All about civil construction knowledge- PARAM VISIONS

Calculating the volume & BBS of plinth beam in a compound wall. / Estimating & costing of a compound wall ( part 4 ).

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 Let us make a 9"×12" plinth beam as shown in the drawing.




Given data :
Plinth beam size 9"× 12",

Main bar dia = 12 mm.,   no. of bars - 4 nos.
Stirrups  8mm @ spacing 150mm c/c , clear cover = 25 mm from all the sides.


5a. The volume of plinth beam concrete

= length × breadth × depth

= [ (perimeter of site - no.of columns × column width) × breadth × depth] 

= [(140ft - 14 nos. × 0.75 ft.) × 0.75 ft × 1 ft.]

= [129.5 ft. × 0.75 ft. × 1ft.]

= 97.125 cu ft. i.e. 2.75 cum.

Note: We have deducted the column width from the plinth beam length, as we have included them in the column volume.


5b. BBS of plinth beam for the compound wall.




The perimeter of site = 140 ft. = 42672 mm.

We will provide Ld at the corner columns of the compound wall, passing the plinth bar through the intermediate columns. 

The cutting length of the main bar 

= [ (perimeter of site)+ (8nos × Ld )+ (4 nos.×lap length)- (8nos × column width) ]


Note: Let us assume one overlapping for each bar & we will provide a lap length of 50d.

 Let us provide development length Ld = 40d for the main bar. 

= [ (42672 mm.) + (8 nos. × 40 × 12mm) + (4nos × 50 × 12 mm)- (8 nos.× 230mm)]

= [42672 mm + 3840mm + 2400mm - 1840mm ]

= 47072 mm. i.e. 47.072 m.


Cutting length of the stirrup

= 2nos. × (a +b ) + hook length - 90° bend - 135° bend




Where a = beam width - 2 × cover, & b = beam depth - 2 × cover

= 2 nos.  ×[ ( 230 mm - 2 × 25mm.) + ( 300 mm - 2  × 25mm ) ]+ (10d ) - (3 nos. × 2d ) - (2 nos. × 3d)

Here, 10d is taken for hook length.

We have deducted 2d for 90° bend -3nos., & 3d for 135° bend -2nos. as shown in the above drawing.

= 2 nos.×[ ( 180 mm ) + ( 250 mm ) ] + (10 × 8mm) - ( 3 nos. × 2 × 8mm ) - ( 2 nos. × 3 × 8 mm.)

= 2 nos. × [ 430 mm ] + 80 mm - 48mm - 48mm.

= 860 mm + 80 mm - 96 mm.

= 844 mm i.e. 0.844 m.


Number of stirrups 

= ( length of the plinth beam ÷ stirrup spacing ) + 1

here, length of the plinth beam = 129.5 ft = 39471 mm

= ( 39471mm. ÷ 150 mm) +1 

= 263.14+ 1

= 264.14 nos.

By rounding off, the no. of stirrups required = 264 nos. 


Now, let us prepare a BBS table for the plinth beam.


sl.    bar                    dia.     no.     length    total      weight       total   
no.                        ( mm)                 (m.)     length     kg/m      weight

1.  main bar              12         4    47.072    188.29         0.89     167.57

2. stirrups                 8        264    0.844    222.816   0.395      88.012

                                                     Total weight of bars  =  255.58 kgs.

                                                    Add 2% wastage          = 5.111  kgs.

                                         Grand total of rebars  =  260.69 kgs.       

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