BIM based Quantity Takeoff – A Review
While Building Information Modeling (BIM) based quantity takeoff is potentially one of the most benefiting applications for BIM, it is still not as frequently used. In this article, let us look at how under-explored yet potentially profitable BIM based QTO is, while simultaneously discussing some of its limitations.
As the construction industry already uses BIM for building projects, there is mounting pressure for cost estimators to manage estimation using BIM for quantity takeoffs. Working with BIM has many benefits such as –
- Consistency and linking of drawings.
- Up-to-date material schedules with every recorded change incorporated in the model.
- Co-ordination amongst building objects to detect clashes between design components.
- Ability to exhibit as a single 3D model as compared to an assembly of 2D illustrations.
- Better information.
The impending question is whether BIM really is the harbinger for quantity takeoff? For BIM to be an alternative to using quantity takeoff software, it is necessary to ascertain if it can support the variations as a two-dimensional drawing. When it comes to designing, planning & constructing a project, BIM is a multi-faceted method to trade & combine information that follows an object-oriented parametric modelling approach.
The following issues concerning BIM make it a debatable arrangement to be used for quantity take off:
- Is the use of BIM method more convenient compared to other conventional methods?
- How accurate is the quantity take-off created via BIM tool?
- If not, what are the deviations from the regular methods, that cause the mistakes?
- How much time is saved when compared to the traditional method?
Answering the questions posed above, more than 75% of contractors still rely on a dated combination of traditional data collection & spreadsheets for their estimating. Although all trade estimators can use BIM, many times they would want to use the two-dimensional drawing to better understand the nuances of what the BIM model represents. With a scaling majority now moving to quantity takeoff tools for the quantity markups, the polar shift to the digital world of estimation isn’t very distant.
Quantity take-off using BIM is all about:
- Visualizing the project using a 3D illustration
- Evaluating the model to display the items being quantified
- Sorting the list of materials in the model for better estimation of quantities
- Segregating the objects in the 3D view to tally the quantification
- Understanding the data provided
Related Read- Transformation of the Construction Industry – An Outline
BIM system generates a takeoff by the act of designing the building with “smart objects” instead of established lines. BIM technologies include collections (libraries) of 3D objects with relevant data that can be brought into the drawings.
While it is possible to modify the models to extract quantities according to given specifications for manual-input based measurements, the adjustments do not come without their implications on other model applications such as drawings or 3D visualizations. Takeoff specifications should hence be revised in order to align with BIM features and thereby reducing their implications.
Discussing the outcomes of this BIM based QTO (Quantity take off), can be helpful in proposing ways to solve the shortcomings particularly from a contractors’ point of view. Spreadsheets, specialized estimating applications can greatly enhance an estimator’s productivity.
In today’s date, it is highly unlikely to expect the industry to resort exclusively to manual calculations, complex spreadsheets, and the cumbersome revision methods for cost estimating. But it is nearly impossible for an individual designer to include the specializations of each trade. Designers deal with the drafting & publishing of drawings. The skills of the draftsmen’s drawing representations have direct impact the quality of work & results anticipated from the BIM product. Although drawing errors lead to calculation-based errors, there is no reliable way to ascertain the point of occurrence of these errors. The window for human interpretation errors is narrow when the professionals’ are skilled but their effects on the final product cannot be neglected. As quantity takeoff is not a basic function of BIM, prior experience of working on BIM is required to properly perform takeoffs.
In common knowledge, it is believed that BIM is more effectively used only for modeling standard designs & not the complex ones. To confront this belief, the primary task is to contemplate the fact that all parts in BIM are related & require 100% modeling which is not subject to the complexity of the design but to the efficiency of the handler/designer. Therefore, this breach between precise representations of building & actual structure has to be filled with technological assistance & manual inputs.
In the cost estimates prepared using a BIM model, objects have descriptive data “attached” so when the object is brought into the drawing, the attached data is automatically brought into the project. This helps in bringing up to an 80% reduction in the time to generate estimates. Outcomes of this product-model-based-system that is BIM, result in savings of up to 10- 15% of total building costs.
Alternatively, there are BIM tools that allow quantity take-off directly from a model. Such a quantity take-off is based on the geometrical properties of the model. It provides information on the surface area, volume, and other dimensions. BIM as a tool for quantity take-off should be used for rendering and streamlining of cost estimation at all stages of the construction.
On the one hand, where BIM has immense versatility; Construction professionals, in this case, estimators might come across certain challenges while adopting BIM for their work, such as –
- Overcoming the resistance to change in technology.
- Developing a new work process from the existing design process.
- Training for the new software & other technicalities.
- Plush hardware.
- Larger files
Nevertheless, as project cost estimates are the criterion to assess the feasibility, productivity, analysis, financial requisites, budgeting & approval, bidding, warranty costs and lastly the operation & maintenance costs involved in a project, estimating indisputably impacts the success of any construction project, commercially. Due to the growing demand for quicker and precise estimates, additional methods such as BIM can be profitably adopted by estimators & companies globally, starting now.