CSI SAFE FINITE ELEMENT DESIGN INTERPRETATION

[spatio99]

Hello Guys,

I'm finding it difficult interpreting the rebar intensity contour plots (direction 1 and 2) generated by CSI SAFE after finite element design of a slab. I'm getting intensities as high as 3500 mm2 per m at column joints. How does one provide reinforcement for a slab using the contour plots. I've attached some screenshots of the model for viewing(can provide more if needed). I designed the structure to BS8110. Any materials (PDFs, Videos, Links, Examples...) to help me understand the concept especially how to provide reinforcements from the contour plots would be greatly appreciated. I've already tried adding strips but the details generated for the slabs in the radial part of the structure is not concise. My plan is to get the reinforcement from the Finite Element Design and detail the slab with Tekla Structures because of the ease of modelling reinforcements in virtually any angle. 

Thanks for your anticipated responses.   

[WR1]

Well yes contours plots are more (comparatively) realistic than strip reinforcement and economical too. Think of it as you have reinforcement in the form of powder. You put the reinforcement powder (or form like that) on slab based on fea contours. Economical, realistic, interesting and easy. Isnt it?

Now unfortunately we dont have powder reinforcement (or atleast i dont know). And we are left with reinforcement bars. For that we need to have some assumptions; width of bar strip width assuming a single bar will cover the contoured reinforcement in a limited width. Complex right.

But you have the ability to draw strips in SAFE. But for that you have to decide things like;

Strip width

Orientation of strips

Layers of strips

Now that is simple for simple slabs and geometry but complex in your case. But there is no other solution. You gotta decide (based on experience, economy etc) how and at what angle you gonna lay your strips (like actual site conditions). Lets say you find out that 45 degree strips give you more economical reinforcement (probably because fea contours are 45 degrees w.r.t. principal axes of plan). So you on actual site you put the same layout plus any additional reinforcement given by SAFE.

So far now we dealt with orientation but what about strip width? Well thats variable practice. According to ACI you can have a very large strip like middle strip averaging all the fea contours in this width. Some people use 1m width (thats very conservative i think, happen a lot in middle east but not in Asia). Because practices worldwide differ due to different economies among other minor factors.

For point loads I would not go for a 1m strip. It depend on the location of point load and its spreadout relative to boundary conditions. Check with your industry practice about this.

For strip layers, SAFE give you total strip area. Its you who then decide (by changing covers) that how to split total reinforcement in how many layers.

Now things really get interesting if you have 12 top and 12 bottoms layers (so cover is huge factor now) with some layers orientation different than others plus extent of some layers is also variable. And I know how crazy this is to handle with SAFE.

But if you do have a better reinforcement software you could use that. So it all depend on your interprettation.

For the high values of concentrated reinforcement it all depend on how would you average out the contours. Is the concentration due to point load? Is it due to a corner, mesh, singularity in FEA solution?

[spatio99]

Thanks Rana Waseem, that is a brilliant response.

I'll remodel the strips with respect to the local axes I assigned to the slab panels to get proper mesh transitioning and re run the analysis, design and detailing. I'll share the results when I'm done. The only problem with the strips is that there is no flexibility in modelling the exact width needed as the start and end points only have (x inputs) would have been better if there was an option for (x,y inputs). More so the Auto Widen Strip function does not help much with strips at angles other than 0 or 90 degrees. I'll try out some configurations and post the results.

Back to the Finite Element Analysis Solution. In this day and age I seriously doubt the possibility of a Structural Engineer not coming across complex floor systems. Architects and Engineers alike can be very imaginative (we can't hide from it ). From The Concrete Centre publication "HOW TO DESIGN REINFORCED CONCRETE FLAT SLABS USING FINITE ELEMENT ANALYSIS by O Brooker" in pg.12 "Interpreting results: The results from an FE analysis will generally be in the form of contour plots of stresses and forces, although a ‘section’ through the contour plots (either bending moment or areas of steel) can usually be obtained. These will show very large peaks in bending moment at the supports. The temptation to provide reinforcement to resist this peak moment should be avoided. This potential error stems from a lack of understanding of the assumptions made in the modelling. The reinforcement in the concrete will yield at the support position and the moment will be distributed across a larger area; it is not therefore necessary to design to resist this peak moment. However, a method is required for distributing this peak moment across a larger area." I would like to share the PDF with you but I'm not sure about the forum rules concerning that. From the above statement, the large intensities I'm getting at the Beam/Column supports (joints) are to be expected but then it still takes me back to my initial question, "How do I use the plot to provide reinforcement for the slab?" More so, the concept of "AVERAGING" keeps popping with FEA Solutions. Let's say for instance we don't have the luxury of adding strips and the only option is to use FEA to design the slab, how would you do it ? If there is an approach on how to get this done it would be greatly appreciated.

Thanks for your anticipated response.

[spatio99]

I tried something else,

From The Concrete Centre publication "HOW TO DESIGN REINFORCED CONCRETE FLAT SLABS USING FINITE ELEMENT ANALYSIS by O Brooker" in pg.13 "An alternative method is to simply average the bending moment over a width of slab. However, if designing to Eurocode 2 the requirements of Cl.9.4.1(2) should be adopted. The widths of these strips can be determined by the designer; an example is shown by the green line in Figure 13. Here the same strip widths as the BS 8110 method have been adopted to show how the results compare. This method has the advantage that it can be used for a slab with irregular geometry, because a fixed bay width is not required. It can also be used with area of steel results, removing the need to calculate the reinforcement areas by hand. It will be seen that both methods give a similar distribution of reinforcement when applied to the same strip widths."

I used a Max. Average width of 1m and got the following results.....