Reasons, terms and tools for material take-off
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[edit] Introduction
The variety of methods used to calculate the amount of material used in a design, building or other structure vary in methodology, accuracy, the information they provide, what they are needed for, the stage of procurement and the tools being used to measure the materials.
[edit] Different reasons for taking off
Take off or taking off construction works in most cases refers to the process of identifying elements of construction works that can be measured and then priced. The process of measurement traditionally would be from the set of project drawings and can be by number, length, area, volume, weight or time, which are then collated.
The primary reasons taking off and calculating the material required for a building is to assist in quantity orders and for price estimates and tendering. However more recently with the higher levels of interest in the embodied carbon investment in materials used in buildings as a whole, material take off has also been used in the calculation of embodied carbon. Furthermore through greater consideration to whole life carbon and circular economy issues, as opposed to only capital carbon investment, detailed material quantity calculations have a further role to play in consideration of replacement cycles, end of life scenarios and their impact on embodied carbon calculations.
[edit] Different terms relating to taking off
There are many different informal terms, used to describe the different methods of estimating which are often country specific, as well as a number of different more formal terms used in guidance, such as for example in the New Rules of Measurement (NRM) or Standard Method of Measurement (SMM7) which are most relevant to UK construction. Here, order of cost estimating refers to methods used prepare order of cost estimates and cost plans, whilst detailed cost estimating is used in the preparation of bills of quantities, quantified schedules of works and schedules of rates in order to obtain tender prices.
Other terms are also used to describe different methods of calculating quantities for buildings or civil engineering projects such as;
- Out to out and in to in method, most detail and similar detailed cost estimating
- Bay method, used by repetition of identical elements
- Service unit method, similar to bay but based on service provision such as number of beds.
- Centre Line Method, used for symmetrical construction on the drawn basis.
- Crossing method, most similar to cost estimating by length, depth, width etc.
[edit] Different tools for taking off
In projects prepared by hand the project drawings are most likely to be scaled sheets of two dimensional plans, sections and elevations, whereby scale measurements are taken to give lengths, thickness', widths and areas of the proposed building. Detailed measurements this way can be a lengthy process which is why alternative rates methods are often used along side other project documentation to make calculated estimates.
In terms of 2D CAD files the same is likely to apply with project drawings most likely to be broken down into two dimensional plans, sections and elevations, but in this case because of the digital format, lengths, thicknesses, widths and areas can be relatively easily calculated digitally which speeds up the process significantly. The supporting information found in the project documentation is likely to be separate but in some cases can be more easily directly associated to the drawings by means of staged packages.
In terms of 3D CAD where a three dimensional model of the building is created, the 2 dimensional sections, elevations and plans can more easily be extracted from the three dimensional model, as can the lengths, thicknesses, widths and area information, as well as volumetric information relating to three dimensional solids and voids. Material identification is possible through coded elements or layers which then relate to separate inventories alongside the project documentation. However in some cases the project documentation can be more easily integrated with the three dimensional model if it is digitally created in stages of construction.
In terms of building information modelling (BIM) the possibilities for greater integration between project programme, project documentation and detailed quantities are vastly increased as any element of three dimensional drawing can have any number of associated variables directly linked within the model. In terms of material take off this means that different solids and volumes can have associated variables alongside dimensional information through families or elements within the model. For example a solid can be associated with a material, which is given a certain density, allowing the weight to be calculated which in turns allows various estimations to be calculated directly, such as the recycled content which might be calculated by volume of material (as opposed to weight).
[edit] Related articles on Designing Buildings
BIM Directory
[edit] Building Information Modelling (BIM)
[edit] Information Requirements
Employer's Information Requirements (EIR)
Organisational Information Requirements (OIR)
Asset Information Requirements (AIR)
[edit] Information Models
Project Information Model (PIM)
[edit] Collaborative Practices
Industry Foundation Classes (IFC)
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