The quantity take-offs generated from BIM models are quick gain and one the most well-known BIM uses.
This is often the first step in the process of Bill of Quantities preparation as well as actual estimation of quantity of material delivered to construction site. So in most cases the data generated from the BIM model requires further processing before it becomes the valuable information.
Way 1- Quantities generated using schedule functionality of BIM (native) software.
The main concept is dead simple. When we have the model of planned or existing building, we are able quickly to extract exact volume of concrete used or number of doors per floor, assign unit prices to them and have a very early cost estimates prepared .
If the BIM model is built in accordance with, and periodically checked against earlier agreed rules ,the scope of quantities taken from it is predictable. We know what data will be available at a given stage. The rules should cover at last modelling principles, naming and LOD requirements.
The most common BoQ position we can get this way are: number of furniture pieces, light points, floor area of a room, length of steel structural elements like beams and roof trusses or volume and dimensions of prefabricated concert beam. We must acknowledge though ,that this approach give us the actual data, which does not always transfer to the quantities purchased by the contractor.
Way 2- Estimation prepared using coefficient.
Great number of material quantities delivered or removed from construction site require incorporation of technological correction.
That accurate data described in Way 1 is excellent starting point but often require incorporation formulas with wastage coefficient (usually using one of specialised software).
This way, we can rapidly get volume of concrete needed for pouring, length of pipes cut on site or volume of paint.
There are also less intuitive cases. For example: the volume of soil removed during excavation work does not equal volume of the designed excavation. Number of track loads planned to remove the soil from the site will be too small if the soil loosening coefficient is not applied in the process. The fact is obvious for any QS (Quantity Surveyor) however I took this example from the real situation.
Way 3- Estimation using professional judgment of quantity surveyor
The situation is different when BIM model does not provide possibility of direct quantity generation. It can happen because of early stage of the BIM model and/or project development ,when information is not available yet or nature of the BoQ position i.e.: time of manual labour.
Despite some software attempts to automatise this stage, the Quantity Surveyor professional knowledge and judgment is essential to get reliable figures. Items such as complex or uncommon type of structure or unique building function may ruin estimation based purely on mathematical formulas.
To benefit from BIM based quantity take off, it is paramount that all involved parties are aware of origin of the quantities and its accuracy.
So, when the BIM software generated quantities cover precise amount of material, allowance needs to be added for wastage. Likewise, when the estimator included wastage in his calculation it needs to be clearly communicated and understood so that the quantities extracted from the model are not contested.
One of the main advantages of extracting quantities form BIM model is its speed and possibility of doing it on demand, allowing for tracking changes in amount during design development.
Unlike in the case manual take off, quantities generated directly from the model can be prepared within minutes or hours instead of days or weeks. In both cases though this is only a starting point for the cost estimation, regardless if we are working on the early budget estimation or preparing material for tender.