- 1. How the effective self-weight of the footing is calculated?
- Effective self-weight of footing is calculated as additional load of footing volume with respect to difference in density of concrete and soil. For more information refer “Technical discussion” of footing from help Content.
- 2. For calculation of two way (punching) shear, footing self-weight is not added to axial load.
- Concept of Footing punching shear criteria is column punches in the footing. Load from the column punches the column in footing. Self-weight of footing, overburden load do not participate in punching of column as it is a uniformly distributed load, thus it is not considered in design. For more information refer “Technical discussion” of footing from help Content.
- 3. As per design, required reinforcement is 1478/1528 sqmm along L/B direction. Provided reinforcement is T12@175 mm which amounts to 1453 sqmm only for 2.25 m width of footing, whereas the calculation report shows Ast provided as 1583sqmm. Please clarify.
- Width = 2250 mm Ast required = 1478 sqmm Reinforcement provided = T12@175 mm In RCDC the number of bars required are calculated and spacing is calculated. While presenting the spacing rounded to 5 mm. In this example, number of bars as per spacing would be calculated as below – (2250 / 175 = 12.86 which is 13 spaces). Hence number of bars would be 14. Thus actual Ast-provided would be (14x113.097 = 1584) 1584 sqmm which is more than required.
- 4. Please clarify the footing type “on raft” given in RCDC.
- RCDC designs the isolated footings. The purpose of this options is to maintain the same thickness of first step for all footings if all footing are connected by common raft. This type of footing mostly use in the basement structures. The concept of this type of footing is similar to stepped footing.
- 5. How is the volume of trapezoidal footing calculated in RCDC?
- RCDC calculates the weight of trapezoidal footing as per following formulations, Self-wt. of footing Volume of footing = ((L*B*D) + ((((L*B) + (Lt*Bt))/2)*(D-d))/1000^3 Where, L= Length of footing B= Width of footing D= Depth of footing Lt= Length of footing top Bt= Width of footing top d= depth of sloping area.
- 6.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%
7. Can RCDC design the footings for If there is tension in the column due to lateral loads?
Yes, RCDC can design the footing if column having tensile forces. User can design the footing with or without Loss of Contact options if there is tension in column OR one of the corners in footing is in tension.
Without loss of contact, RCDC will try to maintain the +ve pressure at all four corners of footing by increasing the self-weight. It is recommended to consider overburden to reduce the footing size in this case.
With loss of contact, RCDC will try to maintain the LOC within permissible limits and it will design the footing.
It will also design and provide the top reinforcement if required in both the cases.
8.Can We design Eccentric Footing in RCDC?
Yes, RCDC can design eccentric footing. Refer below link for more details,
Eccentric Footing Design
https://communities.bentley.com/products/ram-staad/m/structural_analysis_and_design_gallery/274459
9. What is Loss of contact? How RCDC perform the design of footing if LOC is selected?
RCDC allow user to provide permissible limits for the loss of contact. If in any case due to tension in footing or heavy lateral moments, corners of the footings experience the -ve pressure. -ve pressure could be at corner or one of the sides of footing. i.e. two adjacent corners with -ve pressure. Technically, maximum 50% LOC is allowed. Thus, maximum limit in RCDC for LOC is 50%. User can provide the % LOC as per its design requirements. If there is LOC, top reinforcement would be design as per the bending moment at top face of footing due to loss of contact.
For more details, procedure and validation problems, please refer below link.
Footing design with LOC
https://communities.bentley.com/products/ram-staad/m/structural_analysis_and_design_gallery/274312
10.Can you explain the various options available for pressure calculation in RCDC footing design?
RCDC provides following options to calculate base pressure,
Average Pressure
Maximum Pressure
Factored SBC
Above options are explained below with example:
