Bin design with ACI 313-97 1

[mrMikee]

Does anyone have experience with ACI 313-97 and designing steel bins?

I am specifically concerned with calculating loads on bins and the use of the overpressure factor in section 4.4.2.2. This section specifies the overpressure factor to be a minimum of 1.5 unless a lower factor can be shown as satisfactory, but no less than 1.35. An older version of the spec ACI 313-77 had factors up to 2.0 depending on the zone. It seems to me that many bins have been sucessfully designed in the past just using filling pressures per the Janssen equation, but on the other hand, published literature does clearly indicate that pressures during emptying are greater. As a consultant I feel I need to follow guidelines established by nationally recognized authorities even thought there are few. The only USA one I know of is ACI 313.

I am doing an investigation now and will begin writing a program to do these calcs when I can determine the best way to proceed.

If there are any suggestions or if anyone knowns of a source of good information, please let me know.

Any help will be appreciated.

Thanks,
-Mike

 

[tincan]

Try "Useful Information on the Design of Steel Bins and Silos" published by AISC and Steel Plate Fabricators Assn.

Best, Tican

 

[mrMikee]

tincan,

I did a search for that title and it came up with John R. Buzek as the author. I think I saw this years ago and it was a good reference. It's one of the books I had forgotten about, or at least couldn't remember enough to be able to find it again.

Thanks for the help.

-Mike

 

[aggman]

I have used ACI 313-97 quite a bit. I have also used Buzek's book, Troisky's book and then of course the "Bible" Gaylords "Design of Steel Bins for the Storage of Bulk Solids." By far the Gaylord book is the best reference. I think they difference is the horizontal pressure coeficients. Gaylord, and I believe the old ACI 313-77 calculated the lateral pressures based on active pressures with the charted overpressure coeficients. ACI 313-97 utilizes at rest pressures (larger). I pretty much use the at rest pressure with the 1.5 factor with good results and I think most people are going this route now. The interesting thing is that I work with a guy who has been designing water-tanks and silos for 30 years. He uses the Gaylord's infomation. (Unwillingness to change what has worked forever.) My designs are always either comparable or slightly conservative to him. I can live with that.

 

[mrMikee]

aggman,

I also consider the Gaylord and Gaylord the best book I have seen on the subject. Since I have lost my ACI 313-77 (it's been about 15 years since I designed bins) I haven't been able to do comparisons and didn't realize the the active/passive pressure difference in the specs. When I first saw the 77 spec I was not comfortable with increasing the designs by a factor of 2. The current 1.5 (or 1.35) however seem reasonable. It's possible that many of the 'old' designs exist with a reduced factor of safety.

Thanks for your comments.

-Mike

 

[aggman]

Very true about the older designs. 85% of my designs are for aggregate and sand bins so I feel like I have a pretty good grasp of the loads based on experience. When I get into other types of bins I like to have testing done to verify the lateral pressures.

 

[mrMikee]

aggman,

You mention that you work with someone who has been doing this for years that uses Gaylords information and is unwilling to change. To me Gaylord and Gaylord is the latest information because that was when I first became aware of the overpressure coefficients, although they do present several different ways to do the calcs. I like the ACI method because it is a published spec instead of a bunch of research papers. Prior to that the people I knew were using equations published by Ketchum about a hundred years ago based on Janssen's work.

Did I misunderstand?

Regards,
-Mike

 

[aggman]

mrMikee,
He uses the same Janssen equations that I utilize with the ACI 313-97. The difference is that he typically calculates the lateral pressures utilizing "Active" pressures (1-sin(theta) / 1+sin(theta)) verses I use the at "Rest" equations (1-sin(phi)) where phi is the angle of internal friction and theta is the angle of repose. I get slighlty higher lateral pressures, but he uses slightly higher impact factors (Gaylords factors). It seems to work out fairly close. His arguement is that he is confortable with his designs based on his experience and therefore sees no reason to utilize a seemingly more conservative approach.

 

[mrMikee]

aggman,

Ok, now I understand, and I wee why you characterize this as an older approach.

I think I'm with you on this in terms of the K factor and overpressure of 1.5 but the two methods are close enough that it probably doesn't matter. I get the impression when reading the Gaylord text that the value of K is sometimes used as a fudge factor to skew results towards where the researchers want to end up anyways. But that's just a guess. What I want is 'good' final answer. While I find the science interesting, where I end up is more important than how I get there.

Thanks,
-Mike

 

[JStephen]

What I recall from using the Gaylord book is that they give several different equations and factors, and then say "You can use this one, but some people use this one and this third method seems to work okay too". The point to be learned is that there is not just a wide variation in the methods used, but in the loads themselves.

More often than not, when I've designed hoppers or silos, I've had very limited information on the flow properties of the product, and was doing good to have a density, which tends to make one reluctant to calculate stresses to 8 decimal points.

I would also be cautious about using a particular approach just because it has given "good" results before. The structure got built, it didn't fall down, but does anyone really know how safe it is? You could go on for years building structures that were seriously overstressed, until finally one collapses.

 

[mrMikee]

It looks to me at this point that I will be using a combination of methods depending on if it's a shallow bunker or deep bin, filling or emptying, funnel-flow or mass-flow, or a vertical wall or hopper. Based on test results shown in Gaylord the ACI equations are as good as any and it is an actual written specification. For hoppers both the Jenike and Walker equations are probably satisfactory.

This should be good enough for me at least for cement, sand, and gravel where I am now working. There are many other materials that can cause unusual and serious functional and loading problems, so I agree that caution is certainly warranted when designing a bin. I've come to the conclusion that many of the homemade bins and the bins produced by small inexperienced fab shops have less than the desired factor of safety. So be it. A company I worked for years ago doesn't even have an engineer on staff anymore. You can make more money that way.

For those so inclined the Jenike and Johanson site at

http://www.jenike.com

has some interesting reading in the 'papers' and 'cases studies' sections under the educational offerings page. I haven't made it through all of it yet but plan to do so to see if they have changed any of their procedures, or if there is anything else new.

Thanks for the input,

-Mike