for the staad foundation advanced, under the Slab Designer and you click the 'Design', a table of Moments and Rebar Area Required appears. if it says 'Failed' in Area required, do I need to increase the thickness? Kindly elaborate more on this. Thanks
In these kind of situation increasing depth will help you.
thanks for the response. but if I use the moments I can still get the bar spacing by just increasing the bar diameter. that's why I wonder why there is a need to increase the footing depth....
Also, I noticed that the Rebar Area Required in the next column is quite big. How is it computed?
When you generate a moment envelope (default 60 X 60), the program is creating a grid of points on the surface of the mat. It fetches the moments along the longitudinal and transverse directions for each of those points by scanning through the results of all the ultimate load cases that the mat has been analysed for.
Then, at each of these envelope points, a total of 4 moments are fetched, each of which is the maximum value for that location from amongst all the ultimate load cases.
1. for longitudinal direction, tension on top surface of the mat2. for longitudinal direction, tension on bottom surface of the mat3. for transverse direction, tension on top surface of the mat4. for transverse direction, tension on bottom surface of the mat
Note that each of these moments is per unit width of the slab, 1 metre or 1 foot, depending on the design code, which is what a FE analysis of a mat gives you.
Thus, for the purpose of flexure design, at each envelope point, the slab is treated as a beam having a unit width.
Next, the program calculates the maximum permissible moment on the cross section based on compression failure of concrete for the thickness at that location. So, if any of these 4 moments exceeds the maximum moment capacity of the section, then, that moment cannot be designed for, and that is when you get a fail in the design status report. Envelope points that fall directly under a column or a pedestal are not designed.
The moment capacity is based on the crushing of concrete, which if I am not mistaken is also associated with the maximum area of steel that the code permits for that thickness. So, changing the reinforcement arrangement isn't going to help. The only way to resolve it would be to increase the slab thickness. You may not need to increase it all over the slab, just find the areas of high moment and increase the thickness locally by creating regions of higher thickness in those areas.
Regarding your question on why such large steel areas are shown in the screenshot you attached, we need your model. Also, let us know what build of the program you are checking it with.
thanks a lot for this explanation @Kris Sathia, just want also to know how the steel areas are calculated based from those moments? btw, how to send here my model?
Procedure for calculating the area of steel for the ACI code
Procedure to upload an SFA file:
Procedure to upload a file if you want to keep it private : Secure File Upload
mat foundation for checking 0.55 thk.sfa Thanks a lot @Kris Sithia. see attached file for your reference.....previously it was .5m thickness but Rebar area failed in bottom reinforcements so I adjusted to .55m
It is found that in latest released SFA version 9.6.1.74 the attached file is being designed successfully with 0.5 m thickness without any failure. Please let us know the SFA version number you are using.
thanks for the reply @Souvik Munshi.
Really? working with 9.6? I'm currently using version 9.3. May I know the moments that you got with 0.55m and 0.5m thickness? just want to know the difference between the two versions. TIA. This is wat I got with 0.5m thkness.
Here are a few suggestions.
1) Edit the job and remove the primary load cases 1, 2 and 3 from the Selected Load cases list. Thus, the analysis and design should be just for cases 101 to 104, 201 and 202.
2) The screenshot of the "Required Reinforcement Summary" that you attached earlier indicates that 3 of the 4 locations - Long Bot, Trans Top and Trans Bot - are within the perimeter of the columns. The program is supposed to ignore points beneath columns but versions prior to 9.6 had a defect that caused such points to be designed. The "What's New" section for version 9.6 mentions this.
Upgrading to 9.6.1.74 is the logical answer. But if you cannot do that, try the following:
In the Meshing Setup page, there is an option known as "Auto Generate Control Regions for Columns". It will create regions each having the same dimension as the perimeter of the columns.
Select each of them one at a time, edit, and switch off the option called "Design Control Regions".
Re-mesh the mat. Re-analyze. Then perform the design again. It should ignore the points beneath the columns and the 0.5m thickness should pass.