2. Approximate Estimates ,Civil Engineering 5th Sem Notes CE 5002

 

2. Approximate Estimates

Approximate estimates are rough estimations made when the detailed design or quantity survey is not available. These estimates are used during the early stages of a project for budgeting, feasibility studies, or quick decision-making. They are less accurate than detailed estimates but provide a preliminary cost assessment.

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2.1 Methods of Approximate Estimate

There are various methods to prepare approximate estimates, and they can be used depending on the available information, time, and project type. Below are the common methods:

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2.1.1 Service Unit Method

• Definition: The service unit method involves estimating the cost based on a unit that serves as a basis for cost comparison, such as the number of units of a service (e.g., per hospital bed, per classroom, per toilet unit).

• Formula:

  $$\text{Cost Estimate} = \text{Number of Service Units} \times \text{Cost per Service Unit}$$

• Purpose: This method is used when there is no clear design or quantity, and it is applicable to large-scale public buildings (hospitals, schools, etc.), where costs can be related to the number of service units provided.

• Example: For estimating the cost of a hospital, the number of hospital beds can be used as a service unit. If the cost per bed is ₹10,00,000 and the number of beds is 100, then:

  $$\text{Cost Estimate} = 100 \times 10,00,000 = ₹10,00,00,000$$

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2.1.2 Plinth Area Rate Method

• Definition: In this method, the cost is estimated based on the plinth area (the total covered area of the building at the floor level, including walls and columns). The cost per square meter (or square foot) of the plinth area is multiplied by the total area to get an approximate cost.

• Formula:

  $$\text{Cost Estimate} = \text{Plinth Area} \times \text{Rate per unit area}$$

• Purpose: This method is typically used for estimating residential buildings or commercial structures where detailed drawings and bills of quantities are not available.

• Example: Let’s say the plinth area of a building is 200 square meters, and the rate per square meter is ₹2,000. Then:

  $$\text{Cost Estimate} = 200 \times 2,000 = ₹4,00,000$$

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2.1.3 Cubical Content Method

• Definition: This method involves calculating the total volume of the building (length × width × height) and multiplying it by a rate per cubic meter (or cubic foot). It is used when the building's height is significant and when the building has more vertical structure (e.g., multi-story buildings).

• Formula:

  $$\text{Cost Estimate} = \text{Volume of Building (Cubical Content)} \times \text{Rate per cubic meter}$$

• Purpose: It is useful for buildings where the total volume is more relevant than the floor area, such as high-rise buildings or complex structures with varied heights.

• Example: Assume the total volume of the building is 500 cubic meters, and the rate per cubic meter is ₹3,000. Then:

  $$\text{Cost Estimate} = 500 \times 3,000 = ₹15,00,000$$

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2.1.4 Typical Bay Method

• Definition: The typical bay method is based on dividing the building into standard "bays," which are structural units that represent a regular part of the building (such as a section between columns or spans). The cost is calculated by estimating the cost per bay and then multiplying it by the number of bays in the entire building.

• Formula:

  $$\text{Cost Estimate} = \text{Number of Bays} \times \text{Cost per Bay}$$

• Purpose: This method is primarily used for industrial or large commercial buildings where the structure is repetitive and can be divided into modular sections or bays.

• Example: Suppose a factory building has 10 bays, and the cost per bay is ₹5,00,000. Then:

  $$\text{Cost Estimate} = 10 \times 5,00,000 = ₹50,00,000$$

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2.1.5 Approximate Quantity Method

• Definition: The approximate quantity method involves estimating the quantities of materials required for the project based on available design information or similar past projects. This method calculates the total cost based on rough quantity estimates for key components (e.g., cement, steel, bricks, etc.), multiplied by their respective unit rates.

• Formula:

  $$\text{Cost Estimate} = \text{Quantity of Material} \times \text{Rate of Material}$$

• Purpose: This method is used when there are basic drawings, sketches, or assumptions available, but the detailed design is not yet prepared.

• Example: Assume the building requires approximately:

  • 100 cubic meters of concrete
  • 50 tons of steel
  • 100,000 bricks

  And the unit rates are:

  • Concrete: ₹6,000 per cubic meter
  • Steel: ₹50,000 per ton
  • Bricks: ₹10 per brick

  Then:

  $$\text{Cost Estimate} = (100 \times 6,000) + (50 \times 50,000) + (100,000 \times 10)$$ $$\text{Cost Estimate} = 6,00,000 + 25,00,000 + 10,00,000 = ₹41,00,000$$

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Summary of Approximate Estimate Methods:

Method	Description	Formula
Service Unit Method	Based on the number of service units (e.g., beds, classrooms, toilets).	Cost = Service Units × Cost per Unit
Plinth Area Rate Method	Based on the plinth area of the building (floor area).	Cost = Plinth Area × Rate per unit area
Cubical Content Method	Based on the volume of the building.	Cost = Volume × Rate per cubic meter
Typical Bay Method	Based on the number of repetitive bays or modules in the building.	Cost = Number of Bays × Cost per Bay
Approximate Quantity Method	Based on rough quantities of materials required (e.g., cement, steel).	Cost = Quantity × Rate per material