How to Estimate Footing: Calculating Metal Reinforcements

How to Estimate Footing: Calculating Reinforcements

When a footing carries a column load, it needs reinforcement bars (rebars) to resist the tension caused by bending and soil pressure. As an estimator, our job is not to design the footing, it’s for structural engineers. Our job is to figure out how much steel is needed in kg, how long, how many bar, for BOQ (Bill of Quantities) and BOM (Bill of Materials).

Understand the Type of Footing

Before estimating, you need to identify the footing type because each has a different steel arrangement.

Isolated Footing – single column
Combined Footing – two or more columns sharing a slab
Strap Footing – two footings connected with a strap beam
Raft or Mat Footing – large slab under entire structure

In this lesson, we’ll focus on isolated footings, since they’re the most common.

Two Methods in Estimating Footing Reinforcements

Spacing method

There are 10 F1C1 in the plan,
Diameter of the bars = 16 mm

Spacing = 150 mm
compute the number of bars to be ordered.

Applicable if and only if: If there is no definite number of cutbars per footing stated, and the given is spacing instead.

Step 1: Calculate the number of cutbars needed in a footing with this formula:

Estimating footing reinforcements example

$N_{cutbars}=\frac{L-2cc}{spacing}+1$

$\frac{[1500mm - 2(75mm)]}{150}+1$

$\frac{1350}{150}+1$

$9+1=10$

Along x-direction:

$n_{cutbars}=10$

Along y-direction:

since they both have the same length

$n_{cutbars}=10$

Total number of $n_{cutbars}$ for all the footing is:

$10 \times 2 \times 10 footings = 200 cutbars$

Step 2: Solve the Cut Length

$Cut Length = L - 2 cc + 2 hooks$

$Cut Length = 1.3 - 2(0.075) + 2(0.125) = 1.4 m$

So we need 200 cutbars that is 1.4 meters.

Step 3: Choose from the commercial length steel bars.

6 m
7.5 m
9 m
10.5 m
12 m

When choosing between the length of steel bars, it is important for us civil engineers/quantity surveyors to determine which one produces less excess, also known as waste.

In our case,

$Cutbars=\frac{Commercial Length}{Cut Length}$

6 m:
$\frac{6 m}{1.4 m} = 4.28$

4 pcs, with 0.28 waste least waste

$\frac{7.5 m}{1.4 m} = 5.36$

5 pcs, with 0.36 waste

$\frac{9 m}{1.4 m} = 6.43$

6 pcs, with 0.43 waste

$\frac{10.5 m}{1.4 m} = 7.5$

7 pcs, with 0.5 waste

$\frac{12 m}{1.4 m} = 8.57$

8 pcs, with 0.57 waste

Step 5: Divide the total number of Cutbars to the number of bars per footing.

$n_{m RSB}=\frac{n_{cutbars}}{n_{cuts}}$

$n_{6 m RSB} = \frac{200 cutbars}{4cuts} = 50 bars$

$Order 50 pcs 6 m 16 mm \phi$

COUNTING METHOD

There are 17 F1C1 in the plan, compute the number of bars to be ordered.

Step 1: Count the number of the same footing in the plan.

$17 Footings$

Reinforcements for footing in civil engineering and construction

Step 2: Multiply the number of cutbars per footing to the number of the same footings in the plan.

In counting method, the number of bars per footing is given in the plan.

$n_{cutbars} = 9 \times 2 \times 17 = 306 cutbars$

Step 3: Compute the Total length of the cutbars

$Cut length = L_{footing} - 2cc + 2 hooks$

$Cut length = L_{footing} - 2cc + 2[9D]$

$9D = 8(16) = 144 mm = 0.144m$

$Cut length =1.3 - 2(0.075)+2[0.144]$

$Cut length =1.438 m$

Step 4: Commercial length divided by length of the Cutbar.

$n_{cuts} = \frac{L_{commercial length}}{L_{cut length}}$

$n_{cuts} = \frac{6 m}{1.406 m}= 4.17 \approx 4 cuts$

Step 5: Divide the total number of Cutbars to the number of bars per footing

$n_{m RSB}=\frac{n_{cutbars}}{n_{cuts}}$

$n_{6 m RSB} = \frac{306 cutbars}{4cuts} = 77 bars$

$Order 77 pcs 6m 16 mm \phi$

Tie Wires

Step 1: count the number of intersections, no. of footings, and length of each tire wire.

each tie wires can range from 0.2 m to 0.4 m per intersection.

$L_{tie wire} = n_{intersection} \times n_{footing} \times l_{tie wire}$

Continuation of counting method problem:

$L_{tie wire} = (9 \times 9) \times 17 footing \times 0.4 m$

$L_{tie wire} = 550.8 m$

Step 2: Convert to kg.

$W_{tie wire} = 550.8 m \times \frac{1 kg}{53m}$

$Order \: 10.45 kg \: #16 \: G.I. \:Wire$

References

Aci Committee 318, & American Concrete Institute. (2022). Building code requirements for structural concrete (ACI 318-19) : an ACI standard : commentary on building code requirements for structural concrete (ACI 318R-19). American Concrete Institute.

Fajardo, M. B. (1980). Simplified Construction Estimate.

CH 4: Masonry Works

‌