Applies To
	Product(s):	STAAD Advanced Concrete Design
	Version(s):	Connect Edition
	Environment:
	Area:	Concrete Design
	Subarea:	Footing Design
	Original Author:	Suvadeep Acharjee, Bentley Technical Support Group

In RCDC, the minimum rebar’s in Bottom are calculation based on effective Depth of the Foundation (Dff),whereas as per Clause 26.5.2.1 the minimum %Pt  is 0.12 of the Gross cross section area. Ast (minimum) as per IS456-200 Clause 26.5.2.1 is 0.12% of the Gross Cross section area.Therefore,  for  F4 1800 x 1800 x 500 thk. : Ast mini = (0.12/100) x 500 x 1800 = 1080 Sqmm. RCDC Calculations. Area calculation based on Deff. { (0.12/100) x 411 x 1800 = 887.76 Sqmm }. Actually we end up providing less %Pt than required. < 1080 sqmm actual requirement vs 888 sqmm Provided>

• Please refer below snap extracted from IS 456,

                        

Minimum reinforcement is for the total cross section area, thus 0.12% steel is to be provided at top and bottom.

In your case, top reinforcement is selected and below is the reinforcement given,

The minimum bottom reinforcement is given as 0.12% and top as 0.06%. Thus In your case, the RCDC will check the total reinforcement to be provided at a given cross section area as 0.12%. here, 0.06% is already provided at top, thus at bottom it will provided as per the 0.12% given by you with deff. Refer below snap of calculation  report.

Now, don’t select the top reinforcement and maintain the same reinforcement, below is the output

Reinforcement is provided indirectly as 0.12% with total depth. 0.12 x 500 x 1800/100 = 1080 sqmm.  If we take Deff, then 0.141 x 427 x 1800/100 = 1083.72 sqmm. So the final reinforcement required is 1080 sqmm.

Here, the required pt has increased to match with the 0.12% with total D.

If you want to reduce the Bottom reinforcement, please provide bottom reinforcement as 0.06%