METHODS OF CALCULATIONS OF QUANTITIES OF MATERIALS

Methods Of Calculations Of Quantities Of Materials

The calculations of quantities of materials can be done using various methods of estimates. The application of an individual method depends upon the design and shape of the building. The different methods are as under:

 

1. Centre line method.
2. Crossing method.
3. Out to out and in to in method.
4. Bay method.
5. Service unit method.

1. Centre line method

This method is suitable only if the offsets are symmetrical and the building is more or less rectangular in shape. The centre line of the building is determined carefully after doing deductions for repeated measurements (as explained in the next problem). This centre line acts as length for the complete calculations of the estimate.

If the deduction is not cared for the results of estimates may be wrong. All the walls should have the same section.

2. Crossing Method

In this method, lengths and breadths of the masonry walls at plinth level are taken (internal dimension of the room + thickness of the walls) for calculating quantities. The symmetrical offsets are a must as in the case of centerline method.

3. Out to out & in to in Method

This method is most practicable under all circumstances and is generally followed in the P.W.D. for computing the quantities of various items. The estimation in this book has been done using this method.

4. Bay Method

This method is useful and is generally followed in case of building having several bays. The cost of the one class room is worked out and then multiplied by the number of bays in that building. The extra cost of the end walls and difference in framing. If there is any, should be made, so as to arrive at the correct cost.

5. Service Unit Method.

This method is followed in cases such as school building where there are so many class rooms. The cost of one class room us worked out and then multiplied by the number of class rooms to be constrused. In case of Hospitals, the service unit is a bed, in case of Water Tank, it is a litre and in case of Cinema Hall, the service unit is a seat.

ESTIMATION METHODS OF BUILDING WORKS

CIVIL_ENGINEERING

The estimation of building quantities like earth work, foundation concrete, brickwork in plinth and super structure etc., can be workout by any of following two methods:
a) Long wall – short wall method
b) Centre line method.
c) Partly centre line and short wall method.
a) Long wall-short wall method:
In this method, the wall along the length of room is considered to be long wall while the wall perpendicular to long wall is said to be short wall. To get the length of long wall or short wall, calculate first the centre line lengths of individual walls. Then the length of long wall, (out to out) may be calculated after adding half breadth at each end to its centre line length. Thus the length of short wall measured into in and may be found by deducting half breadth from its centre line length at each end. The length of long wall usually decreases from earth work to brick work in super structure while the short wall increases. These lengths are multiplied by breadth and depth to get quantities.
b) Centre line method:
This method is suitable for walls of similar cross sections. Here the total centre line length is multiplied by breadth and depth of respective item to get the total quantity at a time. When cross walls or partitions or verandah walls join with main wall, the centre line length gets reduced by half of breadth for each junction. Such junction or joints are studied carefully while calculating total centre line length. The estimates prepared by this method are most accurate and quick.
c) Partly centre line and partly cross wall method:
This method is adopted when external (i.e., around the building) wall is of one thickness and the internal walls having different thicknesses. In such cases, centre line method is applied to external walls and long wall-short wall method is used to internal walls. This method suits for different thicknesses walls and different level of foundations. Because of this reason, all Engineering departments are practicing this method.

ESTIMATION OF BRICK WORK IN MASONRY BUILDING

CIVIL_ENGINEERING

In the procedure for the estimation of brickwork in masonry building, two approaches – Centre-line, and Long and Short-wall methods – are used. While in the case of centre-line approach (in straight forward cases), the total length dimension remains unchanged with the width and height of the masonry courses (in footings, plinth, and superstructure) varying according to the design as detailed in the given section of the wall; but the length dimensions (as well as width and height) in long- and short-wall method do register a change from course to course (Figure-1).
In fact there are no strict straightjacket rules for arriving at (taking out) dimensions from the plan, elevation, and sectional drawings – experience and suitability (vis-à-vis, each drawing) always guide one dividing a plan into parts so that the dimensions are easily worked out for ultimately computing the quantities.
Three procedures (for the sake of clear understanding of full basics of the mode of mensuration) however, are available concerning the quantification of foundation work – excavation, concreting and masonry – and superstructure in a given building as listed below:
(a) out-to-out and in-to-in method (i.e., long- and short-wall method),
(b) crossing method, and
(c) centre-line method.
Out-to-out and in-to-in method is the most commonly adopted procedure. Here the length of long walls (say for excavation purposes) are reckoned from out to out – AB in Figure 1 – and, the length of short walls measured in between the long walls in-to-in – EF.
These lengths shall, obviously, apply to foundation concreting also. The magnitude of these dimensions changes (in fact, decreases) for long walls, and increases for short walls at every change in the breadth (or, ray, thickness) of a course of brickwork:
AB will decrease to , i.e. for the first footing of the foundation masonry the length of long wall shall be (2 – 3). And EF shall increase by same amount, becoming , i.e. (5 – 3). Here, the width for excavation is b1and height (or thickness, vertically) is d1; while for the first footing, width = b2, and depth = d2. Similarly, in the plinth course (or for next footing if it is there), long wall length shall be [(10) – (11)], and short-wall length shall be [(15) – (16)] – width being b3, and depth = (d3+ d4). And for the superstructure, long-wall will have a length [(18) – (19)], and short wall length will be [(23) – (24)] – width being = b4, and height = height of the room from the top of DPC (of floor top) to the underside of roof slab (or whatever it is).
Here it is important to point out that the width and depth of excavation shall be b1and (d1+ d2+ d3), respectively; while for foundation concrete the values will be b1 and d1, respectively.

Fig.1: A Simple Rectangular Trench Plan of a Building and Section of Wall-cum-Foundation
It is obvious that with the decrease in the thickness of walls of a room, (i.e. proceeding up from the first footing towards the superstructure) the length of a long wall decreases, whereas the length of a short wall increases in accordance with the breadth (or, thickness as it is generally designated).
At the plinth level, the length of long wall = the length of the room (wall to wall, i.e. inner dimension plus twice the wall thickness; and the length of short wall = width of the room (inner dimension). If the thickness of the walls is different, the dimensions are reckoned accordingly.

ANALYSIS OF RATES FOR CIVIL ENGINEERING WORKS

CIVIL_ENGINEERING

In order to determine the rate of a particular item, the factors affecting the rate of that item are studied carefully and then finally a rate is decided for that item. This process of determining the rates of an item is termed as analysis of rates or rate analysis.

The rate of particular item of work depends on the following:
1. Specifications of works and material about their quality, proportion and constructional operation method.
2. Quantity of materials and their costs.
3. Cost of labours and their wages.
4. Location of site of work and the distances from source and conveyance charges.
5. Overhead and establishment charges
6. Profit
Cost of materials at source and at site of construction:
The costs of materials are taken as delivered at site inclusive of the transport local taxes and other charges.
Purpose of Analysis of rates:
Purpose of Analysis of rates:
1. To work out the actual cost of per unit of the items.
2. To work out the economical use of materials and processes in completing the particulars item.
3. To work out the cost of extra items which are not provided in the contract bond, but are to be done as per the directions of the department.
4. To revise the schedule of rates due to increase in the cost of material and labour or due to change in technique.
Cost of labour -types of labour, standard schedule of rates:
The labour can be classified in to
1) Skilled – 1st class
2) Skilled – 2d Class
3) Unskilled
The labour charges can be obtained from the standard schedule of rates 30% of the skilled labour provided in the data may be taken as Ist class, remaining 70% as II class. The rates of materials for Government works are fixed by the superintendent Engineer for his circle every year and approved by the Board of Chief Engineers. These rates are incorporated in the standard schedule of rates.
Lead statement: The distance between the source of availability of material and construction site is known as “Lead ” and is expected in Km. The cost of conveyance of material depends on lead.
This statement will give the total cost of materials per unit item. It includes first cost, conveyance loading, unloading stacking, charges etc.
The rate shown in the lead statement are for metalled road and include loading and staking charges. The environment lead on the metalled roads are arrived by multiplying by a factor.
a) For metal tracks – Lead x 1.0
b) For cartze tracks – Lead x 1.1
c) For Sandy tracks – Lead x 1.4

PREPARATION OF DETAILED CONSTRUCTION ESTIMATE

CIVIL_ENGINEERING

The preparations of detailed estimate consist of working out quantities of various items of work and then determine the cost of each item. This is prepared in two stages.
i) Details of measurements and calculation of quantities:
The complete work is divided into various items of work such as earth work concreting, brick work, R.C.C. Plastering etc., The details of measurements are taken from drawings and entered in respective columns of prescribed proforma. The quantities are calculated by multiplying the values that are in numbers column to Depth column as shown below:
Details of measurements form

ii) Abstract of Estimated Cost:
The cost of each item of work is worked out from the quantities that already computed in the detals measurement form at workable rate. But the total cost is worked out in the prescribed form is known as abstract of estimated form. 4%of estimated Cost is allowed for Petty Supervision, contingencies and unforeseen items.
Abstract of Estimate Form

The detailed estimate should be accompanied with:
i) Report
ii) Specification
iii) Drawings (plans, elevation, sections)
iv) Design charts and calculations
v) Standard schedule of rates.
Factors to be considered while Preparing Detailed Estimate:
i) Quantity and transportation of materials: For bigger project, the requirement of materials is more. Such bulk volume of materials will be purchased and transported definitely at cheaper rate.
ii) Location of site: The site of work is selected, such that it should reduce damage or in transit during loading, unloading, stocking of materials.
iii) Local labour charges: The skill, suitability and wages of local laboures are considered while preparing the detailed estimate.
Data for detailed estimate:
The process of working out the cost or rate per unit of each item is called as Data. In preparation of Data, the rates of materials and labour are obtained from current standard scheduled of rates and while the quantities of materials and labour required for one unit of item are taken from Standard Data Book (S.D.B).
Fixing of Rate per Unit of an Item:
The rate per unit of an item includes the following:
1) Quantity of materials & cost: The requirement of materials is taken strictly in accordance with standard data book (S.D.B). The cost of these includes first cost, freight, insurance and transportation charges.
ii) Cost of labour: The exact number of labourers required for unit of work and the multiplied by the wages/ day to get of labour for unit item work.
iii) Cost of equipment (T&P): Some works need special type of equipment, tools and plant. In such case, an amount of 1 to 2% of estimated cost is provided.
iv) Overhead charges: To meet expenses of office rent, depreciation of equipment salaries of staff postage, lighting an amount of 4% of estimate cost is allocated.

METHODS OF REINFORCEMENT QUANTITY ESTIMATION IN RCC STRUCTURE

CIVIL_ENGINEERING

Estimation of Reinforcement Quantity in RCC Structure
Estimation of steel reinforcement quantity is required for calculating cost of RCC structure along with other building materials as per construction drawing. Accurate quantities of the concrete and brickwork can be calculated from the layout drawings.
If working drawings and schedules for the reinforcement are not available it is necessary to provide an estimate of the anticipated quantities. The quantities are normally described in accordance with the requirements of the Standard method of measurement of building works.
In the case of reinforcement quantities the basic requirements are:
1. Bar reinforcement should be described separately by steel type (e.g. mild or high-yield steel), diameter and weight and divided up according to:
(a) Element of structure, e.g. foundations, slabs, walls, columns, etc., and
(b) Bar ‘shape’, e.g. straight, bent or hooked; curved; links, stirrups and spacers.
2. Fabric (mesh) reinforcement should be described separately by steel type, fabric type and area, divided up according to 1(a) and 1(b) above.

Methods of Reinforcement Quantity Estimation
There are different methods for estimating the quantities of reinforcement;, three methods of varying accuracy are:
Method-1 for Reinforcement Estimation
The simplest method is based on the type of structure and the volume of the reinforced concrete elements. Typical values are, for example:
Warehouses and similarly loaded and proportioned structures: 1 tonne of reinforcement per 105m3

Offices, shops, hotels: 1 tonne per 13.5m3

Residential, schools: 1 tonne per 15.05m3

However, while this method is a useful check on the total estimated quantity it is the least accurate, and it requires considerable experience to break the tonnage down to Standard Method of Measurement requirements.
Method-2 for Reinforcement Estimation
Another method is to use factors that convert the steel areas obtained from the initial design calculations to weights, e.g. kg/M2 or kg/m as appropriate to the element.
If the weights are divided into practical bar diameters and shapes, this method give a reasonably accurate assessment. The factors, however, do assume a degree of standardization both of structural form and detailing.
This method is likely to be the most flexible and relatively precise in practice, as it is based on reinforcement requirements indicated by the initial design calculations.
Method-3 for Reinforcement Estimation:
For this method sketches are made for the ‘typical’ cases of elements and then weighted.
This method has the advantages that:
(a) The sketches are representative of the actual structure
(b) The sketches include the intended form of detailing and distribution of main and secondary reinforcement
(c) An allowance of additional steel for variations and holes may be made by inspection.
This method can also be used to calibrate or check the factors described in method 2 as it takes account of individual detailing methods.
When preparing the reinforcement estimate, the following items should be considered:
(a) Laps and starter bars
A reasonable allowance for normal laps in both main and distribution bars and for starter bars has shall be considered. It should however be checked if special lapping arrangements are used.
(b) Architectural features
The drawings should be looked at and sufficient allowance made for the reinforcement required for such ‘non-structural’ features.
(c) Contingency
A contingency of between 10% and 15% should be added to cater for some changes and for possible omissions.

PREPARATION OF APPROXIMATE CONSTRUCTION COST ESTIMATE

CIVIL_ENGINEERING

Preliminary or approximate construction cost estimation is required for studies of various aspects of work of project and for its administrative approval. It can decide, in 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 necessity and utility of the project and with a site or layout plan. A percentage 5 to 10% is allowed for contingencies.

 Approximate Construction Cost Estimation Methods:
The following are the methods used for preparation of approximate construction cost estimates:

a) Plinth area method
b) Cubical contents method
c) Unit base method
a) Plinth area method:

The cost of construction is determined by multiplying plinth area with plinth area rate. The area is obtained by multiplying length and breadth (outer dimensions of building). In fixing the plinth area rate, careful observation and necessary enquiries are made in respect of quality and quantity aspect of materials and labour, type of foundation, height of building, roof, wood work, fixtures, number of storeys etc.
As per IS 3861-1966, the following areas include while calculating the plinth area of building:
Types of Estimates:
Area of walls at floor level.

Internal shafts of sanitary installations not exceeding 2.0 sqm, lifts, air-conditioning ducts etc.,

Area of barsati at terrace level: Barsati means any covered space open on one side, constructed on one side, constructed on terraced roof which is used as shelter during rainy season.

Porches of non-cantilever type.

Areas which are not to include
Area of lofts.

Unenclosed balconies.

Architectural bands, cornices etc.,

Domes, towers projecting above terrace level.

Box louvers and vertical sun breakers.

b) Cubical Contents Method:
This method is generally used for multi-storeyed buildings. It is more accurate that 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 Local Cubic Rate. The volume of 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 off set.
The cost of string course, cornice, corbelling etc., is neglected.
The cost of building = volume of buildings x rate/ unit volume.
c) Unit Base Method:
According to this method the cost of structure is determined by multiplying the total number of units with unit rate of each item. In case schools and colleges, the unit considered to be as ‘one student’ and in case of hospital, the unit is ‘one bed’. The unit rate is calculated by dividing the actual expenditure incurred or cost of similar building in the nearby locality by the number of units.

WORK CHARGED ESTABLISHMENT

CIVIL_ENGINEERING

WORKK CHARGED ESTABLISHMENT:

During the construction of a project considerable number of skilled supervisors, work assistance, watch men etc., are employed on temporary basis.

The salaries of these persons are drawn from the L.S. amount alloted towards

the work charged establishment. that is, establishment which is charged directly

to work. an L.S.amount of 1½ to 2% of the estimated cost is provided towards

the work charged establishment.

LUMPSUM

​LUMPSUM:

While preparing an estimate, it is not possible to workout in detail in case

of petty items. Items other than civil engineering such items are called lumpsum

items or simply L.S.Items.

The following are some of L.S. Items in the estimate.

1. Water supply and sanitary arrangements.

2. Electrical installations like meter, motor, etc.,

3. Architectural features.

4. Contingencies and unforeseen items.

Ingeneral, certain percentage on the cost of estimation is alloted for the

above L.S.Items

Even if subestimates prepared or at the end of execution of work, the

actual cost should not exceed the L.S.amounts provided in the main estimate.

DEFINITION OF ESTIMATION AND COSTING

CIVIL_ENGINEERING

​ DEFINITION OF ESTIMATING AND COSTING

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 avilable are less than the estimated cost the work is

done in part or by reducing it or specifications are altered, the following requirement are necessary for preparing an estimate.

a ) Drawings like plan, elevation and sections of important points.

b) Detailed specifications about workmenship & properties of materials etc.

c) Standard schedule of rates of the current year.

1.2 NEED FOR ESTIMATION AND COSTING

1. Estimate give 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 in the funds

available or not.

2. Estimate gives an idea of time required for the completion of the work.

3. Estimate is required to invite the tenders and Quotations and to arange

contract.

4. Estimate is also required to control the expenditure during the execution

of work.

5. Estimate decides whether the proposed plan matches the funds available

or not.

1.3 PROCEDURE OF ESTIMATING OR METHOD OF ESTIMATING.

Estimating involves the following operations

1. Preparing detailed Estimate.

2. Calculating the rate of each unit of work

3. Preparing abstract of estimate

1.4 DATA REQUIRED TO PREPARE AN ESTIMATE

1. Drawings i.e.plans, elevations, sections etc.

2. Specifications.

3. Rates.

DRAWINGS

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

1.4.2. SPECIFICATIONS

a) General Specifications: This gives the nature, quality, class and work and

materials in general terms to be used in various parts of wok. It helps no

form a general idea of building.

b) Detailed Specifications: These gives the 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.

1.4.3. RATES:

For preparing the estimate the unit rates of each item of work are required.

1. For arriving at the unit rates of each item.

2. The rates of various materials to be used in the construction.

3. The cost of transport materials.

4. The wages of labour, skilled or unskilled of masons, carpenters, Mazdoor,

etc.,

1.5 COMPLETE ESTIMATE:

Most of people think that the estimate of a structure includes cost of land,

cost of materials and labour, But many other direct and indirect costs included

and is shown below. The Complete Estimate

Cost of land P.s.and

contingencies 

at 5%

Legal expenses 

between owner’s

and contractor

Cost of Structure

Actual

cost of 

land

Cost of 

Surveying

Cost of 

Verification

of deeds and

execution of 

deeds

Brochorage

if any

Cost of 

labour

Permit fees for

constrution water, 

electricity from

concerned autorities

cost of 

materials

Consulting

Engineers

fees

cost for

preparation

of plan,

estimate and

design

Cost of

supervision