FEM Flat Slab Design 4

[Bubik]

Hi everyone

I am really stuck with the design of a 350 mm thick flat slab (no column drops). The structure is a podium supporting five floors above. As the structure has an irregular column layout, line loads and point loads located in random areas I intend to design it with FEM. I previously did some design of a flat slab but with a regular column layout using FEM. I then used the column/middle panel strip method to average peak moments over the columns so I could specify the reinforcement. In this case however I just don’t know how to apply the strip method due to the irregularity of the column layout as I don’t know how to define a panel width. Maybe you guys know some other method?

By the way I am using Tekla Structural Designer software and below there is a plan view of the podium.

 

[Trenno]

350mm seems slim for punching shear of the columns above/below the transfer slab. Have you checked this?

 

[Bubik]

Trenno

You are probably right. We are to use punching shear reinforcement if required ...it is a bit political as the client doesn't want anything above 350 mm. In short, the project is a torture.

 

[Trenno]

It may even exceed code maximum stress rendering shear stud rails unfeasible. I would definitely check this first, as this usually governs transfer flat slabs.

 

[Bubik]

Trenno,

Thank you

 

[JAE]

Three reasons column and middle strips are typically used:
1. They mimic the difference in general magnitudes of bending and shear between the two
2. They allow a fairly simple way of concentrating more reinforcement where it's needed
3. They allow a fairly simple way of communicating to a contractor how to place the reinforcement.

For an irregular support and load layouts, the value in using column and middle strips, to me, sort of goes away.

Your FEM analysis should allow you to place the reinforcement exactly where it is needed. But this may be a fairly complex layout.
You just have to find patterns, and layouts, that work and yet are kept as simple as possible.
In some cases you may find that the overall slab simply needs a common, uniform distributed area of steel everywhere and you end up with a simple mat of bars.

 

[WARose]

The structure is a podium supporting five floors above. As the structure has an irregular column layout, line loads and point loads located in random areas I intend to design it with FEM. I previously did some design of a flat slab but with a regular column layout using FEM. I then used the column/middle panel strip method to average peak moments over the columns so I could specify the reinforcement. In this case however I just don’t know how to apply the strip method due to the irregularity of the column layout as I don’t know how to define a panel width. Maybe you guys know some other method?

I don't think any (code prescribed) "strip method" is applicable if you are using a FEA method to obtain forces. (That is, if I am understanding you correctly.) Any time I've used FEA for concrete slab design, the moments I've obtained from my analysis are what I've use to figure (flexural) steel.

 

[Bubik]

JAE,

It occurred to me that with such a layout it might be more practical just to use a blanket reinforcement, however some moments over the supports are very high due to the nature of FEM design( peak moments), this is to be avoided as far as I know and averaging such moments is required. Also there is no amount of reinforcement that could be used for the peak moments as the are of reinforcement required is literally off the scale. Second reason is that it is to be as economical as possible as the client is extremely difficult. Having said that I am wondering if there is any way to rationalize the design.

 

[Bubik]

WARose,

I am including a technical guide for this sort of thing

 

[Guest262653]

Regarding the FEM analysis and design of irregular flat slabs, try to get a copy of the Concrete Society's guide "TR64 - Guide to the design & construction of reinforced concrete flat slabs" - Link.

A 350mm thick transfer slab supporting 5 storeys? I can't see that happening. Depends of course on spans, top storeys' loads and columns location, etc., but it seems quite a stretch.

 

[Trenno]

Bubik,

With these transfer slabs, I've done them in the past with a modest typical mat of rebar (say 20mm bars at 200mm centres, each direction, each face) then add supplementary bars where required midpspan/supports for the peak moments. The peak moments integrated across a minimum width of column width + 2d.

 

[Bubik]

Trenno,

2d , you mean 2 times the depth of the slab? Did you average the peak moments?

 

[Bubik]

avscorreia

The structure above is a light , metal frame( a kind of modular thing). I got the T64 and as many other guides it mentioned irregular layouts but...not so specific.

 

[structSU10]

When I didnt have a robust two way slab program that handles the strip averaging for me, I would go in and select nodes across a reasonable strip, say 4*t on each side of a column or so, and then get the total moment across that strip. It is cumbersome, but it at least gives you something to go by.

 

[Bubik]

In my software, I can apply patches ( i.e. strips) over columns, panels, walls and beam. The patches can average the moments. The question is what size of the patches?

 

[JoshPlumSE]

Bubik -

Below is something that I wrote up during the development of RISAFoundation for users that ran into similar questions about what the widths of their strips should be:

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 Tech, Inc. makes no endorsement on what methods would be most appropriate.

ACI Definition of Strips (ACI 318-14 Section 8.4.1/ACI 318-11 Section 13.2)
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.​

 

[JAE]

Yes, to some extent you have to average out the peak moments from an FEA model. This takes some judgement, of course, as to how far you can go to average them out.

 

[molibden]

In Dlubal RFEM you can define average region above columns and reduce those peak negative moments. Size of the region is at least your column size. I use concrete module to design reinforcement on irregular slabs. I don't imagine doing it by hand.

 

[Bubik]

Great input guys

JoshPlumSE, great guide

Does anybody know any practical example by any chance?

 

[Trenno]

Trenno,

2d , you mean 2 times the depth of the slab? Did you average the peak moments?

2 * depth to tensile steel.

Yes, integrated is another way to say averaged in FEM terms.