gasma1975
I have a steel tank with diameter of 16' and height of 32'. The base is flat and anchored to the slab with 16 anchors (1.25" dia = 0.104 ft). The weight of the tank without any content is 5000 lb.
I'm not sure how to use the equation in API-650 (Section 3.11.3) Where the tension per anchor is given by: tB=4M/dN-W/N
if I have 20 psf of wind pressure, the total effective surface (ft^2) of the tank is 2*pi*(16/2)*32=1608.5 ft^2
The moment is M=(20*1608.5)*32/2=25.74 kips*ft
tB=(4*25736)/(0.104*16)-5000/16=61.55 kips per anchor
The tension force in the anchor is excessive, I really don't think the value is O.K. What I'm doing wrong ?
Thank you,
Gasma1975






RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
You have a couple big issues with which to contend. First, your copy of the standard is not current. The current section of the standard relating to wind overturning is 5.11.3. Second, 5.12.4 requires that the stress in the anchorage be determined using the root area of the bolt as opposed to the tensile area.
If you are able, obtain a current copy of the standard and check the anchorage loading conditions. It has seen significant changes in recent years and may clear up many of your issues.
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
BR,
Patrick
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
Overturning Moment = projected area * psf
Righting Moment = Weight*Deadload Factor*(0.5*Base)
Tension=(Overturning Moment - Righting Moment)/Base
(81920ft*lbf - 24000ft*lbf)/16ft ~= 3620lbf
Tension per Anchor = Tension/Number of Anchors
3620lbf/8= 453 lbf tension (laughably small with a 1-1/4" anchor)
You can get into bolt inset etc, but I doubt that number will get bigger than 500lbf tension per bolt, limit your Demand Capacity Ratio of the anchor to 0.8 and you'll be safe as you can be.
Keep in mind: ESR supported 1-1/4" anchors do not exist for seismic/wind loading. If it's just wind, don't sweat it. If you are looking at both types of loading I would reduce to 13/16" hole so you can use a stainless steel mechanical anchor (Hilti KB TZ, expensive, but easy to find). Loading is well within acceptable means.
Also, seismic lateral loading probably will govern if the tank is full (no overturning, but, depending on where you are, shear can be massive). 10240 lateral from wind is great and all, but a lateral G of 50% on a 40000lb tank will win (if you're in Cali, Kentucky, Missouri...).
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
The process that you outlined in your post is just right. But, I think that you left off the lever arm in your Overturning Moment.
Regards
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
Thank you for that and nice catch.
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
1. As has been mentioned already, use the projected area for wind loads.
2. I personally never use the weight of the liquid to resist the overturning moment. This is a suggestion out of "The Structural Engineering Handbook" by Gaylord and Gaylord. They mention that in order for the liquid to weigh down the tank it must rely on bending action in the relatively flexible tank floor plate. Usually the anchor will see the full tensile load long before the floor plate deflects enough to provide the resisting load.
3. The API 650 equation you mentioned above gets you close enough, but it does not properly take into account the effect of the "compression zone". If you really want to get accurate, "Pressure Vessel Design Handbook" as well as "Tubular Steel Structures - Theory and Design" by M.S. Troitsky have better equations that take this effect into account. You can get the latter for a cheap price from the Lincoln Arc Welding Foundation's website (I think it's only like $30). It's old, but it's a good reference for anyone doing pressure vessels (although it's riddled with math errors and typos - watch out).
4. Even low seismic coefficients often govern over wind, due to the huge mass of these tanks.
5. New seismic provisions for tank anchorage rolled out in ASCE 7-10 (Section 15.7.5). You are now required to use anchor chairs and a 8 bolt diameter stretch length on the bolts if you're in Seismic Design Category C or higher. If you're using ASCE 7-05 you can still use the overstrength factor instead.
6. Make sure you're using the 0.6D + 1.0W (for ASCE 7-05) load combination to check uplift (or the appropriate seismic combination if seismic controls the anchorage design). These load combinations check overturning for you. There's no need to compare an overturning moment to a righting moment.
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)
Per the current API-650, some allowance is made for liquid weight in evaluating wind overturning, and it is always included in seismic overturning, but it is not included in the weights used for sizing anchor bolts.
RE: API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)