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Reinforced Concrete Footing Design using RCDC CONNECT Edition V9 Update 4

Swetha Ajith Mathew
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Dear Bentley,

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:-

SL.NO. ISSUES REMARKS
1 RCDC assumes the support reactions from STAAD CONNECT Edition to act at footing bottom instead of pedestal bottom. This issue leads to underestimate the forces.
2 RCDC assumes 16mm diameter of rebar for calculation of effective depth. This issue leads to a difference in effective depth calculation.
3 RCDC always by default provide top reinforcement in both directions as follows;
a) 50% of minimum reinforcement (i.e. 0.06%) for footing depths less than 1.0m
b) 100% of minimum reinforcement (i.e. 0.12%) for footing depths greater than 1.0m		 If design doesn't require taking care the overall section minimum rebar as per code, this will lead to conservative results.
4 Even when RCDC options ‘Use Gross SBC’ & ‘Consider overburden’ are selected, the software neglects downward weight calculation in design This issue leads to increase in design force & area of steel leading to uneconomical footing sizes.
5 Eventhough ‘Average Pressure’ input is selected for footing , RCDC considers maximum uniform pressure in both direction through out the footing instead average as per actual. This will lead to higher values of the design moment and shear at critical locations.
6 For isolated footing with depth greater than 1m, software does not automatically calculate the required side face reinforcement as per IS 456:2000 Clause 34.5 Side face rebar is required as per code

Kindly help me understand if the issues stated above are valid & if yes, when these issues can be resolved?

Regards,

Swetha

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  • 0 Offline

    Hi,

    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.

    • Ex: if user has provided depth of footing above which top steel to be provided then RCDC works on following cases,
           1 --- Actual footing depth is less than specified - top steel ignored
           2 --- Actual footing depth between specified by user and 1000mm – top steel would be calculated as per % steel defined in reinforcement setting
           3 --- Actual footing depth more than specified and 1000mm – top steel would be maximum of 360sqmm / m and % steel provided in reinforcement setting.

    This option is available in general setting.

    Further -- 

    Footing design for column in tension:

    • For columns with axial tension in sizing combinations, RCDC takes following steps. RCDC works out the optimum size of the footing along with overburden and surcharge to balance tensile force. Top steel shall be designed for load on footing. Two way and one way shear shall be performed at face of column. Footing design for column in tension is applicable only for pad footing. In any case if user has ignored top steel and footing is designed for column in tension, RCDC will provide the top steel for calculated bending moment.

    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 –

    1. User chooses ‘Yes’ to option – Consider Overburden – In this option we add load of soil and self-weight of footing
    2. User chooses ‘No’ to option – Consider Overburden – In this option we add self-weight of footing

     

    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

    1. Average Pressure – Pressure along the edges are averaged and used for the design of the footing section
    2. Maximum Pressure – The largest corner pressure is used to design the footing section
    3. Factored SBC – The footing section is designed for a factored SBC as upward pressure. The factor can be specified in the software.

                       

    Reply to Point 6 ---

    Face reinforcement will be provided only when the option to provide Face reinforcement in the General Settings is selected. 

Reply
  • 0 Offline

    Hi,

    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.

    • Ex: if user has provided depth of footing above which top steel to be provided then RCDC works on following cases,
           1 --- Actual footing depth is less than specified - top steel ignored
           2 --- Actual footing depth between specified by user and 1000mm – top steel would be calculated as per % steel defined in reinforcement setting
           3 --- Actual footing depth more than specified and 1000mm – top steel would be maximum of 360sqmm / m and % steel provided in reinforcement setting.

    This option is available in general setting.

    Further -- 

    Footing design for column in tension:

    • For columns with axial tension in sizing combinations, RCDC takes following steps. RCDC works out the optimum size of the footing along with overburden and surcharge to balance tensile force. Top steel shall be designed for load on footing. Two way and one way shear shall be performed at face of column. Footing design for column in tension is applicable only for pad footing. In any case if user has ignored top steel and footing is designed for column in tension, RCDC will provide the top steel for calculated bending moment.

    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 –

    1. User chooses ‘Yes’ to option – Consider Overburden – In this option we add load of soil and self-weight of footing
    2. User chooses ‘No’ to option – Consider Overburden – In this option we add self-weight of footing

     

    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

    1. Average Pressure – Pressure along the edges are averaged and used for the design of the footing section
    2. Maximum Pressure – The largest corner pressure is used to design the footing section
    3. Factored SBC – The footing section is designed for a factored SBC as upward pressure. The factor can be specified in the software.

                       

    Reply to Point 6 ---

    Face reinforcement will be provided only when the option to provide Face reinforcement in the General Settings is selected. 

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