Column and Middle strip design for flat slab

[drasticxxxx]

I found some comments for you and references regarding the above subjects,but the links not working now

So I need your help

What I have is the following:

Flat slab with beams here and there(but not fully beams)

I need to understand the best way to design column and middle strip for such slab ,and what are the best to pick the widths of strips.

Please provide example and references to follow

Any comments are welcomed

Note: I am using Safe Software

 

[drasticxxxx]

http://files.engineering.com/getfile.aspx?folder=4915e8f0-e136-4af5-aa24-062b02eeae4f&file=11111.PNG

 

[Guastavino]

JoshPlum works for RISA, so I'll speak for him (since he's very professional and doesn't market here) and say you should get RISA Floor ES and do all of your FEM slab designs with that. It even comes with semi-rigid diaphragm modeling. And, if you bundle it with RISA-3D you'll save!

In all seriousness, RISA is a great program, and RISA-floor ES was made for that. I'm not familiar with Safe, but if you watch the Youtube videos on RISA Floor ES, the same concepts likely apply.

 

[jayrod12]

Whichever code you are designing to (ACI, CSA, AU, Euro) will have some impact on the design methodology. Having a program is great. But if you can't understand the results you are getting (or check them by hand) then a program does more harm than good.

 

[drasticxxxx]

ACI code

 

[JoshPlumSE]

Programs like SAFE, RISAFloor ES, Adapt or such will handle a lot of the strip creation automatically. But, can (hopefully) be overriden by the user's engineering judgment. I can't speak to how the other programs work exactly, but RISA sets this based on ACI 318 requirements related to the "strip method". And, we force you to define "lines of support" that we use as the baxis of our strips.

For what it's worth, I'm not sure what thread you're thinking of exactly. Or, what sites I may have linked to that may no longer be valid. There is another thread in the RISAFloor forum where someone asked a similar question recently related specifically to the RISAFloor ES program. In this latest thread, I did re-post some text from the RISAFoundation help file concerning the setting of strip widths. This is more important in RISAFoundation because the program doesn't currently make any attempt to set strip widths. Therefore, we added this information to the help file because lots of people were asking similar questions about the subject. Here it is again:

Setting the Width of Design Strips
One of the most important design considerations is how wide to set the Design Strip. If the width is set too large, then the program will average 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. RISA Technologies, LLC makes no endorsement on what methods would be most appropriate.

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 requires engineering judgment for column grids that are not perfectly aligned and rectangular. In addition, when the column strip becomes very small then the middle strip may become very wide so that the entire slab is included in either a column strip or a middle strip.

The ACI strip method listed above is based on essentially 1/2 of the mid-span tributary lines. The hand calculation methods would have you design for the full tributary moments over this smaller width which should be conservative. Computer methods (like RISAFoundation) will design for the average moment over the assumed design width which should result in a more efficient design.

Zero Shear Transfer Method
The Zero Shear Transfer method used 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. Ideally, this method should give design strips of similar width to the ACI strip method. However, it is more rationally derived and should work better for cases where uneven column spacing makes the strip method difficult to apply.

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. This is similar, though not identical, to a method given in the NEHRP document GCR 12-917-22 (Seismic Design of Reinforced Concrete Mat Foundations).

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. This is especially true for situations where the column grid is not aligned or rectangular.

In addition, when the middle strip widths get too large, they could be set to values closer to the column strip width. The middle strip would normally be centered on the area with the highest mid-span 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 into the lower moment regions between strips. Or the user could set up another design strip for these lower moment regions.