RISA 3D Model Q 2

[FiniteElemet]

I am analyzing a circular slab in RISA 3D. Plates are of different sizes. Am I right to say that my reinforcement will be designed for max moment / ft from the results. How will I apply the results of Qx and Qy from the output to design for shear. Say my Qx and Qy are -100 kips and -103 kips respectively. Also, looking for some help on using Fx and Fy from output. I know the Plate Forces outputs are in per ft. I have almost three thousand plates and looking for a fast way to come up with reinforcement instead of going into individual plates.

Thanks,

 

[JoshPlumSE]

Plate force results are a good bit more complicated than beam results. So, expect to experience a bit of a learning curve as you go through this the first time. Some comments:

1) FEM analysis results will tend to show localized stress risers at points of discontinuity (support conditions and points of applied load). For this reason designing to the maximum moment per foot may be overkill. I tend to use a variation of the ACI strip method. By that I mean that I look at a representative strip width and use the average design force over that width.

2) For moment design or flexure, don't forget about the Mxy moment. In many cases, this moment will be very low compared to Mx and My. If not, then you need to add or subtract the absolution value of Mxy from the Mx and the My moment to get a total design moment for each direction.

3) Pay attention to sign convention. Plate moments do not follow the right hand rule. Therefore, an Mx moment is not a moment about the local x axis. Instead it is a moment which procduces a stress in the local x direction....which ends up being a moment about the local y axis. This trips up a lot of people when they are first working with plate elements.

4) Qx and Qy are out of plane shears and are generally given on a per foot basis. Similar to item #1, I will tend to use a representative width for designing the shear reinforcement or doing a shear code check for the slab.

 

[FiniteElemet]

Thanks Josh for taking time to respond to the question. I have pretty good grasp of underlying program requirements such as explained in 2) and 3). I am having difficulty selecting representative width or for that matter how to select that within the model screen?

Thanks

 

[JoshPlumSE]

There is a lot of engineering judgment associated with setting the representative width of your design strip. If the width is set too large, then you are averaging out the moments and shears over too wide of a region. This would result in unconservative design moments and shears. Similarly, if the strip is set too small, then the effect of stress risers in the FEM analysis will be over estimated and the design will be over conservative.

The setting of the design strip widths is truly a matter of engineering judgment. I can make no endorsement on what methods would be most appropriate. However, I have assembled a brief summary of the methods that I have seen used. To some extent, the below discussion is geared towards elevated slabs. But, it should still give you some good guidance on your slab.

ACI Definition of Strips (Section 13.2 of ACI 318-11)
This section of the ACI code is really intended for elevated slabs. But, the concepts can be extended into mat foundations as well. The requirement for "column strips" is that the width on each side should be set to 25% of the span length or width whichever is smaller. Then the "middle strip" is defined to span between the edges of the column strips.

This method works best when you have clearly defined (and rectangular) column grids. But, it requires engineering judgment for column grids that are not perfectly aligned and rectangular.

The ACI strip method listed above is based on essentially 1/2 of the midspan tributary lines. The hand calculation methods would have you design for the full tributary moments over this smaller width which should be conservative.

Zero Shear Transfer Method
The Zero Shear Transfer method uses the shear force contours perpendicular to the span of the slab to set the design width. This should provide a result very similar to using the mid-span tributary lines, but is a bit more theoretically derived for non-rectangular column layouts. This method is described in greater detail in the PTI publication Design Fundamentals of Post-Tensioned Concrete Floors.

Zero Moment Method
In a similar fashion to the zero shear transfer method, the Zero Moment method uses the moment contours to identify where the moment changes sign. This can be used to set the design strip width approximately equal to the distance between zero moments.

Shear Perimeter Method
Another basis would be to set the design width equal to the pedestal width plus a distance 'd' or 'd/2' on each side. This will end up being a more conservative assumption for flexure than the other methods listed. As such, it would be more appropriate for situations where shear or punching shear failures are a primary concern. Examples would also include cases where the pedestal is very large such as for a vertical vessel or grain silo.

Hybrid Method / Engineering Judgment
A variation on these methods would be to start off setting the column strip using the ACI strip method. Then, if necessary, the width could be modified based on considering the other methods...especially for situations where the column grid is not aligned or rectangular.

In addition, when the the middle strip widths get too large they could be set to a values closer to the column strip width. The middle strip would normally be centered on the area with the highest midspan moments. This would neglect lower moment regions between the column and middle strips. Hence the strips would designed for a higher moment per unit width. This reinforcement could then be extended somewhat into the lower moment regions between strips. Either that, or the user could set up another design strip for these lower moment regions.

 

[FiniteElemet]

Josh,
Will that be correct to say that for reinforcement calculation within slab model, design for the maximum moment/ft along a selected design strip. Check shears, moment and axial capacities along this longitudinal strip in given x or z direction.
For stress riser locations, check punching shear. I was getting mixed up with multiplying per ft moment with the applicable plate dimension and figuring out reinforcement that way.