Despite the fact that applying constructability concepts in building designs have led to savings estimated within a range of 1% to 14% of the capital cost, the construction industry still lacks an advanced tool to assess and check for constructability Building Constructability implementations in designs. Therefore, the objective of this research is to propose a quantitative assessment of building constructability, which transforms the subjective assessment of constructability knowledge to a quantified value so that it is easy to analyze and improve building design. The proposed methodology uses the advancement of object oriented Building Information Model (BIM) and 4D simulation model to serve as a data repository models for the constructability assessment platform. Factors affecting constructability of building designs have been identified and relatively weighted using Analytical Hierarchy Process (AHP) technique based on a questionnaire survey collected throughout the Canadian provinces. Evaluation criteria are also developed to assist the designer to evaluate the design proposals. The outcome shows that the proposed method provides the designer an accurate and faster mode in evaluating project constructability by using the parametric features from BIM and taking the advantage of spatial relationship between building components.
Construction is a dynamic, complex, as well as fragmented process, and the traditional system of construction separates the two main disciplines of design and construction, where designers and contractors rarely communicate before the initiation of the construction phase. Early in 1982, The Business Roundtable defined a “constructability program” where a potential return on investment of 10:1 was reported by applying constructability  . National Institute of Standards and Technology (NIST) conducted a study in 2004 and reported that the lack of Architecture/Engineering/Construction (AEC) interoperable software was costing the industry $15.8 Billion annually  .
With the continuous advancement in technologies, new techniques are constantly being developed in order to improve engineering implementations for the construction industry. One of these promising developments is Building Information Modeling (BIM) where it facilitates a more integrated design and construction process that results in better quality buildings at lower cost and reduced project duration  . The American Society of Civil Engineer (ASCE) reports that BIM technology can be used to validate a new constructible tool  . BIM provides the construction community a complete 3D database that can be used for estimating, scheduling, detailing, advance bill production, automated shop drawing, and construction planning for all of the trades. In addition, by adding time information to the BIM model, 4D models facilitate the testing of different design and execution sequencing alternatives so as to achieve better constructability. Therefore, by integrating BIM and 4D simulation, quantitatively assessing the constructability of a building design becomes a novel, feasible and necessary method. The present paper describes the proposed new method and relative implementation and development.
The benefits of improved constructability have been associated with the time, cost, quality, and safety performance of a project, together with other intangible benefits. Quantifying assessment of designs, constructability review, and implementation of constructability programmers are the three most commonly employed approaches. Quantifying assessment of designs enables an objective evaluation of constructability attributes since results are comparable. Using this approach, two issues were addressed. On one hand, it is reasonably practicable and manageable for assessments to be made because it is done using the finished design product rather than evaluating the design process. On the other hand, it is difficult to comprehensively take into account all relevant factors affecting constructability into the assessment system  .
Singapore has pioneered in the field of quantifying constructability based on a scheme known as the Buildable Design Appraisal System (BDAS). The concept is based on a “3S” design principle in which a design is tested for constructability issues with respect to Standardization, Simplicity and Single Integrated Elements. A similar approach was used for assessing constructability proposed for the Hong Kong construction industry in 2007  . A constructability multi-attribute system (BMAS) was developed for assessing constructability of designs in Malaysia  . The assessment was based on 6 principles with identified factors for each principle which were developed through a literature review, questionnaire survey and brainstorming sessions. Another assessment method based on cognitive models (CM) was proposed for constructability assessment for steel frame structures  . It was based on the use of interviewing techniques to understand problem solving and the development of knowledge models for automated constructability assessment.  developed a fuzzy Quality Function Deployment (QFD) system to support constructible design decision making. It provided a systematic and structured method to support the integrated decision-making process of constructible designs, and applied a fuzzy set theory to facilitate the processing of design-relevant QFD information.
It was found that previous assessment platforms were in complex calculations, dependent on governmental benchmarks, and lack of time factor. Detailed comparison can be found in  . In addition, working schedule was not considered in any of the previous work introduced above. The analysis of the time module throughout all construction phases was essential for any constructability evaluation, as indicated by  . Therefore, there is a need for an integrated assessment platform to provide the user a project specified model, with clearly defined rules, understandable criteria, and flexible interactive system.
As discussed in the Introduction, BIM is playing a crucial role in research and development fields of construction information integration and interoperability. BIM is defined as the creation and use of coordinated, consistent, computable information about a building project. These various forms of information had a parametric nature where they can be used for design decision making, production of high-quality construction documents, prediction of building performance, cost estimating, and construction planning  . By having the possibility to build the whole project virtually before physical construction begins, BIM adds a level of accuracy to both quantities and quality issues that overcomes the shortenings found when traditional processes of design and documentation are used. Traditional 2D and 3D CAD programs don’t represent a space because it doesn’t exist as a distinct physical entity. However, a space entity is a fundamental part of a BIM model, and includes the proper relationships to its corresponding walls, ceilings, floors, etc. Thus, spatial information needed for constructability analysis can be easily obtained from an application using a BIM data model, whereas several complex calculations had to be required to derive the same information from an application using a traditional geometric data model  .
As stated earlier, certain constructability factors are of a qualitative nature which makes it exceptionally difficult to quantify their impact on design. For example, many studies have been done on analyzing material access throughout the construction site, and all the contributions included only ideas and knowledge bases to improve proper material accessibility (   ). There is a little evidence in literature that reported a formal procedure or standard process for material access analysis. In order to analyze qualitative constructability factors, the present research argues that visual analysis can be used as basis for studying such qualitative factors if they were properly associated with constructability knowledge.
In addition, time information can be integrated into the BIM model, which utilizes a three-dimensional project geometry into an environment where the effect of time schedule over construction can be visualized. A virtual object-oriented 4D model had the potential to support automated constructability assessment and to assist a project team in identifying constructability issues early in the design and construction phases  . This improved the identification of possible mistakes or conflicts done unknowingly at an early stage of a construction project, and thus enabled project stakeholders to optimize the construction schedule as much as possible  .
The authors of the present paper have been exploring the integration of quantitative constructability assessment with BIM and 4D models for a long time. Based on our previous research results (   ), a complete methodology and implementation are described in the present paper.