Author: Mickael Rodrigues; António Ruivo Meireles

Country: Portugal

Company: ndBIM Virtual Building

1. INTRODUCTION

This article aims to demonstrate the importance of the definition of clear modeling processes and use of BIM models by construction companies to avoid cost overruns that can compromise the integrity of the project. Assuming the advantages and effectiveness of BIM in the design phase we will focus our attention in the construction area. This paper is the result of several month of work as BIM consultant and service provider firm for construction companies.

The cost overruns can have dramatic consequences not only with regard to the feasibility of the project to be built, but on the sustainability of the construction companies. Although there are no official figures for the private sector, it is possible to look at the public sector and quickly draw some conclusions. In Portugal, for example, over the last decade several public works have exceeded the initial expected value, having the most critical a cost deviation, in averaged, 94% higher than the initially set value. Besides shocking, this value is an exorbitant sums that demonstrate a real problem in the construction sector. In order to try to answer the purpose of this work was necessary to understand the main causes of cost overruns.

2. COST OVERRUNS – CAUSES AND CONSEQUENCES

According to a report presented in 2007 in the 3rd National Congress of Construction (1), the three main causes of cost overruns in Portugal are: first, Design errors; Second, Direct change orders; and finally, Different site conditions. The last one will be ignored regarding the subject of this abstract.

Keeping in mind the two main reasons outlined above, we will try to draw a summarized chronological succession corresponding to the various stages of the process that culminate in the construction of a building, in order to understand the impact, or rather the consequences of the problems mentioned above. At first, we have the design phase. It is precisely at this stage (consisting of several moments corresponding to the various legislative stages and respective degrees of definition) that occur most of the project errors. Assuming, as already mentioned, the effectiveness of BIM in the design phase, we will certainly take into account that the designers remain human beings. Mistakes were, are and will be made, what we want is to control and minimize the impact of these errors in the following phases.

Once all trade designs are validated, these are delivered to the companies responsible for its construction. Before starting the construction, projects go through several phases that we will describe. Initially, projects go through the measurement phase. All the components of the project - in this case the BIM models from the designers - are measured and quantified in order to understand exactly the consumption of materials and workmanship required for the construction. Once completed the measurements and based on historical information of each company - such as productivities – the deadlines to complete each task are calculated. Once calculated the amount of workmanship and materials as well as the deadlines, it is calculated a budget that indicates the expected value of construction. Once validated this budget, begins the construction of the project.

Being these three stages interconnected, any error in one of them will have direct and negative influence on the next one.

Unfortunately, the reasons stated above as the main causes of cost overruns difficult the entire process since design errors often lead to quantification errors and consequently planning and budgeting errors while making changes requires to conduct further measurements, which makes the process last longer.

Ideally, with the use of BIM tools, this temporal succession may seem simple: based on BIM models delivered by designers to construction companies, would be automatically calculated the necessary resources and in turn planned deadlines and project costs.

In order to minimize the impact of errors and changes described above, it is necessary to ensure a perfect connection between the models of the designers and the following steps leading to the construction of the building. It is precisely this moment, situated between the design phase and the measurement phase that corresponds to a time window in which it is possible to intervene and minimize the risks. Getting a perfect connection between the models of the designers and the measurement and quantification phases quickly and efficiently would dramatically reduce the cost overruns. Any change to the project, in this case the model would immediately ensure an update of the quantities extracted from the models and consequently an update of the time planning and budget.

Although it seems simple, our experience in the construction has shown that the real issue is not only about the quality of the models for quantity-takeoff but also about the type of quantities that we want to calculate from the model.

3. CASE STUDY

If we think about measurements, we will immediately think about national rules and standards that vary from one country to another. The way a quantity of material or workmanship is calculated in Portugal for example is different from the way it is calculated in Brazil, so the results will be different. The most inexperienced in this area may think that each country has its own document that dictates the measurement rules to be applied by all the professionals of the construction area in the country.

Unfortunately, besides the fact that these documents do not have a legal character - since in most cases they are only recommendations - our experience has shown that the measurement rules and quantification methods vary much more than expected.

As an example, we will use information collected from various implementations that we were involved over the past years in order to demonstrate the difficulties that construction companies have been facing. By analyzing the structure of a WBS (that changes according country and client involved) and quantity measurement rules we have found considerable variations of methodologies applied for the measurement and quantification of the elements. For example, to calculate the workmanship required to build a simple brick wall - measured in square meters - and that included a window, the following measurement rules were found:

• Calculated area = Gross wall area - Window area
• Calculated area = Gross wall area - (Window area / 2)
• Calculated area = Gross wall area - (Window area - 2)

It is important to note that these three methods of calculation and quantification were found in several construction companies in a single city. Unfortunately, this is a reality that happens in every city and every country, which creates communication problems when a construction company has to subcontract another one since both use different measurement rules. To ensure good communication between all stakeholders, it was necessary to find a way to extract the quantities - according to the measurement rules used by each company - from the models provided by designers without compromising these models so that all stakeholders could use these same models ensuring a single coherent database.

The methodology that we applied with our clients consisted initially to clearly define the measurement rules used internally. Understand the operation of each company is crucial to achieve this. To this end, a quantitiy measurement rule standard is created, from information collected over the months, for all items quantified in a WBS and involving the various departments of each company. This knowledge is essential to the creation of internal procedures which allow the extraction of quantities directly from BIM models without errors. Then we implemented a methodology that allows us to use the same model in different companies (designers, contractors, subs, etc) and extract from it quantities that obey to each ones quantities extraction rules. In the end, with this methodology all of them will have the same model, but can have different outputs from it as shown in picture 1.

4. CONCLUSIONS

What we argue at ndBIM Virtual Building is that the definition of internal procedures is equally or more effective than the BIM model itself. What is really important is to understand the inner workings of each company once that a single BIM model won’t be efficient if you do not know what you want to do with it. From the moment is clearly defined what kind of information is intended to extract from model, this information combined with a strong knowledge of BIM software allows the definition of procedures and methodologies that enable the use of a single model for design, quantify-takeoff, planning, budgeting and construction, and this, by all stakeholders.

What we have learned over the past few months with our clients is that working all together with BIM models, servers, clouds, openBIM files etc., is not enough. The most important in each and every company is working all the same way.  

5. REFERENCES
   
   

1 – MOURA, H; TEIXEIRA, J. (2007). “Competitividade e incumprimento das funções de gestão na construção”. Congresso Construção 2007, Coimbra.