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

[1] Doloi, H.K., Understanding stakeholders’ perspective of cost estimation in project

management, International Journal of Project Management, 29, pp 622-636, 2011.

[2] Forgues, D. et al, Rethinking the Cost Estimating Process through 5D BIM: a Case Study,

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.

[5] Shen, Z., Issa, R.R.A., Quantitative evaluation of the BIM-assisted construction detailed

cost estimates, Journal of Information Technology in Construction (ITcon), Vol.15, pp 234-

257, 2010.

[6] Laboratório Nacional de Engenharia Civil, Medições em Construção de Edifícios, LNEC,

Lisboa, 1983.