---
title: "Underground RCC tank: brickwork, steel (fig a)"
subject: "Estimating and Costing"
chapter: 6
key: "semester_7/estimating_and_costing:ug-rcc-tank-a"
source: https://www.bctengineer.com/q/semester_7/estimating_and_costing/ug-rcc-tank-a
site: BCT Engineering
---

# Underground RCC tank: brickwork, steel (fig a)

Estimating and Costing · Chapter 6: Detailed Estimate

- Asked in: 2082 Bhadra
- Video: [Underground Water Tank Design | RCC and Brick Underground Water Tank with Cost Estimate](https://www.youtube.com/watch?v=myGOpFPUa-8) — Civil Gyan
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/estimating_and_costing/ug-rcc-tank-a
- Whole chapter: https://www.bctengineer.com/md/semester_7/estimating_and_costing/chapter-6

## Question

Calculate quantities for the following items of works for an underground RCC water tank given below:
a) First class brickwork in 1:4 c/s mortar
b) Steel reinforcement in top RCC cover slab
[Figure a: Underground RCC water tank. Plan: outer 300 cm wide, brick wall 23 cm thick with 12 cm and 35 cm offsets/projections, inner opening 200 cm × 100 cm, 200 cm. Section A-A: overall 276 cm wide, top RCC cover slab 20 cm thick with 10 mm dia all-bent-up bars and 10 mm dia main bars @ 15 cm c/c, 10 mm dia distribution bars @ 15 cm c/c, internal height 60 cm + 137 cm + 60 cm as marked, brickwork (1:4) 23 cm at top and 35 cm at base, M20 RCC floor, PCC (1:3:6) and flat brick soling below. Figure dimensions are hard to read in the scan.]

## Short answer

Brickwork in 1:4 mortar = 2.780 m³. Steel in top RCC cover slab: 10 mm bars at 15 cm c/c both ways, with hooks and alternatte cranked bars, total weight = 33.3 kg.

## Answer

#### Assumptions (figure dimensions are hard to read)

- Clear internal size of tank $2.00 \times 1.00$ m; internal clear height $1.37$ m.
- Brick wall (1:4): bottom part $0.35$ m thick and $0.60$ m high (12 cm offset on the outer face), top part $0.23$ m thick and $0.77$ m high; total $1.37$ m.
- Top cover slab: $20$ cm thick RCC, resting on the 23 cm wall at both ends, so its size is $(2.00 + 2 \times 0.23) \times (1.00 + 2 \times 0.23) = 2.46 \times 1.46$ m. Clear cover $25$ mm.
- 10 mm bars at 15 cm c/c in both directions; main bars in the short direction, alternate bars cranked up; distribution bars in the long direction; hooks $9d = 0.09$ m at each end of main and distribution bars; crank extra $= 0.42 \times (0.20 - 2 \times 0.025 - 0.010) = 0.059$ m at each of the two cranks.

#### (a) Brickwork in 1:4 mortar (centre-line method)

| Part | Centre-line length (m) | Thickness (m) | Height (m) | Volume (m³) |
|---|---:|---:|---:|---:|
| Lower wall | $2\,[(2.00+0.35)+(1.00+0.35)] = 7.40$ | 0.35 | 0.60 | 1.554 |
| Upper wall | $2\,[(2.00+0.23)+(1.00+0.23)] = 6.92$ | 0.23 | 0.77 | 1.226 |
| **Total** | | | | **2.780** |

#### (b) Steel in the top cover slab

- Number of main bars $= 1 + (2.46 - 0.05)/0.15 = 17$; of these 8 are cranked and 9 are straight.
- Straight main bar length $= 1.46 - 0.05 + 2 \times 0.09 = 1.590$ m; cranked bar $= 1.590 + 2 \times 0.059 = 1.708$ m.
- Number of distribution bars $= 1 + (1.46 - 0.05)/0.15 = 10$, each $= 2.46 - 0.05 + 2 \times 0.09 = 2.590$ m.
- Weight per metre of 10 mm bar $= d^2/162 = 100/162 = 0.617$ kg/m.

| Bars | Number | Length each (m) | Total length (m) | Weight (kg) |
|---|---:|---:|---:|---:|
| Main, straight | 9 | 1.590 | 14.31 | 8.83 |
| Main, cranked | 8 | 1.708 | 13.66 | 8.43 |
| Distribution | 10 | 2.590 | 25.90 | 15.99 |
| **Total** | | | | **33.25** |

**Answer: brickwork = 2.780 m³; steel in top slab = 33.3 kg.**

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. For the brick walls in a rectangular tank, the centre-line length of a wall is
   A. 2[(L + t) + (B + t)] using the clear size L x B and wall thickness t
   B. 2(L + B) using the clear size
   C. 2[(L - t) + (B - t)]
   D. L x B
   Answer: A. The centre line of the wall is half a thickness outside the clear face on each side, so length = 2[(L+t)+(B+t)]. (Key point: Centre-line method)

2. Why is the top cover slab 2.46 x 1.46 m?
   A. It is the clear size plus 2 x 0.35
   B. It rests on the 23 cm walls, so size = clear size + 2 x 0.23
   C. It equals the clear internal size
   D. It is the clear size plus the slab thickness
   Answer: B. The slab bears on the top wall (0.23 m thick) at both ends. (Key point: Slab size)

3. What is the extra length of a cranked (bent-up) bar over a straight bar?
   A. 0.42 x slab thickness exactly
   B. 9d hook at each crank
   C. 0.42 x (depth - covers - bar dia) per crank, about 0.059 m here
   D. Nothing, it is the same length
   Answer: C. A 45-degree crank adds about 0.42 x the vertical rise of the bar. (Key point: Crank length)

4. Weight per metre of a 10 mm bar is
   A. d/162 = 0.062 kg/m
   B. d^2/16 = 6.25 kg/m
   C. d^2/1000 = 0.1 kg/m
   D. d^2/162 = 0.617 kg/m
   Answer: D. The standard formula W = d^2/162 kg/m gives 0.617 kg/m for 10 mm. (Key point: Weight of bar)

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## Related in other subjects

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