Concrete Tank Moment Distribution 1

[jcali]

I'm designing an open-top rectangular concrete tank. The walls are fixed to a base slab and free at the top. I determined fixed end moments of the wall and slab using "Moody Tables". Now I want to perform a moment distribution to determine the final moment at the base slab/wall interface.

Question: How is the Distribution Factor determined for the wall with bottom fixed and top free?

-JCali

 

[aggman]

I am not sure exactly what the "Moody Tables" are but here is my approach to rectangular tanks. First have a copy of PCA (Rectangular Concrete Tanks) on hand. It has most of the relevant information you need to design tanks. If you don't want to get that book then you can take two or three approaches. One is to just design the walls as cantilever neglecting the fixity due to end walls. Second is to design the walls as two-way elastic plates using Roarks formulas or any other book which has solutions for elastic plates. Third you can design the walls as two-way strips utilizing a cantilever beam vertical and fixed beams horizontal. This will give you conservative results and require less reinforcement than designing as a cantilever. Best advice I can give is to just get the PCA book, as it will walk you through it.

I am not sure how you can do the moment distribution. I would assume that you are looking at a 1' wide strip around the top of the tank and trying to look at it that way. I guess that isn't the approach that I would take. I think that you will find for many walls that you can just design as a cantilever and move on.

 

[miecz]

I don't think you can do a moment distribution, as the supporting soil and outside soil defeat the assumptions.

For your base moment (and horizontal wall steel away from the corners), use figures 21, 22 and 18 for plates with a fixed base and hinged edges. You can't use fixed edges unless your tank is square.

For your corner steel, use figures 1, 4, and 5. Keep in mind that your corner steel has to resist tension in addition to bending.

 

[jcali]

Thanks! So, what table would you use to design the bottom slab? I could see using figure 34 for steel at corner, but what figure would you use for top steel away from the corner?

-JCali

 

[miecz]

You can use figure 34, with some adjusments. Depending on whether your tank is above ground or below, full or empty, deep or long and narrow, the adjustments may be additive.

Since the edges af the mat are connected to the walls, they will rotate with the base of the walls, and have the same moment. Figure 34 will probably give you different edge moments than your wall analysis. You'll have to adjust your mat analysis for the difference. As you can see, your analysis becomes pretty approximate, so be conservative.

Also, keep in mind that your mat may carry tension as well as bending as a result of the shear at the bottom of the wall.