BIM METHODOLOGY AS A SUPPORT TO THE
QUANTITY TAKE-OFF
Bernardo Ferreira e Silva
Extended Abstract
Master’s Degree in Civil Engineering
Supervisors
Profª. Alcínia Zita de Almeida Sampaio
Jury
President:
Supervisor: Profª. Alcínia Zita de Almeida Sampaio
Member:
October 2016
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1. Introduction
The Quantity Take-Off (QTO) is an essential process in project, because it allows to
manage the costs involved in construction, supporting the investment analysis, as well as the
decision-making and resources planning [1]. QTO can be applied through the construction
process, where valid results can be obtained in the early stages that will assist the estimation of
a preliminary cost. Before construction, it can be used as a planning tool with the construction
activities. The BIM model guarantees the automatic update of results obtained in QTO after any
change in the model [2]. During the works, it can be used to control the economic part of the
construction. The traditional quantity take-off, through manual measurement of the different
project elements, is based on 2D drawings, that can present inconsistencies and, therefore,
might produce errors on the results obtained [3].
The level of automation in the industry has been a major concern over the last years with
the academic community working hard to raise it [3]. Nowadays, BIM technology presents a
valid alternative to the traditional process, allowing better results to the quantity takeoff process
and raising the automation present in the project, while facilitate the information management in
construction [4]. BIM is an automated tool based on a virtual model where information is
automatically generated from it. BIM tools allow various uses, like visualization and clash
detection, based on the automation and transfer of information using the virtual model. Using
BIM to create bills of quantities will reduce the time spent on obtaining them and, therefore, will
reduce costs on the project [5]. Despite these considerations, this feature is overlooked and
BIM is mainly used to visualization.
The utilization of BIM in QTO process is still target of reservations in the industry, due to
some lack of information about the advantages provided in BIM utilization as a QTO tool. The
strengths of using BIM in a QTO process are:
Faster execution;
QTO in design phase;
Costs analysis through the project;
Reliable Results;
Competitive Advantage.
But there are some problems identified, such as:
Lack of technical standards;
Insufficient interoperability between systems;
Changes in the company, due to the necessity of collaborative work and new
work methods;
Additional work in obtaining quantities that can’t be obtained automatically.
This work aims to contribute to the knowledge of the BIM utilization in the QTO process,
while demonstrating that it has more advantages and provide some solutions to the problems.
Particularly, it compares both methods, traditional and BIM, used in the process, applied to a
real structural project.
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2. Case Study
2.1. Guidelines to BIM modelling
The 3D model is the base of the QTO process with BIM. The modeling should follow
certain guidelines, suitable for the objective of the model. In order to guarantee that the BIM
model will provide reliable results, the following standards should be applied to the modeling
process:
It is important that all the objects and components of the model are modeled under
the same circumstances and standards. Consistency of the model is necessary to
ensure homogeneity on the results;
Each object should be created using the appropriate tool, depending on which
software is used. All the information relevant to the QTO should be entered on the
tool, such as dimensions and materials.
Some objects can have two function types: structural and non-structural. Is
important to differentiate the layers by type to obtain bills of quantities classified in
function types;
Interoperability is a main concern when exchanging information between different
software. The person responsible for the QTO need to know the limitations of the
file format used and be able to overcome them, in order to maintain the results
quality;
Clash Detection is important to ensure the quality of the model, and the model
should be adapted accordingly to that analysis. This tool is available in most of the
software available.
2.2. Case Study Modelling
In this research, a real case study was used: a project of a school in Faro, Portugal. The
documents provided consists in 2D drawings of the structural foundations (Footings, Columns
and Lintels) with rebar detailing.
The modeling process was divided in phases, listed on the Table 1
Table 1 - Modeling phases
Activity
1º Creation of levels and grids
2º Column insertion
3º Footing placement
4º Lintel definition
5º Footing rebar detailing
6º Model Checking
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Before modeling, it’s necessary to import, through Revit, the 2D drawing from AutoCAD
with the elements that will be modeled, to be used as a basis for the BIM model.
The first step is to create levels and grids that will support the 3D modeling. The levels
are useful to limit the height of columns and footings, as it is possible to pin the top of the
columns to the level and change the height of those columns automatically. The grids provide a
place to attach columns and assist their placement. The Figure 1 shows a representation of the
grids defined.
Figure 1 - Grids Defined
As it can be observed, the intersection between grids normally coincide in the center of
the columns, to ease the placement.
Next step is to create the different columns as a parametric object, duplicating a
standard concrete column and adapting the dimensions. With the help of grids and the layout
imported from AutoCAD, inserting the columns is an easy process, where the level that limits
the column height is chosen in the software. For this work, all the columns have the same
height (3 meters). The Figure 2 shows a representation of the columns on Revit.
The process to create footings is analogous to the column process. Standard footing is
duplicated and all the footings with different dimensions are created. To facilitate the placement
of footings, the columns were created first so it is possible to use them as a reference. The
software identifies the base of the column and place it correctly. Revit has the capability to
facilitate the modeling and to place adjacent elements in a simple way.
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Figure 2 - Columns representation
The way to create and place lintels is, equally, based on the standard element
duplication. Taking into account that all the lintels have a square section and do not exist, in
Revit, a tool to create lintels, the beam tool was used due to the similarity between those
elements. After creating the different types of section, the lintels were placed 1.25 meters above
the top of the footings, to ensure that no lintel conflicted with the footings. For quantity take-off
results, the columns pass through the lintel. The Figure 3 shows a 3D model of the foundations.
Even using the columns as a reference to place the lintels, each individual placement was
checked to guarantee that there weren’t errors in the alignment.
Figure 3 - Foundations 3D model
To illustrate and study the rebar detailing in structural elements only the footings were
considered. An add-on from Revit was used in this process, Reinforcement. This allows an
automatic detailing, after providing measurements and information like steel grade. The add-on
calculates the number of bars and the spacing between them, placing them inside the element.
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This process needs to be executed on each case and a manual verification is needed to ensure
the correct rebar bending and placement. The Figure 4 illustrates a footing rebar.
Figure 4 - Footing rebar detailing
To finish, is necessary to check the model to verify that all the elements are modeled
with the correct dimensions and in the correct location, based on the drawings provided. This
way, the results from the automatic quantity take-off are rigorous and represent the reality. This
process guarantees the reliability of the model, and is the last step to finish the modelling
process. The Figure 5 represents the final model of the foundations, with all the elements
modelled.
Figure 5 – Foundations BIM Model
The creation of the BIM model is simplified by the advanced modelling capabilities of
Revit, however, some limitations were identified, particularly on the rebar detailing. Despite the
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limitations, modelling in BIM brings unique advantages that compensate the initial difficulty of
working with BIM.
3. Bill of quantities
3.1. Traditional method
The traditional quantity take-off consists in measuring all the elements of the building
manually, using the technical drawings. In this paper, the measurement regulations used were
from Laboratório Nacional de Engenharia Civil, “Medições em Construção de Edifícios” [6].
The elements of the case study are simple, with common geometric shapes, so the
volume formula is multiplying the three dimensions (length, width and height). By analyzing the
foundation plan, the quantity of concrete needed to build the footings, columns and lintels was
obtained.
To obtain the steel quantities from the footing rebar, it is necessary to measure the
length of the bar, taking into account the overlaps and hooks. This process takes a lot of time
due to the necessity of measuring both rebar (superior and inferior) and guarantee the correct
quantity take-off.
The time taken and the quantities measured can be observed on the Table 2.
Table 2 - Quantities and time taken
Element Time spent (min) Quantities measured (m3) Material
Footings 25 119,52 m3
Concrete Columns 35 33,31 m3
Lintels 60 102,53 m3
Footing Rebar 90 4239 kg Steel
Total 210
Related to rebar modelling, due to the difficulties found in the BIM modelling process of
the footing rebar, only this rebar was quantified. The process of modelling the columns and
lintels would require a great amount of time and the results would be different between the
traditional method and BIM.
3.2. BIM method
Using a Revit functionality, called “Schedules”, a list of quantities is created based on the
element type chosen and the properties needed, such as area, volume or material. It is possible
to require a sum of quantities of the same property, represented on the list, that will facilitate the
delivery of results like quantity of concrete needed to build the footings. The Table 3 represents
a list of footings classified by type, showing the number of footings of each type, the material
and the volume. This information is useful to support the planning of formwork. The process to
obtain this table took 5 minutes.
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Table 3 - Footings Schedule
To obtain the other elements, the process is analogous. After the modelling phase, the
quantity take-off is a simple process and the results obtain are trustworthy if the modelling was
correctly executed.
4. Results Analysis
The Table 4 presents the values obtained through both methods. The footings and
columns returns the same result due to simplicity of the take-off in the traditional method.
Table 4 - Quantity take-off (Traditional and BIM)
Element Traditional method
(m3) BIM method (m3) Difference (m3)
Footings 119,52 m3 119,52 m3 0,00 m3
Lintels 102,53 m3 103,11 m3 0,58 m3
Columns 33,31 m3 33,31 m3 0,00 m3
Rebar (Footings) 4239 kg 4199,03 kg 39,97 kg
In the case of the Lintels, the difference is not significant. The justification to this result is
that the project has some uncommon intersections between columns and lintels, where the lintel
wraps the column and, per measurement rules, it is not accounted in the column, as is seen in
the Figure 6.
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Figure 6 - Intersection zone between column and lintel
Regarding the footing rebar, the difference verified is due to the length of the bending,
confirmed with the manual comparison between both methods. It is possible to correct this
situation on Revit, but the increase in time spent doesn’t compensate the difference of less than
1%, considering the steel wastes in rebar.
Constraints and Limitations
The production of a BIM model requires training to the modeler to acquire knowledge
and experience to conceive proposals, manipulate information and obtain results. This training
requires a great amount of time to adapt to a new methodology and the results aren’t visible
initially, a situation that can move away potential interested parties.
The initial investment, that requires software licenses, new hardware and training is a
constraint that will limit the successful implementation of BIM.
The modelling process requires good practice and experience to return reliable results,
and a model created to QTO may not be an accurate representation to 3D visualization.
The rebar modelling in Revit is a difficult process and not user-friendly, although all the
applications developed and incorporated in BIM software. The results obtained aren’t reliable
and the rebar modelling is a limitation to BIM and a point to improve to the software companies.
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5. Conclusions
This document presents an experimental study in order to evaluate the advantages of using
BIM on the QTO process, and make a comparison between the traditional method and BIM.
The results obtained in this research allows drawing the following general conclusions:
1) The results obtained from the BIM model are, in most cases, reliable and easily obtained,
reducing the time spent on the QTO process.
2) The results obtained with BIM are as accurate as traditional method and aren’t affected by
the human error in the QTO process, if the modelling process was well executed.
3) The study of alternatives to the project is almost instant and automatic, allowing to evaluate
a wide variety of options and choose the less expensive, reducing costs.
4) The rebar modelling needs further development by the software companies and the results
returned should only be representative of the quantities required.
5) Even with the initial constraints, such as a high up-front investment and training required to
the professionals, the utilization of BIM in the QTO process is beneficial, in place of the
traditional method.
References
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Construction Research Congress, ASCE, 2012.
[3] Monteiro, A., Martins, J.P., A survey on modeling guidelines for quantity takeoff-oriented
BIM-based design, Automation in Construction, 35, pp 238-253, 2013.
[4] Parreira, J.P., Implementação BIM nos processos organizacionais em empresas de
construção – um caso de estudo, Dissertação Mestrado. Faculdade de Ciências e
Tecnologia, Universidade Nova de Lisboa, 2013.
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257, 2010.
[6] Laboratório Nacional de Engenharia Civil, Medições em Construção de Edifícios, LNEC,
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