PCA tank design tables

[LOKSTR]

Does anybody knows why in PCA tables for rectangular tank design example,the calculation do not take into account Mxy( twisting) moment in addition to Mx moment (wood Armer equations)while calculating final design moment/coeff. for vertical steel?
Same is true for horz. steel.

The only place Mxy moments are used is while designing top "simply supported" slab.

The tank tables in section 3 which include effect of wall/frame stiffness for rectangular configuration do include table for Mxy values.

 

[FEM4Structures]

Even though design example uses Mx or My, I would use controlling of Mux or Muxy and Muy or Muxy specially for thin plates where the forces are high. My logic is at given section, conventional design suggest using uni-directional forces, however in reality the forces are acting in two directions such as in x and y directions. To account for two directional forces at any given point in concrete section, you take resultant of moments in both directions. Muxy coefficients will account for resultant moments when multiplied by q*a*a / 1000.


Thanks,

FEM4Structures

 

[JAE]

FEM4Structures - I think there is a misunderstanding here. LOKSTR is talking about concrete design - where you specifically detail/design reinforcement in orthogonal directions.

Adding Mx and My in concrete design - or using their resultant - doesn't make any sense.

 

[hokie66]

JAE,
I think LOKSTR is talking about the twisting or torsional moments that are encountered at the corners and other supports of slabs (and walls). Those moments do have to be added in some way to the Mx and My moments, and there are various methods of doing this. Using computers, some programs do it automatically, some make it an option, and some ignore the twisting, which is not correct.

There are a variety of threads on the site about this topic, mostly for flat slabs and flat plates.

 

[JAE]

hokie66 - yes typically we use Mx + Mxy and My + Mxy.

I was responding to the idea of using the resultant of Mx and My in design of concrete - which we at least never do.

 

[LOKSTR]

JAE/hokie66,

Thanks. I am using Mx+Mxy and My+Mxy in my spreadsheets using the values taken from PCA table for particular b/a and c/a ratio,
but I am not sure why in the example given in "PCA rectangular Concrete tanks 5th edition" the author do not show the addition of Mx and Mxy in the sample calculation for vertical and horizontal steel for long side and short side of tank.

In the example he ignored Mxy totally.

 

[Splitrings]

On page 1-4, it states, "These moments should be considered for safe design wherever their effect is deemed to adversely affect the steel requirement". As you mention they only use them in the example of the two way slab where they are significant in comparison to the principle moments. Its up to you to determine when to included the orthogonal moments. In the tank walls these orthogonal moments are much less significant when compared to the principle moments. The author chose to consider them insignificant.