Importance of Estimation and Types of Estimates

QUANTITY SURVEYING

The profession of a person who estimates the cost of the materials and labor required for a construction job.

A Quantity Surveyor (QS) is a construction industry professional with expert knowledge on construction costs and contracts. They work on behalf of contracting organizations or representatives of the client, aiming to maximize value and ensure the project is completed within the proposed budget.

Estimate

An estimate of the cost of a construction job is the probable cost of that job as computed from plans and specifications.

For a good estimate the, actual cost of a project should not differ by more then 5 to 10 % from its estimated cost, provided there are no unusual, unforeseen circumstances.

Estimation

Estimation is the scientific way of working out the approximate cost of an engineering project before execution of the work.

Estimation is totally different from exact cost calculation after completion of the project. Estimation requires a detailed Knowledge of the construction procedures and cost of materials & labour .

IMPORTANCE & NEED OF ESTIMATION

It help to work out the approximate cost of the project in order to decide its feasibility with respect to the cost and to ensure the financial resources, if the proposal is approved.

Requirements of controlled materials, such as cement and steel can be estimated for making applications to the controlling authorities.

It is used for framing the tenders for the works and to check contractor’s work during and after the its execution for the purpose of making payments to the contractor.

From quantities of different items of work calculated in detailed estimation, resources are allocated to different activities of the project and ultimately their durations and whole planning and scheduling of the project is carried out.

EFFECT OF SITE CONDITIONS ON OVERALL COST

Each type of work requires a different method of construction techniques, type of machinery, and formwork.

Construction may be of an ordinary house, Multistory building, Dam, Tunnel, Airport, Bridge etc.

Weather conditions affect the output and, hence, the overall cost.

Ground conditions vary and change the method of construction. e.g. excavation may be dry, wet, hard, soft, shallow or deep.

The work may be in open ground such as fields or it may be in congested areas such as near or on the public roads, necessitating extensive watching, lightening, and controlling efforts, etc.

The source of availability of a sufficient supply of materials of good quality is also a factor.

The availability of construction machinery also affects the method of construction.

Access to the site must be reasonable. If the access is poor, temporary roads may be constructed.

QUALITIES OF GOOD ESTIMATOR

Estimator must have good knowledge regarding the important rules of quantity surveying and construction work.

He must thoroughly understand the drawings of the structure, for which he is going to prepare an estimate.

He must also be clearly informed about the specifications showing nature and classes of works and materials.

Having information regarding the materials required, machinery needed, overhead problems, and costs of all kinds.

Good judgment with regard to different localities, different jobs and different workmen.

Selection of a good method for preparing an estimate.

Ability to be careful, thorough, hard working and accurate.

Ability to collect, classify and evaluate data relating to estimation.

Ability to visualize all the steps during the process of construction.

For example, if the construction of a large building is planned, a good estimator or his representative visit the site and:

Note the location of the proposed building.

Get all data available regarding the soil.

Make a sketch of the site showing all important details.

Obtain information concerning light, power, and water.

Secure information concerning banking facilities.

Note conditions of streets leading to railway yards and to material dealers etc.

Investigate general efficiency of local workman.

TYPES OF ESTIMATES

Rough Cost Estimate

Detailed Estimate

ROUGH COST ESTIMATE

Estimation of cost before construction from plans or architectural drawings of the project scheme, when even detailed or structural design has not been carried out, is called Rough Cost Estimate.

These estimates are used for obtaining Administrative Approval from the concerning authorities.

Sometimes, on the basis of rough cost estimates, a proposal may be dropped altogether.

Rough cost estimate may be prepared on the following basis for different types of projects:

Cost per square foot of covered area (plinth area) is the most commonly adopted criterion for preparing rough cost

estimate for most of the residential buildings.

For public buildings, cost. Per person (cost per unit) is used.

For example,

Students hostel———————---Cost per student

Hospitals————————————Cost per bed

Hotel—————————————----Cost per guest

Rough cost estimate may be prepared on the following basis for different types of projects:

Cost per cubic foot is particularly suitable for commercial offices, shopping centers, and factory buildings, etc.

For water tank/reservoir, cost may be worked out on the basis of capacity in gallons (cost per gallon) of water stored.

For roads and railways, cost may be found out per mile/kilometer of length.

For streets, cost may be per hundred feet/meters of length.

In case of bridges, cost per foot/meter of clear span may be calculated.

DETAILED ESTIMATE

Detailed estimates are prepared by carefully and separately calculating in detail the costs of various items of the work that constitute the whole project from the detailed working drawings after the design has been finalized.

The mistakes, if any, in the rough cost estimate are eliminated in the detailed estimate.

Detailed estimates are submitted to the competent authorities for obtaining Technical Sanction.

The whole project is sub-divided into different items of work or activities. The quantity for each item is then calculated separately from the drawings as accurately as possible. The procedure is known as “Taking out of quantities".

The quantities for each item may be estimated and shown in the pattern which is called "Bill of Quantities."

The unit, in which each item of the wok is to be calculated, should be according to the prevailing practice as followed in various departments of the country.

Each item of the work is then multiplied by its estimated current rate calculated by a fixed procedure to find out cost of the item.

At the end, a total of all items of the work are made to get the total estimated cost.

The rates are usually as per Schedule of Rates for the locality plus a premium to allow for rise in labor and material rates over and above the schedule of rates.

Contingencies charges is added 3% to 5% of estimated cost of the project.

Work charge establishment is added 1.5% to 2% of estimated cost of

the project, also called establishment charges.

Supervision or departmental charges is added 5% to 10% of estimated cost of the project.

DATA FOR DETAILED ESTIMATE

Drawings

Plan, elevation, sectional elevation, complete dimensions

Specifications

General Specifications: It gives nature, quality and class of work and material in general terms.

Detailed Specifications: Gives detail description of various items, quantities and qualities of material, their proportions, method of preparation, workmanship and execution of work. Describe each item of work separately.

Rates

Rates per unit of various items of work, rates of various materials, wages of skilled and unskilled labors etc.

DOCUMENTS REQUIRED FOR TECHNICAL SANCTION

Besides drawings and details of measurements and calculation of quantities (Bill of Quantities), the following documents are also usually submitted with the detailed estimate for obtaining Technical Sanction:

A report explaining History, necessity, scope and main features of the project, its design, and estimate, etc.

Specifications lying down the nature and class of work and material to be used in various parts of the work.

The abstract of cost (priced Bill of Quantities) showing the total quantities under each sub-head, rate per unit of measurement, and cost.

Calculation sheets showing calculations for important parts of the structure.

In fact, in estimating the art and skill lies only in the computation of details without any omissions, of all parts of the building or work.

TYPES OF DETAILED ESTIMATE

CONTRACTOR ESTIMATE:

It is made by the contractor for determining the price or prices to be bid. It is usually a carefully prepared detailed estimate.

ENGINEER’S ESTIMATE:

This type of estimate is made by the Engineer (Consultant) usually for the purposes of financing the work and for checking bids and running bills submitted by contractors.

PROGRESS ESTIMATES:

These are made by the Engineer at regular intervals for the completed parts of the project during the progress of the work for determining the amounts of partial payments to be made to the contractor.

On large contracts, such estimates are commonly made each month and hence, are called monthly estimates.

UNFORESEEN ITEMS IN DETAILED ESTIMATE

While preparing a detailed estimate, one had to be very careful to see that all items of the work are incorporated.

It is likely that a few Items, though unimportant in nature, might have been overlooked and which may result in raising the estimate of the project.

There may be also certain unforeseen circumstances affecting the project.

Hence, a certain allowance usually 3% to 5% of the total cost, is made in the estimation which will take care of all these items that are unforeseen or are overlooked and are known as "Contingencies".

About Author:

I am Thomas Britto here to share my experiences in the civil engineering field to all my readers.Today many students are struggling to buy books at high prices. So I decided to start a blog and share my experience and knowledge with all my readers.

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