Estimate Of Buildings | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

General

Estimating is the technique of calculating or computing the various quantities and the expected Expenditure to be incurred on a particular work or project. In case the funds available are less than the estimated cost, the work is done in part or by reducing it, or specifications are altered, the following requirements are necessary for preparing an estimate. 

• Drawings like plan, elevation, and sections of important points.
• Detailed specifications about workmanship& properties of materials, etc.
• Standard schedule of rates for the current year.

Units of Measurements 

The units of measurement are mainly categorized for their nature, shape, and size and for making payments to the contractor. The principle of units of measurement normally consists of the following:

• Single units, like sanitary fittings, electrical points, electrical appliances, etc., are expressed in numbers.
• Works consist of linear measurements involving length like cornice, fencing, handrail, pipe length with details, bands of specified width and skirting, etc., are expressed in running meters (RM)
• Works consist areal surface measurements involving areas like plastering, whitewashing, partitions of specified thickness, glass of specified thickness, flooring up to the thickness of 40mm, Tiles flooring, wall tile finishing, painting of doors and windows, A.C Sheet roofing, Weathering tiles, Doors and windows shutter with required specifications, Half brickwork, Honeycomb Brickwork, Brick on edge work etc.,  are expressed in square meters (m²)
• Works consist of cubical contents that involve volume like earthwork, Earth fill, cement concrete, Masonry, etc., which are expressed in Cubic meters.
• Steel for RCC works is expressed in Kilogram, Killonewton, or tonne.

Rules for Measurement 

• The rules for the measurement of each item are invariably described in IS- 1200.
• However, some of the general rules are listed below.
• Measurement shall be made for the finished item of work, and description of each item shall include materials, transport, labor, fabrication tools, and plant, and all types of overheads for finishing the work in the required shape, size, and specification.
• In booking, the order shall be in sequence of length, breadth, and height or depth or thickness.
• All works shall be measured subject to the following tolerances.
• Linear measurements shall be taken to the nearest 0.01m.
• Areas shall be measured to the nearest 0.01 sq. m
• Cubic contents shall be worked out to the nearest 0.01 cum and wood for door and window frames to the accuracy of 0.0001 mt.
• Killogram to the accuracy of 0.01

The same type of work under different conditions and nature shall be measured separately under separate items.
The bill of quantities shall fully describe the materials, proportions, and workmanship and accurately represent the work to be executed.

In the case of masonry (stone or brick)  or  structural  concrete,  the  categories  shall be measured separately and the heights shall be described:

• From foundation to plinth level
• From the plinth level to the first-floor level
• From First floor to second-floor level and so on.

Requirements Of Estimation And Costing 

The estimate gives an idea of the cost of the work, and hence its feasibility can be determined, i.e., whether the project could be taken up with the funds available or not.

• Estimate gives an idea of the time required for the completion of the work.
• An estimate is required to invite the tenders and Quotations and to arrange a contract.
• An estimate is also required to control the expenditure during the execution of work.
• The estimate decides whether the proposed plan matches the funds available or not.

Types of Estimates 

• Preliminary Estimate
• Detailed estimate
• Abstract
• Revised Estimate

Preliminary Estimate: The estimate is a rough estimate that is normally estimated at an approximate square feet rate. In this estimate  the specifications and Area are only for the temporary purpose. Sometimes, the cost may differ by up to 50%.

Detailed Estimate: The estimate, which is in detail, will be provided with specifications of material, method of work, detailed measurements, and drawings. The quantities of the item of the works may vary up to 10%.

Abstract: The estimate includes only the total quantities of the item of works, Rates either as per PWD schedule or market values, and the total cost of the project.

Revised Estimate: The revised estimate includes revised quantities or specifications and rates.

The conditions for the  preparation of Revised estimates are

• When the area or measurements of the approved plan changes 
• When the specification of the material of the method of construction changes 
• When the rates of material labor change over and above 10% 
• When the location of the work changes

Steps Or Procedure of Estimation
Estimating involves the following operations

• Preparing detailed Estimates.
• Calculating the rate of each unit of work 
• Preparing abstract of estimate

Requirements to Prepare An Estimate 

• Drawings, i.e., plans, elevations, sections, etc., with complete measurements 
• Detailed Specifications, if possible, with brand name 
• Scheduled Rates or Market rates

Drawings 

If the drawings are not clear and without complete dimensions, the preparation of estimation becomes very difficult. So, it is essential to know before preparing an estimate.

Specifications 

• General Specifications: This gives the nature, quality, class work, and materials in general terms to be used in various parts of work. It helps to form a general idea of building.
• Detailed Specifications: These give a detailed description of the various items of work, laying down the Quantities and qualities of materials, their proportions, the method of preparation workmanship, and execution of work.

Rates 

• For preparing the estimate, the unit rates of each item of work are required, as given below are the rates of various materials to be used in the construction.
• The cost of transport materials.
• The wages of labor, skilled or unskilled of masons, carpenters, Amador, etc.,

Lumpsum 

While preparing an estimate, it is not possible to work out in detail in case of petty items. Items other than civil engineering are called lump sum items or simply L.S. Items.
The following are some of the L.S. items in the estimate.

• Water supply and sanitary arrangements.
• Electrical installations like meters, motors, etc.,
• Architectural features.
• Contingencies and unforeseen items.

In general, a certain percentage of the cost of estimation is allotted for the above L.S. Items. Even if sub-estimates are prepared, or at the end of the execution of work, the actual cost should not exceed the L.S. amounts provided in the main estimate.

Work Charged establishment 

• During the construction of a project, a considerable number of skilled supervisors, work assistants, watchmen, etc., are employed on a temporary basis. The salaries of these persons are drawn from the L.S. amount allotted towards the work-charged establishment or in the category of contingencies.
• During the construction of a project, a considerable number of skilled supervisors, work assistants, watchmen, etc., are employed on a temporary basis. The salaries of these persons are drawn from the L.S. amount allotted towards the work-charged establishment or in the category of contingencies.

Methods of Estimation

The quantities like earthwork, foundation concrete, brickwork in the plinth and superstructure etc., can be worked by any of the following two methods:

• Longwall - short wall method
• Centre line method.
• Partly center line and short wall method. 

Long Wall-Short Wall Method 

• In this method, the wall along the length of the room is considered to be a long wall, while the wall perpendicular to a long wall is said to be a short wall. To get the length of a long wall or short wall, calculate first the center line lengths of individual walls. Then, the length of the long wall (outside dimension) may be calculated after adding half the breadth at each end to its center line length. 
• Thus, the length of the short wall is measured and may be found by deducting half breadth from its center line length at each end. The length of a long wall usually decreases from earthwork to brickwork in superstructure while the short wall increases. These lengths are multiplied by breadth and depth to get quantities.

Centre Line Method 

• This method is suitable for walls of similar cross-sections. Here, the total center line length is multiplied by the breadth and depth of the respective item to get the total quantity at a time. When cross walls or partitions or verandah walls join with the main all, the center line length gets reduced by half of the breadth for each junction. Such junctions or joints are studied carefully while calculating the total centerline length. The estimates prepared by this method are the most accurate and quick.

Partly Centre Line Partly Cross Wall Method 

• This method is adopted when the external (i.e., around the building) wall is of one thickness and the interior walls have different thicknesses. In such cases, the center line method is applied to external walls and the long wall-short wall method is used to interior walls.  This method is suitable for different thicknesses of walls and different levels of foundations. This method is a popular method in practice.

Detailed Estimate

The preparation of a detailed estimate consists of working out quantities of various items of work and then determining the cost of each item. This is prepared in two stages.

Details Of Measurements And Calculation of Quantities 

• The complete work is divided into various items of work such as earthwork, concreting, brickwork, and R.C.C., plastering, etc. The details of measurements are taken from drawings and entered in respective columns of prescribed performed. The quantities are calculated by multiplying the values that are in the numbers column by the Depth column, as shown below:
  Details of measurements form
  
• Abstract of Estimated Cost :
  The cost of each item of work is worked out from the quantities that are already computed in the details measurement form at a workable rate. However, the total cost is worked out in the prescribed form, which is known as an abstract of the estimated form. 4% of the estimated cost is allowed for Petty Supervision, contingencies, and unforeseen items.
  

Data

• The process of working out the cost or rate per unit of each item is called data. In preparation of Data, the rates of materials and labor are obtained from the current standard scheduled of rates while the quantities of materials and  labor  required  for one unit of item are taken from the standard Data Book

Fixing Of Rate Per Unit of An Item 

The rate per unit of an item includes the following:

• Quantity of materials & cost
  The requirement of materials is taken strictly in accordance with the standard data book(S.D.B).  The cost of these includes first cost, freight, insurance, and transportation charges.
• Cost of labor
  The exact number of laborers required for a unit of work is multiplied by the wages/ day to get labor for unit item work.
• Cost of equipment (T&P)
  Some works need special types of equipment, tools, and plants. In such a case, an amount of 1 to 2% of the estimated cost is provided.
• Overhead charges
  To meet expenses of office rent, depreciation of equipment salaries of staff postage, and lighting, an amount of 4% of the estimated cost is allocated.

Methods of Preparation Of Approximate Estimate 

A preliminary or approximate estimate is required for studies of various aspects of the work of the project and for its administrative approval. It can decide, in the case of commercial projects, whether the net income earned justifies the amount invested or not. The approximate estimate is prepared from the practical knowledge and cost of similar works. The estimate is accompanied by a report duly explaining the necessity and utility of the project and with a site or layout plan. A percentage of 5 to 10% is allowed for contingencies. The following are the methods used for the preparation of approximate estimates.

• Plinth area method 
• Cubical contents methods 
• Unit base method

Plinth area method

• The cost of construction is determined by multiplying the plinth area by the plinth area rate. The area is obtained by multiplying the length and breadth (outer dimensions of the building). In fixing the plinth area rate, careful observation and necessary inquiries are made with respect to quality and quantity aspects of materials and labor, type of foundation, the height of the building, roof woodwork, fixtures, number of stories, etc. As per IS 3861-1966, the following areas are included while calculating the plinth area of the building.

Types of Estimates

• Area of walls at floor level.
• Internal shafts of sanitary installations not exceeding 2.0m2, lifts, air-conditioning ducts, etc.,
• Area of barsati at terrace level: Barsati means any covered space open on one side constructed on a terraced roof, which is used as shelter during the rainy season.
• Porches of non-cantilever type.
• Areas that are not included are Areas of lofts.
• Unenclosed balconies up to 1.80mt
• Architectural bands, cornices, etc.
• Dome towers projecting above terrace level.
• Box louvers and vertical sun breakers.

Cubical Contents Method
This method is generally used for multi-storeyed buildings. It is more accurate than the other two methods viz., plinth area method and unit base method. The cost of a structure is calculated approximately as the total cubical contents (Volume of buildings) multiplied by the Local Cubic Rate.
The volume of the building is obtained by Length x breadth x depth or height.
The length and breadth are measured out to out of walls, excluding the plinth offset. The cost of string course, cornice, corbelling, etc., is neglected. The cost of building volume of buildings x rate/ unit volume.

Example 1: Prepare an approximate estimate of building project with total plinth area of all building is 80 sqm. and from following data.
Plinth area rate Rs. 40000 per sqm
Cost of water supply @7½% of cost of building.
Cost of Sanitary and Electrical installations each @ 7½% of cost of building.
Cost of architectural features @1% of building cost.
Cost of roads and lawns @5% of building cost.
Cost of P.S. and contingencies @4%  of  building cost.
Determine the total cost of building project.

Data given:
Plinth area = 80m²
Plinth area rate = Rs. 40000 per Sq.m

Example 2: Prepare the rough estimate for a proposed commercial complex for a municipal corporation for the following data.
Plinth Area = 500m2/floor Ht of each story = 3.5m No. of storey’s = G+2
Cubical content rate = Rs. 1000/m³
Provided for the following as a percentage of structured cost water supply & Sanitary arrangement -8%
Electrification - 6%
Fluctuation of rates - 5%
Contractors profit - 10%
Petty supervision & contingencies - 3%

Cubical content = No. of storey’s (Plinth Area x height of each storey)
= 3(500 x 3.5) = 5250m³
Structural cost = Cubical content x cubical content rate = 5250 x 1000 = 52.5 Lakhs

Unit Base Method

Example 3: Prepare an approximate estimate or rough cost estimate of a hospital building for 50 beds. The cost of construction altogether for each bed is Rs. 60,000/ -. Determine the total cost of hospital building.

No. of beds = 50
Cost of construction = Rs. 60,000/-
Total Cost of Hospital building = 50 x 60,000 = Rs. 30,00,000/-

Example 4: To prepare the rough cost estimate of a hostel building which accommodate 150 students. The cost of construction including all provisions is Rs. 15,000/- per student. Determine total cost of building.

No. of students = 150
Cost of construction including all L.S. provisions = Rs. 15,000/- Total Cost of hostel building =150 x 15000 = Rs. 22,50,000/- (Rupees twenty two lakhs, fifty thousand only).