PRELIMINARY PROJECT ESTIMATES of project.pdf
- 1. PRELIMINARY PROJECT ESTIMATES M.D.N.Sisilaksha (B.Tech (Hons) in Quantity Surveying NCM-IQSSL)
- 2. PRELIMINARY PROJECT ESTIMATE/BUDGET/APPROXIMATE ESTIMATE An approximate estimate is an attempt to forecast the cost of a building at the inception of the project.
- 3. At the very inception of a construction project, the prospective Employer required know to what will be the budget/approximate estimate of the prospective project and other cost impacts to get a decision whether to go ahead or to amend the scope or to post phone or cancel the project.
- 4. The Estimator with his vast knowledge in construction cost and considering the present day market conditions and changes in social , political and economic conditions he will give his cost prediction.
- 5. TECHNIQUES OF PREPARING PROJECT ESTIMATES
- 6. The Cube Method This is the traditional method. When a contract was completed the agreed final account amount will be divided by the cubic content of the building . By this way the Quantity Surveyors are maintaining these information for future use. When an estimate was required for a new project of similar character ,the cubic metreage would be calculated & multiplied by and appropriate rate per cubic metre taken from a previously maintained records to obtain the approximated cost prediction of the proposed project.
- 7. After considering the present day market conditions and changes in social , political and economic conditions .
- 8. The following rules will apply when calculating the cubic content External area (from out side of the external walls to out side of the external walls) Height of the building is measured from top of foundation to 600 mm above the roof level if the roof is a flat roof and mid of the slope if the roof is sloping
- 9. However,the cube is unreliable unless two very similar buildings are being compared. Even then there are grave dangers, as shown in the following example concerning two factories “A” & “B”, shown in Fig. 2.
- 10. 5m 6m 30m 30m 36m 36m FIG. 2 Factories “A” & “B”
- 11. Factory “A” cost Rs.24,000,000/= & an estimate is required for factory “B” which is of similar design & construction but slightly larger. The cube of the first factory is 30m × 30m × 5m = 4,500m3, which when divided into the cost of Rs.24,000,000 gives a rate of Rs.5333.33 per cubic metre.
- 12. The comparable estimate for factory “B” using the same rate would be 36m × 36m × 6m = 7,776m3 × Rs.5333.33 = Rs.41,469,408 This is 72.79% more than the cost of factory “A”, but a quick inspection shows that the areas of floor & roof have only increased from 900 to 1296m2 and that the area of external walls has increased from 120m × 5m = 600m2 to 144m × 6m = 864m2; in both cases an increase of 44%.
- 13. As the building cost will be proportionate to the quantities of the floor, roofs & supports & walls (there are no other major components in this simple factory) it follows that factory “B” will cost Rs.24,000,000 plus 44%, that is Rs.34,560,000/=; & therefore the estimate of Rs.41,.469,000/= obtained by using the cube rate of Rs.5333.33 will be badly wrong. In fact the correct cube rate would be Rs.4444.44 (Rs.34,560,000÷ 7,776m3).
- 14. Would we have guessed this without working out the floor, roof & wall areas which fortunately are easy to calculate in this instance? This is the basic fault of the cube, that it is just a measurement of the air which the building displaces & does not truly reflect the quantity of labour & materials in forming the enclosures & divisions which generally have superficial & not cubic dimensions. As the size of a building increases or decreases the cubic measurement will fluctuate far more violently than the measurements of these components.
- 15. Another important objection to the cube as a means of expressing the size of a building is that it takes no account of the number of storeys. Let us consider a hypothetical office block 50m long, 15m wide & 30m high, containing seven storeys. The total cube (allowing 0.60m above the roof & 1.40m down to foundations) will be 24,000m3 & we may say that the cost will be in the region of Rs.300,000,000/= which is Rs.12500/= per cubic meter. But let as now suppose exactly the same building but with eight storeys each of 3.75m overall instead of seven storeys of 4.29m. The size has not changed, the cube is still 24,000m3.
- 16. It is immediately obvious that the same cube rate cannot be applied. There is more of everything in the second building; not merely the extra floor itself with its expensive floor & ceiling finishes but more doors, windows, skirting’s, additional toilets, extra lift landings, greater heating & lighting load, the strengthening of the structural frame & foundations to take the dead weight & superimposed loads of the additional floor. Once again the volume of space occupied by the building is no real guide to its cost.
- 17. The Superficial Area Method This is an alternative single-price rate method in which the total floor area is measured. The usually accepted rule is that the area of the building inside the external walls is taken at each floor level. It is a more recent method than cubing, & first came into use for schools local authority housing etc., where storey heights are constant. It is more accurate than the cube wherever storey heights are reasonably similar; in the case of the factory example which was worked out earlier in this chapter it would have given a completely accurate answer, although to an extent this is fortuitous.
- 18. Factory“A”costRs24,000,000;superficialarea900m 2 ;=Rs.26,667perm 2 Factory“B”superficialarea1,296m 2 @Rs26,667 =Rs.34,560,432 Inthecaseoftheofficeblockthesuperficialareamethodwouldhaveallowedfortheadditional floor: Seven-storeybuilding 7×750m 2 =5,250m 2 5,250m 2 @Rs57142 =Rs300,000,000/=. Eight-storeybuilding 8×750m 2 =6,000m 2 6,000m 2 @Rs.57142 =Rs342852000/=
- 19. This method has slightly over-valued the additional floor; the increase in cost will not be as high as the direct proportion of the floor area because some items of cost will hardly be affected, for example the area of the external walls has not altered. Nevertheless it has done much better than the cube method. The superficial area also has the virtue of being closer to the terms in which the client’s requirements are expressed; the accommodation is more likely to be related to floor area than to cubic displacement. Sometimes when a very early estimate is required the only data available may be the approximate floor area
- 20. However, the superficial area is very unreliable as a basis for the comparison of buildings such as churches which may have enormous differences in storey heights. A single storey church may be anything from 5m to 15m high & a rate based purely upon plan area will not enable this to be taken into account. The cube on the other hand will allow for the difference in height, though as usual it will exaggerate the cost effect. If the grandeur of the specification rises as well as the roof (there is often a tendency towards this) the cube will appear to be doing quite well!
- 21. The Storey-Enclosure Method About twenty years ago it began to be painfully apparent that the traditional methods of single price estimating were not producing sufficiently accurate results, & a number of quantity surveyors who were worried about this set up a study group. The fruits of their labours appeared in a paper entitled “A New Approach to Single Price-Rate Approximate Estimating” which was read to a General Meeting of Quantity Surveying members of the R.I.C.S. in April 1954 by Mr. J.G. Osborne, a member of the group. The paper which was extremely interesting & is still worth reading by anyone interested in the problems of approximate estimating,* introduced the “Storey-Enclosure System”.
- 22. In this method the areas of the various floors, roofs & the containing walls are measured, each is weighted by a different percentage & the resultant figures totalled to give the storey-enclosure area. The rules are briefly stated here in order to work out an example: 1) Twice the area of the lowest floor. 2) The area of the roof measured on the plan. 3) Twice the area of the upper floors, plus an addition of 15% for the first floor, 30% for the second floor, 45% for the third & so on. 4) Area of external walls.
- 23. Let us apply this method to our factory & office block examples. Factory “A” m2 m2 Lowest floor = 900 ×2 = 1,800 Roof = 900 ×1 = 900 External walls = 600 ×1 = 600 Storey-Enclosure Area = 3,300 Cost = £45,000 representing £13.6364 per square metre of storey-enclosure area. Factory “B” m2 m2 Lowest floor = 1,296 ×2 = 2,592 Roof = 1,296 ×1 =1,296 External walls = 864 ×1 = 864 Storey-Enclosure Area = 4,752 4,752m2 @ £13.6364 = £64.800.
- 24. As we have seen previously this is the correct answer. Let us now tackle the office block, noting that unlike either of the previous methods this will allow both for the additional floor &for the unchanged external wall area. It also makes an allowance for the extra cost of high building.
- 25. Sevenstoreyofficeblock m2 m2 Lowestfloor 750 ×2 1,500.0 Firstfloor 750 ×2.15 1,612.5 Secondfloor 750 ×2.30 1,725.0 Thirdfloor 750 ×2.45 1,837.5 Fourthfloor 750 ×2.60 1,950.0 Fifthfloor 750 ×2.75 2,062.5 Sixthfloor 750 ×2.90 2,175.0 Roof 750 ×1 750.0 Externalwalls 3,900 ×1 3,900.0 Story-EnclosureArea 17,512.5 Cost=£504,000representing£28.78persquaremetreofstoreyenclosurearea.
- 26. Eightstoreyofficeblock m2 m2 Lowestfloor 750 ×2 1,500.0 Firstfloor 750 ×2.15 1,612.5 Secondfloor 750 ×2.30 1,725.0 Thirdfloor 750 ×2.45 1,837.5 Fourthfloor 750 ×2.60 1,950.0 Fifthfloor 750 ×2.75 2,062.5 Sixthfloor 750 ×2.90 2,175.0 Seventhfloor 750 ×3.05 2,287.5 Roof 750 ×1 750.0 Externalwalls 3,900 ×1 3,900.0 Story-EnclosureArea 19,800.0 19,800.0m2 @£28.78=£569,844.
- 27. This is more likely than the £576,000 obtained by the superficial area method; it will be remembered that it was impossible to obtain any real comparison between the two office buildings using the cube method. Here was a system representing an enormous advance on anything existing previously; a system which took into account plan shape (a very important factor), storey heights & overall building heights. A table submitted with Mr. Osborne’s paper showed that when a number of buildings of all sorts had been investigated the new system gave reasonably consistent result in 60 cases, as against 51 with the superficial area method & 43 with the cube. (It was suggested that lifts & other engineering services should be excluded from the calculation & worked out separately).
- 28. Elemental Estimating Although the storey-enclosure method brought increased accuracy it was still no more possible to relate the working drawings (when received) to the estimate than it had been with the earlier methods. The omnibus “rate” bore no definite relationship to the actual cost of any of the measured areas but was only a theoretical average of them all. In the discussion which followed Mr. Osborne’s paper, James Nisbet (who subsequently became Chief Quantity Surveyor to the War Department) put his finger on the only real weakness in the storey-enclosure approach to the problem. He asked whether (if the walls, floors & roofs were all going to be measured separately) it would not be simpler & more accurate to price each separately according to an assumed specification.
- 29. This basically is the “elemental” approach, whereby the building is split up it to a number of major components, or elements, which can be easily measured from sketch drawings & which are priced separately; these elemental costs are added together to give the total of the estimate. It is therefore possible to compare the working drawings with the estimate by considering each element in turn instead of only being able to compare the whole building, & the cost can be watched as the design develops.
- 30. 8000 15000 300 Specimen 300 3000 PLAN 1000 X 1000 Aluminium glazed window 1000 X 2100 Aluminium glazed door 230 mm wall thicknes s ELEVATION
- 34. Functional Unit Method The Quantity Surveyor has to predict the future cost of some sort of projects without seen a sketch drawing. Using the predetermined similar types of unit rates he will predict the future cost. Water tanks/ Water pumps - Using Literage Roads - Mileage/ Kilo mate Hotels - Rooms Pavilions - No of users Cinemas - Seats Schools - Desks Hospitals - Beds
- 35. Updating Predetermined Information Whatever above method used when arriving preliminary project estimates the Quantity Surveyor has to modify his predetermined data considering the following. 1. Market conditions 2. Size, number of storeys, etc. 3. Specification level 4. Inclusions & exclusions 5. Services 6. Site & foundation conditions 7. Other factors
- 36. 1. Market conditions This adjustment deals with the changes in building prices & in tendering conditions between the time when the previous job was priced & the anticipated date of tender for the new project. There are various published indices of building costs which will help the surveyor to calculate the right allowance, & we will deal with these first. 2. Size, number of storeys, etc. The new provincial block will have a larger plan area; this will tend to reduce the proportion of external wall area to floor area (assuming a similar plan shape) & hence should reduce the cost per square meter a little.
- 37. 3. Specification level The specifications will be roughly similar, but the architect has decided that the cheep flooring used in the officers on the London block was a mistake & wants to allow an extra Rs.225 per m2 for this. The client also wants Portland stone facing instead of cast stone used on the previous job. 4. Inclusions or exclusions The client wants the office tenants to do their own partitioning & decorations, whereas on the London job these were provided by the building owner.
- 38. 5. Services Services are the major part of a modern building, & represent a large proportion of the cost. On a project of any size there will most likely be consulting engineers for heating, plumbing & electrical work, & they will normally provide an estimate of the cost of the services for which they are responsible. 6. Site & foundation conditions Both sites are in congested city areas. The new provincial job will probably required pile foundations, which must be allowed for. Either the cost of pile foundations per square metre can be obtained from a suitable analysis or a certain no of piles.