We have been trying to validate RC footing design as per IS 456:2000 using RCDC to use in a MEGA project.
While checking we found following issues:-
Kindly help me understand if the issues stated above are valid & if yes, when these issues can be resolved?
We will respond to your queries soon.
Refer to our reply below for each point ---
Reply to Point 1 ---
RCDC reads the Member end forces of the columns from STAAD. While reading the member end force, the node that is attached with the Support, the force from that location will be considered for the Footing design. Eg. Referring to Column C1, the node 1 of column is attached with the support, hence RCDC will consider the forces from Location at node 1 for footing design. Below image for reference --
If you are facing any major difference for any / multiple columns, you can share the STAAD file with us so that we can help you to match the forces in RCDC-STAAD Model.
Reply to Point 2 ---
Currently RCDC assumes 16mm diameter for calculation for following reasons --
1. To begin with, RCDC starts with the minimum depth of the footing mentioned in the General Settings form. It goes on increasing the depth of the footing until the one way shear and punching shear requirement is satisfied.
2. The sizing of the Footing is finalized from the Pressure checks and Depth is finalized for Shear requirements.
3. While checking the depth requirement for one way shear at 'deff' from face of column and punching shear at 'deff / 2' from face of column, the 'deff' needs to be determined by assuming some diameter and here 16mm diameter is assumed.
The Flexure design and Shear design is performed considering this depth itself and in any case the consideration of this depth is further used for determining the Ast requirement and the final Ast provided is always + 2 to 5% higher than the As required.
If you need any detailed clarification / case where in you think that 'deff' calculation is hampering the Shear check, kindly let us know.
Reply to Point 3 ---
Top Steel in footing:
This option is available if user wants to provide the top reinforcement for thickness/depth more than that wanted by user.
This option is available in general setting.
Footing design for column in tension:
Reply to Point 4 ---
For sizing check of footings where maximum and minimum soil pressure (or loss of contact) is checked (and also other checks), we add the weight of footing and weight of soil. Following are possible cases –
When it comes to design of cross-section, the BM and shear is calculated using the pressure from bottom. In this case, the vertical load due to self-weight of footing and soil is uniformly distributed and it gets off-set by equivalent pressure from below. So net pressure causing the bending from bottom is the pressure due to reactions from super-structure load. In other words, if one considers pressure from bottom (including soil and self-weight) for calculation of BM and shear then, one will have to deduct the load from top (due to self-weight and soil) as it would oppose it. Hence in the design of cross-section, the weight of footing and soil have no impact.
Reply to Point 5 ---
Three philosophies for the design footing cross sections are supported
Reply to Point 6 ---
Face reinforcement will be provided only when the option to provide Face reinforcement in the General Settings is selected.