In any construction activity, two basic things are involved, namely, the quantity aspect and the quality aspect. Te quantity aspect is governed by the study and analysis of drawings which are prepared with respect to the design of the project. The quantity aspect covers the quantum of works involved in and estimation of the materials and labours required for construction. The quality aspect is governed by the specifications for the materials and workmanship. These two aspects lead to an estimate for a work or a project. Thus an estimate is a tool for planning and controlling the construction activities of any project with regard to quality, quantity, time and costs.
An estimate of a project is a forecast of its probable cost for the due fulfillment of the project objectives, to the prescribed workmanship covered by specifications for various items of works and to the stipulated time schedule. For a given project fully described by drawings and specifications, estimating involves quantity take off for various items of work, rate analysis and costing.
In general, estimates can be broadly divided into two categories- detailed and approximate or preliminary. Strictly speaking, all estimates are more or less approximate as the very name implies, and the actual cost of construction would only be known when a work has been completed in all respects and all costs allocated. However, a detailed estimate is prepared on the basis of detailed drawings supported by specifications in accordance with established norms of measurements. Using unit of costs for the different items of work arrived at by rate analysis. Thus, a detailed estimate gives a realistic estimate of final cost and is generally required for obtaining sanction/approval of sponsor of the project or competent authority in respect of public bodies like govt. organizations. It is also used in the course of the execution of the work to control progress and costs.
The detailed estimate will compromise:
1. The direct cost of various items of work;
2. Provision for contingencies;
3. Direct supervision charges
4. Miscellaneous costs such as service charges for water supply connection, sanitary arrangements and electric supply;
5. Labour hutments and camps, field office, etc.
Preliminary Estimates
An approximate or preliminary estimate, or an abstract estimate as it is sometimes referred to, is prepared before a detailed estimate is taken in hands for one of the following reasons:
1. To obtain a rough idea of the cost as part of a preliminary study for feasibility.
2. To rank competing projects for allocation of funds, cost-wise, as part of an investment decision exercise.
3. To make advance arrangements for public utility projects.
4. To fix premia for insurance against risks etc.
In such cases, the cost involved in preparing a detailed estimate may not be justified or the time involved would be substantial or data may not be available or preparation of a detailed estimate may not be otherwise justified.
The approximate costs are prepared based on unit costs of major sub-works established by a survey of costs of similar completed sub-works. The unit costs are updated using cost indices published by govt. bodies or reputed bureaus. For instance, the Central Public Works Department have prescribed cost indices for various cities based on 100 for a particular base year for buildings. The actual costs are known for a particular year from records, then the current costs would be determined using the appropriate indices.
Some normal practices employed for preliminary cost estimates are given below:
i) Projects –based on unit costs
Example:
Cost of construction of a classroom – Rs.100000/-
Total number of classroom planned - 12
Cost of primary school building -Rs.12,00,000/-
Example:
Cost of one furnished room in a 3-star hotel- Rs. 3 Lakhs
Total number of rooms - 100
Cost of hotel - Rs.3 crore
ii) Buildings – based on area.
Example:
Cost of hospital, fully equipped per square meter of floor area- Rs. 3,500/-
Total floor area - 3000 sq.m
Cost of hospital - Rs. 1.05 crores
iii) Bridges- based on estimated quantities
Example:
Cost of a 30m railway B.G. girder bridge (2 panels)- steel work
Steel work weight= 60x2= 120 tonnes
Cost of supply, fabrication and erection per tonne- Rs. 12000/-
Cost of steel work - Rs.14.4 lakhs.
Quantity takeoff
Different countries follow different procedures for taking out quantities. The method given below is generally followed by the public works department in India.
The processes involved are:
· Taking-off quantities
· Grouping, and
· Billing
The takeoff and grouping are carried out on the quantity sheet. The measurements are recorded item wise and the items are arranged in the sequence of execution.
A typical quantity sheet will be as under:
S.No. / Description of item of work / Unit of measurement / Number / L / B / H / Quantity / TotalBilling is done in an abstract form known as Bill of Quantities (BOQ) where costing is done. A typical bill of quantities will be as under:
S.No. / Description of item work / Measurement unit / Quantities / Rate / AmountThe unit of measurement of a particular trade of construction activity depends upon its nature, size and shape. The general principles for decides upon the measurement unit areas follows:
(i) massive, voluminous, bulky items measured by cubical contents.
Example- foundation concrete, brickwork masonry.
(ii) Thin, surface oriented items with large superficial areas measured in terms of area
Example: tile roofing, mosaic tile flooring.
(iii) Items which are long, narrow and thin, having dimensions in other perpendicular directions difficult of measurement, measured in linear units.
Example: hand railing
(iv) Items which are difficult of measurement and which are distinct units, measured in units
Example: supply and fixing wash basin
(v) Items which are heavy and are generally difficult of measurement in terms of linear dimensions, measured by weight.
Example: Steelwork
Indian standard Institution have standardized the units of measurement and these are given in their code IS: 1200 (Parts 1 to 25)
General Rules for Measurement
(i) The description of items should be self-contained and self explanatory and should include all materials, labour, transport, cost of tools and plant, formwork, etc.
(ii) The dimensions should be entered in the order of length, breadth, and depth or height or thickness.
(iii) The measurement for the same item of works under differing conditions should be entered separately, bringing out clearly the differences in the descriptions.
(iv) The measurement should be for finished items of work.
(v) The method of measurement should be consistent and should reflect the special provisions, if any, in the specifications.
(vi) The nomenclature of an item of work should be such as to bring out any special features involved in the execution of work and establish a link with the specifications.
Degree of Accuracy
The degree of accuracy of measurements would depend on the unit of measurement and rate for a particular item.
Rate Analysis
The process of determining the rate of an item is termed as rate analysis. A rate analysis should take into account:
i) Materials incorporated in the works
ii) Direct labour required for the output
iii) Incidence of costs of tools, plant, machinery, ancillary requirements such as formwork, etc.
iv) Direct overhead such as contractor’s site supervision
v) Costs arising out of site conditions
vi) Costs arising out of requirements of specifications (example- curing of concrete)
vii) Contractor’s profile and overheads.
1. Work in Extreme Weather Conditions:
Concreting operations done at atmospheric temperature above 400 C, need special attention. IS 7861(part-1)-1975 gives the recommended practices that would result in concrete possessing improved characteristics in the fresh as well as hardened state. Good practices of concreting require special care with respect to following:
1. Temperature control of concrete ingredients.
1. Aggregates- stored under shade or cooled by water
2. Water- used in the form of ice or in near freezing temperatures.
3. Cement- Temperature restricted to 770 C
2. Mix Design- use low cement content and cements with low heats of hydration. Use approved admixtures for reducing the water demand or for retarding the set.
3. Production and Delivery:
1. Temperature of concrete at the time of placement should be below 400 C.
2. The mixing time should be held at minimum, subject to uniform mixing.
3. Period between mixing and delivery should be kept to a minimum.
4. Placement and Curing:
1. Prior to placing concrete formwork, reinforcements and subgrade should be kept cool by spraying with cold water first. If possible, concreting may be restricted to evenings and nights.
2. Placement and finishing should be speedy.
3. Immediately after compacting and finishing, concrete should be protected from evaporation of moisture.
2. Under-water Concreting:
Inspection of concrete during placement under-water is difficult. Therefore, it is essential to evaluate the proposed mix proportions, inspect the equipment and review preparation prior to the start of underwater concreting.
Underwater concrete should have a slump of 100 to 180mm. The water cement ratio should not exceed 0.6 and may need to be smaller, depending upon the grade of concrete or the type of the chemical attack. For aggregates of 40mm maximum particle size, the cement shall be atleast 350kg/cubic m of concrete.
Cofferdams or forms shall be sufficiently tight to ensure still water if practicable, and in any case to reduce the flow of water to 3m/min through the space into which concrete is to be deposited. Coffer dams or forms in still water shall be sufficiently tight to prevent loss of mortar through the walls. De-watering by pumping shall not be made while concrete is being placed or until 24 hours thereafter, otherwise it may disturb the concrete and may lead to undesirable results.
Concrete cast under water shall not fall freely through the water. Otherwise it may be leached and become segregated. Concrete shall be deposited by, continuously until it is brought to the required height. While depositing, the top surface shall be kept as nearly level as possible and the formation of seams avoided.
The methods to be used for depositing concrete under water shall be one of the following:
1. Tremie
2. Direct placement with pumps
3. Drop bottom bucket
4. Bags
5. Grouting
The void content of the coarse aggregates should be kept as low as possible. The code assumes a maximum void content of 55 percent.
3. Concrete in Sea Water:
In addition to the grade of concrete specified, it will be necessary to control the minimum cement contents and the maximum water-cement ratio.
Portland slag cement may be used but it will be necessary to seek specialists advice.
Precast members are to be preferred because then it will be possible to achieve dense concrete and eliminate those with porous or defective concrete by inspection before installation. Unreinforced elements should be used if practicable, as reinforcing steels are susceptible to corrosion caused by chlorides present in sea water.
Construction joints are potentially weak and the problems of durability are accentuated in the zone subject to alternate drying and wetting that is, between upper and lower planes of wave actions.
IS 4082-1977 recommends a coat of cement water over the reinforcing steels stored in coastal areas.
4. Concrete In Aggressive Soils And Water
This refers to concrete placed in soils and waters, containing sulphates, nitrates and other salts which may cause deterioration of concrete. Naturally occurring aggressive chemicals such as sulphates of sodium and magnesium, are sometimes found in soils and waters. Sea water is mildly aggressive to concrete because of soluble sulphates it contains. The decomposition of sulphide minerals contained in colliery waters may cause the formation of F2SO4 which can cause severe sulphate attact. Durability problems may arise also when concrete is exposed to acids.
Two types of precautions given in the code:
1. Those in the proper attention to the concrete itself will provide sufficient immunity.
2. Those in which additional precautions are to be taken to prevent contact between the aggressive chemicals and the concrete
There are different modes of failures of structures due to various reasons. Here we will discuss about the different modes of failures of structures and its reasons. The different modes of failure are, failure due to:
- Buckling
2. Creep
3. Fatigue
4. Fracture
5. Impact
6. Rupture
7. Thermal shock
8. Wear
9. Corrosion
10. Yielding
11. Overload
· Buckling
Buckling is a failure mode characterized by a sudden failure of a structural member subjected to high compressive stresses. Where the actual compressive stress at the point of failure is less than the ultimate compressive stresses that the material is capable of withstanding. This mode of failure is also described as failure due to elastic instability. Mathematical analysis of buckling makes use of an axial load eccentricity that introduces a moment, which does not form part of the primary forces to which the member is subjected.
Buckling of Columns
The ratio of the effective length of a column to the least radius of gyration of its cross section is called the slenderness ratio (sometimes expressed with the Greek letter lambda, λ). This ratio affords a means of classifying columns. All the following are approximate values used for convenience.
· A short steel column is one whose slenderness ratio does not exceed 50; an intermediate length steel column has a slenderness ratio ranging from 50 to 200, while a long steel column may be assumed to have a slenderness ratio greater than 200.
· A short concrete column is one having a ratio of unsupported length to least dimension of the cross section not greater than 10 12 as per IS 456: 2000). If the ratio is greater than 10 (12 as per IS Code 456:2000), it is a long column (sometimes referred to as a slender column).
· Timber columns may be classified as short columns if the ratio of the length to least dimension of the cross section is equal to or less than 10. The dividing line between intermediate and long timber columns cannot be readily evaluated. One way of defining the lower limit of long timber columns would be to set it as the smallest value of the ratio of length to least cross sectional area that would just exceed a certain constant K of the material. Since K depends on the modulus of elasticity and the allowable compressive stress parallel to the grain, it can be seen that this arbitrary limit would vary with the species of the timber. The value of K is given in most structural handbooks.