API-650 - How to verify for anchorage tension in WIND LOAD ON TANKS (OVERTURNING STABILITY)

[gasma1975]

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

 

[JedClampett]

For one thing, your effective surface is wrong. You don't need the whole tank surface, just a projected area subject to wind. In your case, it's 16 x 32 or 512 square feet.

 

[structSU10]

the d is not the diameter of the bolts, its the moment arm between tension and compression bolts, and i would imagine you would you n = number of bolts in tension - which may be assumed at 1/2 of them. your moment value is way off too.

 

[JStephen]

d is the diameter of the bolt circle, n is the total number of bolts. The terms are defined in the standard, along with the units. If you don't have a copy of it, get one.

 

[fegenbush]

gasma1975,

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.

 

[gasma1975]

I just found the API-650 11th edition 2007 is it the latest ?

 

[dig1]

There are 3 Addendums/Erratas which can be purchased from a variety of sources. I have included a link to Techstreet and from IHS Global Engineering Documents (but there are others).

http://www.techstreet.com/cgi-bin/results?searchText=api 650

http://global.ihs.com/search_res.cf...d=2125462A530A&country_code=US&lang_code=ENGL

BR,

Patrick

 

[jmcoope3]

Quick and dirty:

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...).

 

[IFRs]

jmcoope3 has given an excellent answer. API 650 11th edition 2007 has been extensively revised, ADDENDUM 1: NOVEMBER 2008, ADDENDUM 2: NOVEMBER 2009, ADDENDUM 3: AUGUST 2011, ERRATA, OCTOBER 2011. The latest effective date is February 1, 2012

 

[msquared48]

Based on the curved shape of the tank with respect to the wind force, unless you have done so already, you can also lower the applied wind pressure by 30% or so if my memory serves me correctly.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[IFRs]

API 650 includes a shape factor in their calcs. Here is a snippet of the code: The design wind pressure shall be [18 lbf/ft2][V/120]^2 on vertical projected areas of cylindrical surfaces and [30 lbf/ft2][V/120]^2) uplift on horizontal projected areas of conical or doubly curved surfaces, where V is the 3-sec gust wind speed in MPH.

 

[OldPaperMaker]

jmcoope3,
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

 

[jmcoope3]

OldPaper, you're correct, forgot it in the OTM definition but i think i got it in the calculation.

Thank you for that and nice catch.

 

[fegenbush]

In order to use the formulae that IFRs posted, be sure that the wind speed is from the 2005 version of ASCE 7, not the 2010 version. They are different.

 

[DCBII]

I've done quite a few tanks. A few things to keep in mind for on anchoring them:

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.

 

[DCBII]

I've attached a spreadsheet I created that finds the anchor bolt load for circular bolt patterns. Inputs are blue. There are comments to help you use it. You'll need to enable macro's. Don't substitute this spreadsheet for good judgement... I've been known to make mistakes.

 

[JStephen]

Note that the current API-650 gives specific equations for calculating anchor bolt loads. It's difficult to use the ASCE load combinations as they don't include all the pertinent loads.

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.

 

[IFRs]

Note also that API 650 requires 30 psf uplift on the horizontal projected area - this can mean anchors are required when before they were not. For an API tank it is best to use API code - it's easy and lowers the risk.