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

How the effective depth is calculated in beam design and what is the impact of “maximum Aggregate 2 Size” on effective depth?

Reply:

RCDC assumes the number of rebars layers for the effective depth calculation based on the following,

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:

D < 400 mm                                                                   1 layer

400 mm >=   D <=1200 mm                                        2 layers

1200mm > D <= 1350 mm                                          3 layers

1350 mm > D <= 1800 mm                                        4 layers

1800 mm > D <= 2250 mm                                        5 layers

2250 mm > D <= 2700 mm                                        6 layers

2700 mm > D <= 3250 mm                                        7 layers

D > 3250 mm                                                               8 layers

Effective depth is calculated as follows,

Example-1

Beam Depth (D)                               = 600 mm

Clear cover (Cc)                               = 25 mm (this includes diameter of link/stirrups/fitment)

Maximum Aggregate 2 Size (M)  = 25 mm

 Assumed Diameter (d1)                 = 20 mm

Number of layers                            = 2 Nos (for 400 mm >=  D <=1200 mm)

Space between two rebars (S)         = if  -- d1 > 2 * M / 3

                                                        Then  d1 is considered

                                                        Else, 2 * M / 3  is considered

In the example considered, 

               20 > 2*25/3

               20 > 16.67mm

Thus, spacing (S) between rebars is 20mm.

& 

Effective depth = D – Cc - d1 – S/2

                              = 600 – 25 – 20 – 20/2

                              = 545 mm

 

Example-2

Beam Depth (D)                               = 600 mm

Clear cover (Cc)                               = 25 mm (this includes diameter of link/stirrups/fitment)

Maximum Aggregate 2 Size (M)  = 40 mm

 

Assumed Diameter (d1)                 = 20 mm

Number of layers                            = 2 Nos.

Space between two rebars (S)          = if  --- d1 < 2 * M / 3

                                                        Then  d1

                                                        Else, 2 * M / 3            

In the example considered, 

               20 < 2*40/3

               20 < 26.67mm

Thus, space (S) between rebars is 26.67 mm

 &

Effective depth = D – Cc - d1 – S/2

                              = 600 – 25 – 20 – 26.67/2

                              = 541.665 mm

 

Thus, if user expects clear spacing between rebars to be 25mm, it is recommended user to provide value of Maximum Aggregate 2 Size (M) equal to 37.5mm. (2 x 37.5 / 3 = 25mm)

You may also refer the attached excel sheet for quick check.

communities.bentley.com/.../Deff-and-Ast-calculation.xlsx