RCDC Beam design - effective depth Calculation

Bentley Team,

How is the effective depth for beam being calculated for a beam with two or more numbers of layers in RCDC software?

Please provide the basis and validation for the same.

I could find some formula related to effective depth in FAQ in the link below, but that is not giving correct value for the design I have.


  • Hello,

    In RCDC the effective depth of a beam is calculated in the fllowing manner.
    If D is the total depth and d is the effective depth, then
    d = D - Clear Cover - 16mm -12.5mm
    d= D- Clear Cover -28.5mm

    Here, clear cover thickness includes stirrup dia. For effective depth calculation it is assummed that 2 layers of reinforcement is present and effective depth is calculated from the top most fibre to the mid depth of spacer bar. Though it is conservative in some cases, but we are in safer side. Considering this fact, a value of 16mm is considered as the bar dia of bottom most layer reinforcement. Then 25mm spacer bar is considered for which we take half the depth in claculation as 25/2=12.5mm.

    After getting the d value it is again rounded down to nearest 5mm interval, which in turn make the aforementioned 28.5mm value, 30mm.

    Can you please check and let me know if this matches with your calculation?

    Thanks & Regards,
    Abhisek Mandal
    Global Technical Support

  • d = D - Clear Cover - 16mm -12.5mm
    d= D- Clear Cover -28.5mm

    Issue 1 : Does it take 16mm and 12.5mm (as quoted above) as the fixed values or these are based on User Input?

                   Also, referring to the link below, Q30 which says the assumed diameter of the bar, d1 as 20mm. how is it assumed.


    Issue 2 : Can you please tell how the effective depth is calculated for say 3 number of layers. 

    Issue 3 : Can you confirm, if RCDC does or does not calculate effective depth based on the rebars and layers actually provided? 

    Issue 4 : Also, at beam different locations (say top left and top right) different rebars in different layers may be provided, say at top left one layer is provided and at top right 2 layers are provided. But RCDC is giving only one effective depth for entire beam. How is that being considered?

  • Hi

    Request you to kindly note following:

    • All cover is assumed to be cover to longitudinal reinforcement.
    • Assumption of number of layers of reinforcement:

    At design time the number of layers of reinforcement is assumed as under to arrive at the effective depth, d:

    1. D < 400 mm                                                    1 layer
    2. 400 mm  >=   D <=1200 mm                           2 layers
    3. 1200mm  > D <= 1350 mm                            3 layers
    4. 1350 mm > D  <= 1800 mm                           4 layers
    5. 1800 mm > D <= 2250  mm                           5 layers
    6. 2250 mm > D <= 2700  mm                           6 layers
    7. 2700 mm > D <= 3250  mm                           7 layers
    8. D > 3250 mm                                                 8 layers
    • Detailing Philosophy:
    1. The reinforcement required is computed as per the effective depth for the number of layer of reinforcementas per table above.
    2. Rebars will be arranged in multiple layers if requiredand the style of detailing selected. The maximum number of layers usedwill be limited to thoseassumed when computing the effective depth.
    3. In case it is not possible to provide the required reinforcement using the largest rebar arranged in the maximum possible layers the number of layers will be automatically increased. With the increase in number of layers the effective depth and the reinforcement required will be computed again.
    4. When providing rebars across the section of the beam, the minimum and the maximum spacing criteria and the preferred number of rebars will be considered. The rebars wil be symmetrically arranged across the section.
    5. In case the number of bars are varying across the section, the auto detailing will ensure that the symmetry will be maintained.eg. 6 bars will reduce to 4 and not 5 bars OR 5 bars will reduce to 3 and 4 bars.
  • Hi,

    Agree of the part on how RCDC takes no. of layers into account during auto design. But what I observed is the following. Say my beam depth is 550mm as per which 2 layer will be assumed by RCDC. So effective depth is calculated based on 2 layers of rebar that is,

    d = 550 (overall depth) - 30 (clear cover) - 16 (bar dia) -12.5 (spacer bar dia/2) = 491.5 = 490 (rounded off)

    But as per design (auto-design), only 1 layer of reinforcement is provided consisting of 3 no. of 12 dia bars. But still effective depth corresponding to 2 layers is being used for design. 

    Also the usage of 16mm and 12.5mm is doubtful as provided is only 12mm bars. 

    If you can again refer to my previous reply and provide solutions issue-wise, it will be highly appreciated. Thank you.

  • Hi,

    It would be useful if we can have an example based in which we can take this further. 

    You can provide us with a live example which you are doing manually and we will help you with validation oh how RCDC is doing it. You can share your own manual calculation by taking an example into consideration.