Circular Tanks in RISA 3D vs Portland Cement Association Circular Concrete Tanks without Prestress 2

[ENGINEER92]

In our office we typically design concrete circular tanks using the Portland Cement Association Circular Concrete Tanks without prestressing book. This book provides different force coefficients for different situations of concrete tanks. I am wanting to use RISA 3D to get my ring tension force and flexure force in my tank. I have a few questions about the plate forces in RISA?

1. Currently all of my plates has their local x axis parallel to the ground and the local y axis perpendicular to the ground. So that means my fx+fxy force in my plates spreadsheet would represent the k/ft ring tension force in my tank. So to calculate the ring tension force I would would need the fx+fxy and the length between corner b and c in my plate. So the equation below would be my way of obtaining the ring tension force in my tank:

Fx(Lb-c)=T

Fx:fx+fxy (k/ft)
Lb-c: length between corner b and c in plate (ft)

And the equation below would be my way of obtaining the moment I have to resist with vertical reinforcement.

My(Ld-c)=M

My=my+mxy (k-ft)
Ld-c=length between corner d and c in plate (ft)

2. Also when I compare my flexure forces and moment forces in RISA vs the forces I would get in the Portland Cement Book, I am around 15%-30% less in RISA. Should I be concerned that I am doing something wrong in RISA or is the book most likely a conservative approach to get the loads.

Any help on this would be greatly appreciative and thank you for any response.

 

[JoshPlumSE]

Some of my thoughts on the subject:
1) I'm not too familiar with the PCA publication. So, I can only assume what it contains.
2) PCA has been known for years (in PCA Mats and other places) to use a plate element based on a thin element formulation. One of our engineers wrote a paper on this a few years ago when we came out with RISAFoundation. See this link for the article. I believe that "Software A" in the article was (like PCA Mats) a software that doesn't not include shear deformation in the element formulation. Leading to lower (and in this case, more accurate) moments in RISA-3D.
3) The thinner the tank walls the less of a consideration this would be.... obviously. But, for a plate element that is as thick as it is wide, a difference of 5-10% is about what I would expect. Though I could probably manufacture cases where I could get a 20 or 30% difference.

4) I'm more familiar with the old Bureau of Reclamation technical publication for rectangular plates from the 1960s. As I have compared RISA's numbers to it on a couple of occasions (both when I was a design engineer and during my days as a tech support engineer).
5) That document uses a "finite difference" method, not a Finite Element Method.
6) They suggest an accuracy of +/- 5 percent. However, they don't say whether it was based on a thin or thick plate assumption. However, I suspect thin plate theory just based on the date of publication. If so, maybe add another 5 to 10% on top of the original 5% error. Therefore, numbers from RISA that are within 15% of the Bureau of Reclamation document would be common.... With RISA generally being considered the more accurate numbers.

Of course, all of this depends on proper modeling within the program.

I hope this helped....

 

[ENGINEER92]

Thank you for responding Josh. I found a mistake in my hand calculations following PCA book. My loads are now ending up within 5%-10% of what RISA is telling me.

If I am sizing my horizontal reinforcement based off of my fx forces and my vertical reinforcement based off my my moments. What value in RISA plates would represent the shear in my concrete?

 

[ENGINEER92]

RISA PCA Percent Difference
Tank Dia. (ft) Fx (k) My (k-ft) Fx (k) My (k-ft) Fx (%) My (%)
44' 30.1 10.8 30.86 14.99 2.46 27.95
70'-4" 36.5 16.1 37.37 21.52 2.33 25.19
95'-8" 41.5 27.7 42.9 36.34 3.26 23.78


I am designing three different tanks and if I am using RISA right and doing the correct procedure for PCA. My ring tension forces come out to be about within 4% of each other. My vertical moment is where I am having the greatest difference. I am getting an average difference of 25%.

 

[JoshPlumSE]

In plane shear would be Fxy... that's like a shear wall force. Out of plane shear forces are the Qx and Qy values.

Keep in mind that it's the lack of shear deformation in the plate element formulation that is the issue (or assumed issue) with the PCA element formulation. This actually leads to a more flexible model in RISA and likely lower moments. Especially as the thickness of the plates increases.