APPROXIMATE ESTIMATE AND ITS TYPES

APPROXIMATE   ESTIMATE

Approximate Estimate (ing) is a challenging task, if only that accuracy is required when the cost information available is scant. Various approximate estimating techniques exist for arriving at desired answers.

 

The different types of cost estimates, those are prepared during various phases of a project are already stated in Lecture 2 of this module. As already mentioned in Lecture 2 of this module, the approximate estimates (also known as order-of-magnitude estimates) are prepared during initial stages of the project life cycle. The different methods used in the preparation of approximate estimates are described below. In addition to use in the preparation of estimates during early stages of project development, these methods may also be sometimes used for cost estimating in the detailed design phase.

 

INTERPOLATION METHOD: Interpolation is a techniques used in the early stage of the design sequence when information drawn or otherwise on the proposed project is in the short supply .It requires a good deal of skill and experience and is the process of adding in or deducting from the cost analysis figure to arrive at a budget for a new project. Therefore in preparing a budget for a new project assume, a cost analysis has been chosen as the basis for the estimate as the basics for the estimate. However cost analysis will contain items that are not required for the new project and these must be deducted, for example, in the new project the client wishes to exclude the installation of air conditioning, included in the cost analysis project, from the estimate and this will have to be deducted from the budget; but on the other hand the client wishes to include CCTV thought and added in it is important as describe later, to adjust cost to take account of difference in price level. The process continues until all identified difference have been accounted for.

UNIT METHOD: The unit method is a single price rate method based upon the cost per functional unit of the building, a functional unit being, for example, a hotel bedroom. The method is often regard as a way of making a comparison between building in order to satisfy the design team that the cost are reasonable in relation to other building of a similar nature. It is not possible to adjust the single rate price and therefore is very much a ball park approach. Suitable for client who specialise in one type project; for example, a hotel or supermarket chains, where it can be surprisingly accurate. Other example where unit cost may apply are:

• School – cost per pupil
• Hospital – cost per bed space

SUPERFICIAL METHOD: The superficial method is single price rate method base on the cost per square metre of the building. The use of this method should be restricted to the early stage of design sequence and is probably the most published cost data is express in this form. The method is quick and simple to use through, as in case of unit method, it is important to use data form similarly design project. Another advantage of the superficial method is that the unit measurement is meaningful to both the client and the design team. Although the area for method is relatively easy to calculate, it does require skill in assessing the price rate. The rules for calculating the area:

• All measurement is taken from the internal face of external wall. No deduction is made for internal walls lift shaft, stairwells e.t.c
• Where different parts of the building vary in functional, then the areas are calculated separately.
• External works and non-standard items such as pilling are calculated separately and then added into the estimate figure for specialist works may be available for sub-specialist contractor and specialist contractor.

APPROXIMATE QUANTITIES: Regard as the most reliable and accurate method of estimating, providing that there is sufficient information to work on. Depending on the experience of the surveyor, measurement can be carried out fairly quickly using composite rate to save time. The rule of measurement are simple, although it must be said, they are not standardised and tend to vary slightly from one surveyor to another

• Approach involves grouping together items corresponding to a separate operation and relating them top a common unit of measurement unlike the measurement for a bill of quantities, where items are measure separately.
• All measurement are taken as gross over all but the very lager opening
• Initially, the composite rate requires time to build up rate, but one calculated they may be used on a variety of estimating needs

CUBICAL METHOD: In this method, cost per unit volume is taken as the criterion for rough estimate. Approximate cost of proposed building = cubical content of building multiply by cost per unit volume of similar structure.

In arriving at the cubical content, no hard and fast rules are as follows

1. Whether the space of open veranda, porch, should be taken into account,
2. How to measure the height of the building and
3. Whether to take into account the basement and the foundation and parapet above ceiling.

The RIBA (Royal Institute for British Architect) rules that are generally followed give the followings procedure.

Cubical content = Plan area measure over the external walls x height from top of concrete foundation footing to halfway up the roof if pitched, or to 600cm above the roof if flat

Generally the cubical method can be said expected to give more accurate estimate for single storey building than the area method, as it takes into accounts the ceiling height ; But the cubical method is very unreliable unless two very similar building are compare for example comparison of geometrically proportionate building can give a very wide fluctuation and lead to an unreal estimate. For as all the dimension (size) of the building increase or decrease proportionately, the cubical content fluctuate for widely than the area of the component structure. Also the volume method does not take into account the number of floor or storey. Also such items, as lift are generally, included as lump sum since their cost to not bear any relation to their volume

TYPICAL BAY METHOD: This method is useful for application in frames structure consisting of a number of similar bays, as for example, industrial building. A typical interior bay (between to successive columns) is selected and its total cost worked out.

Then the approximate cost of the building = number of bay in the building x cost of one bay suitable correction factors may have to be used to taken into account the dissimilarities in end bay (and in different floor, if applied to multi storied construction)