Flat Slab Guidelines

The modelling, analysis and design of flat slab is described in detail here :

1. For ProtaStructure 2022/ ProtaStructure 2024: Flat Slab Modelling, Analysis and Design
   
2. For Flat Slab and Raft Design with Slab Patch Panel: Flat Slab and Raft Design with Slab Patch Panels 

The following are additional topics of interest related to flat slab.

Flat Slab Insertion methods

• For the flat slab, it is unlikely that your slab would be fully bounded/surrounded by beams, hence the more appropriate method would be “Axis Region” or “Pick Axis”.
• For Axis Region, you simply click on any point bounded by axis and a slab will be automatically with those axes as insertion axis. You can combine them by holding down the CTRL key while clicking successively locations and a single slab panel will be created after you release the key.
• For Pick Axis method, simply picking on the axes that bound the slab in any order (clockwise or anti-clockwise), starting from any axis and finally re-select the first axis you have shown to end the insertion action.  For more information, please refer to Working with Slabs - Pick Axis section.
• When inserting slabs for the flat slab, make sure that all columns and shear walls lie at the edges or corners of the slab.
  

In other words:

1. The slab boundary must coincide with the insertion axis of column/wall. For example, if the insertion axis of the column is A/1 then the slab boundary at the column must be either axis A or axis 1.

2. Try to keep your slab four-sided, i.e. rectangular or squarish.

3. For raft or flat slab, the slab type does not matter as it is only applicable for slabs designed using yield line, i.e. Analytical slab strip.

Flat Slab Reinforcement Design

• The preferred method to show reinforcement for the flat slab is to “User Defined Contours” or "Threshold Contour" as explained in the Flat Slab Training Manual and Finite Elements Floor Analysis
  
• Alternatively, you can try to show rebars on the model plan view, but it is important that you define and cut the correct Finite Element Slab Strip – for you model that would be “Fixed Width Band strip”. More details are available under Slab Strip Type (Working with Slab Strips and FE Strip-Fixed Band Strip sections)
  
• “Fixed band strip”, will take a single max positive and negative moment to design for the entire length and width of a strip. Hence you only get one top and bot reinforcement, with may lead to some over design if the width of the strip is too large. Hence, you need to examine or evaluate how wide and how many fixed width band strip you want to cut as that has an influence on how efficient the steel design is.
• In the FE tab of the FE slab strip, there is an option to use "integral".   As the name implies, the moments across the width of the strip is “averaged out” or “spread out” by integration so that a lower maximum moment is obtained. This assumes that the slab is sufficient stiff so that the load tend to “distribute itself” across a certain width. With “integral” unchecked, ProtaStructure scans for a single max moment to design across the span and support region.
  
The FE fixed band strip can only be used if cut across slabs with the same thickness.  The reason is the design will only detect the slab thickness nearest to the first end of the strip. If you have many slabs with different thicknesses, then do not use the FE Slab Strip.  Use the other method of design using the As required contours as outlined in the Flat Slab training manual
• Please be aware that cutting strips may not be able to give you a full solution, you will still need to combine or supplement it by investigating the steel contours.

In a nutshell, if you wish to use FE Slab Strip, the procedure is as follows:

a) Model/define slabs

b) Cut FE Strip across of the appropriate type (refer to more notes below)
      

Above : Select FE Strip and choose fixed band strip in Slab Strip properties. Then select the Analysis result source and check “Integral Strip”.

Above: Cut strip through the desire area. The scope of width can be adjusted by input value in corresponding cell and click "Update".

Above: Scope of width also can be adjusted using  and click "Update"

c) Run Building Analysis or FE Floor Analysis and open Analytical Model (Analysis tab > Analytical Model)

d) Without select any members, right click > Arrange All Steel Bars> Update All Steel Bars

a) If you have defined your slab edges correctly, you will notice that edges of shell elements coincide perfectly on the red dot represented by the column and shear walls.

b) The edges of shell element of one slab coincide perfectly with that of the adjacent slab.

Slab Design with Slab Patch Panels

1. (Optional Step) Perform an FE Floor Analysis or Building Analysis with slabs meshed and examine reinforcement contours to decide on the slab panel areas where you will put additional rebars. 
   
2. Insert FE Fixed Band Strips for based reinforcement calculation. Fixed band Strips allow you to decide and calculate the base reinforcement regions. Hence, it is important to first decide how many base reinforcement regions you will use. 
   
3. Insert Slab Patch Panels. Do not worry about the size of the patch panels yet. Ideally, they should cover the support moment contours. You can use an approximate size of 1.5x15 m or 2x2 m to start with. An FE Fixed Band strip should completely or partially cover slab patch panels in its tributary area. 
   
4. (Optional Step) View the FE Reinforcement Contours on the physical floor plan view (such as Asd1-top, Asd2-top for the upper floors, Asd1-bot, and Asd2-bot for raft foundations). This will help you understand whether the patch sizes are large enough to cover the negative support moment regions. 
   
5. Perform an FE Floor Analysis or Building Analysis with the slabs meshed. 
   
6. Design the slab reinforcements using Design > Slab ribbon menu. The designed rebars will also be automatically drawn on the physical floor plan view with the correct lap splices and extensions. The additional bars in the patch panels will be automatically extended outside the designated patch area by anchorage length.
   
7. (Optional Step) Modify the size of slab patch panels if you need to. The most usual scenario would be to extend the patch panel region so that it covers more negative support moment contours. Optimizing the path panel size can further reduce the base reinforcement (not less than the minimum required, Asmin)
   
8. (Optional Step) Redesign the reinforcement using Design > Slabs ribbon menu. 
   
9. (Optional Step) Modify the base reinforcement and/or additional reinforcements on the Design > Slabs ribbon menu if you need to. If the edited reinforcement is not sufficient, ProtaStructure will issue a warning.

Important Remark

Fixed band strips should completely or partially cover the slab patch panels. Otherwise, the Fixed Band strips will continue to collect bending moment results from column nodes, which may yield an uneconomical design. If you think a column support region does not have a significant negative moment, you have the freedom not to insert a patch panel for that column.

If you choose not to insert patch panels around a column, you must cover the column area with an FE Fixed Band. If you leave a particular column region uncovered by a Patch Panel or an FE Fixed Band Strip, the results in the column region will not be considered, and that particular column region will not be designed. You may end up with an unsafe design.

Column Punching Shear Design

• Finally, do not forget to check for punching shear. Try not to run all the columns punching shear shear check altogether, as you need to define the correct type of column location (interior, edge, column).
  
• To do this, you can select a group of columns having the same type location by successively selecting each column while holding down the ctrl key.
  
• Then right click and choose 'Column Punching Check'.
  
• Be reminded you must first run FE floor analysis and post processing, and click 'merge column results with building analysis results' so that slab loads based on FE analysis are transferred to the columns.
  
• Otherwise, column punching will be based on building analysis (i.e yield line slab analysis) rather the FE floor analysis.
  
• You should not perform punching shear check for the columns connected to beam as you will need to design the beam to take the shear.
  
• ProtaStructure does not take into account any beam connected to a column for punching shear check. 

Drop panels can be inserted added through the column properties > Drop tab.

The depth of the drop panel defined as h-Drop is inclusive of the slab thickness, NOT measured from soffit of the slab. (Example: slab is 150mm, drop panel is 200mm. Therefore the h-Drop is 350mm.)

Note that drop panels works best to improve punching shear capacity. They are less effective in reducing moment at the column as a thicker section may attract more moment, hence only model drop panel if punching shear is a problem.