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The estimation of quantities is an important aspect of civil engineering design projects. Estimates are typically provided to a client in the bid package which is prepared by the design team. It involves not only the estimation of the materials of which the completed project is composed, but also the estimation of the construction labor and equipment which is necessary for the completion of the project work.
1. Types of Estimates
Estimates performed throughout the design process help the project team and owner gain an understanding of project costs and feasibility, keeping the proposed construction within the owner's budget. For this reason, estimates are typically provided at each design submission. Types of estimates include approximate (also known as rough) estimates and detailed estimates. The development of a project typically lends itself to estimation procedures of greater detail and accuracy at the later stages of design.
2. Approximate or Rough Estimates
Schematic level design, for example, lends itself to rough estimates, which can be based on an engineer's previous experience with costs. One might, for example, have a general idea of the typical cost of a parking lot at grade on a square footage basis and apply that number to the particular area which is proposed on a given project. This may not be a very accurate estimate, but it may be good enough to allow the design team to work with a general sense of an item's cost while detail on the design elements has yet to be determined. Given the uncertainties of a design during the early stages of development, design contingencies are typically added to early estimates to reduce the risk of going over the construction budget in the project design at a later stage. This is often provided as a percentage increase in the overall project cost. As the design progresses, the amount of design contingency may be lowered since there is typically less uncertainty about the design as a project progresses.
3. Detailed Estimates
Detailed estimates break down design items into their various components to gain greater accuracy of estimation. This includes separate cost components of material, labor, and equipment. For this reason, detailed cost estimation is sometimes referred to as the unit cost method. The use of unit prices and the components of an estimate which apply to them are discussed below.
4. Material Component of Estimation
The material component of the estimation is often referred to as a "take-off," as it involves the calculation of material quantities based on plans, sections, elevations, or other design drawings. The calculations for an estimation depend upon the typical unit of measure which is used for the material being estimated. This could be based on volume, area, length, unit count, or overall weight, depending on the material. The cost component of an estimation is determined by multiplying the material quantity by the corresponding unit cost to determine the overall cost for the material. For example, a calculated volume of soil in cubic yards would be multiplied by a unit cost defined as cost per cubic yard. Likewise, a calculated number of doors on a project would be multiplied by unit cost defined as a cost per unit to determine the overall cost for the doors on a project. Units of measure for typical items include: units for items such as doors, windows, drains, catch basins, manholes, and plumbing fixtures; feet or meters of length for items such as pipes, guide rails, and striping; square feet or square meters of area for items such as clearing and grubbing, paint, and flooring; cubic yards or cubic meters of volume for items such as concrete, aggregates, earthwork excavation, soil, and backfill; and tons of weight for items such as structural steel and reinforcement.
5. Determining Unit Costs
Unit costs are typically determined from industry estimating databases. Web-based services can provide the most accurate and up-to-date information. Books with published values are also utilized, though less frequently than in the past. Unit costs change over time and location due to factors such as supply and demand, inflation, and labor availability. The databases utilize recent historical data to determine a value for use in estimates. It should be noted that an estimator should review the values found in the databases and evaluate their appropriateness for the project under consideration, as the specifics of the project may warrant an adjustment to the values to gain greater accuracy in the estimate.
6. Allowances
Some items may be indicated in an estimate with an allowance rather than a unit cost basis. The allowance, however, is typically determined based on some type of assumption as to the amount of material or work required, though a precise amount may be unknown whether because of uncertainties in the design or for a lack of sufficient detail in the current stage of design.
7. Non-Material Components
Non-material components of work such as labor typically have a unit of measure in overall labor hours. Multiplication of the labor rate by the anticipate labor hours yields the labor cost. It should be noted that off hours or overtime work may need to be considered as this will increase the unit cost for the labor.
8. Earthwork
Civil engineers should be familiar with the common methods of earthwork calculations related to grading work. Earthwork consists of both cutting and filling operations. The methods of estimation for these include the average method, the block (or grid) method, and the section method, among others. Each may be most appropriate for a given type of project or stage of design.
- Average Method: In the average method, one would first determine the average level of existing conditions, then the average level of the final proposed conditions, and finally multiply this difference by the area of the work. This would roughly determine the overall amount of fill or excavated soil that would be necessary to transport to or from the site. It would be most appropriate to use this method as either a preliminary estimation method or on a smaller scale project.
- Block or Grid Method: In the block (or grid) method, one would divide an area up into smaller areas and determine the amount of difference between the existing and finish grade in each of those areas. One would then multiply the area of each of these blocks by the difference determined from each of these blocks and the sum of these numbers to determine the total amount of cutting or filling. The difference between the two totals would indicate the total soil either required to be brought in or removed. This method would result in more accuracy than the average method.
- Section Method: The section method is most appropriate for infrastructural projects, such as new highways where there is a linear area under consideration. Sections are taken at regular intervals along a path through the project area, such as at the centerline of a roadway. The existing terrain and the proposed terrain shown within these sections allow for the area difference between the two to be calculated using calculation techniques such as the trapezoidal method, wherein complex shaped sectional areas are determined by first breaking them down into simpler areas. These areas would then be multiplied by the distance between the sections taken along the path to determine the volumes of cutting or filling. Computer software is often used to generate these sections once survey data of the terrain is imported into the software.
- Soil Swell: It should also be noted that a given volume of soil, once excavated, typically expands to a larger volume, and this should be taken into consideration when determining the number of vehicle trips required for transporting the soil. This is sometimes referred to as "swell." When soil is brought into a site and then compacted, it occupies a smaller volume of space. This is sometimes referred to as "shrinkage."
Conclusion
In summary, understanding how to do quantity take-offs and determining estimated costs is a key task in civil engineering. In order to develop the most accurate cost estimates, civil engineers should be familiar with the various types of estimates and how they are made. They should also be familiar with the particular methods for estimating common work items in civil engineering such as for earthwork. Accurate quantity and cost estimates are essential for successful projects that are delivered within budget.
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