Automated Cost Analysis of Concept Design BIM Models

AUTOMATED COST ANALYSIS OF CONCEPT DESIGN BIM MODELS Interoperability: BIM models and cost models This paper introduces the automated cost analysis developed for the General Services Administration (GSA) and the analysis results of a case study involving a concept design courthouse BIM model. The purpose of this study is to investigate interoperability issues related to integrating design and analysis tools; specifically BIM models and cost models. Previous efforts to generate cost estimates from BIM models have focused on developing two necessary but disjoint processes: 1) extracting accurate quantity take off data from BIM models, and 2) manipulating cost analysis results to provide informative feedback. Some recent efforts involve developing detailed definitions, enhanced IFC-based formats and in-house standards for assemblies that encompass building models (e.g. US Corps of Engineers). Some commercial applications enhance the level of detail associated to BIM objects with assembly descriptions to produce lightweight BIM models that can be used by different applications for various purposes (e.g. Autodesk for design review, Navisworks for scheduling, Innovaya for visual estimating, etc.). This study suggests the integration of design and analysis tools by means of managing all building data in one shared repository accessible to multiple domains in the AEC industry (Eastman, 1999; Eastman et al., 2008; authors, 2010). Our approach aims at providing an integrated platform that incorporates a quantity take off extraction method from IFC models, a cost analysis model, and a comprehensive cost reporting scheme, using the Solibri Model Checker (SMC) development environment. Approach As part of the effort to improve the performance of federal buildings, GSA evaluates concept design alternatives based on their compliance with specific requirements, including cost analysis. Two basic challenges emerge in the process of automating cost analysis for BIM models: 1) At this early concept design stage, only minimal information is available to produce a reliable analysis, such as space names and areas, and building gross area, 2) design alternatives share a lot of programmatic requirements such as location, functional spaces and other data. It is thus crucial to integrate other factors that contribute to substantial cost differences such as perimeter, and exterior wall and roof areas. These are extracted from BIM models using IFC data and input through XML into the Parametric Cost Engineering System (PACES, 2010) software to generate cost analysis reports. PACES uses this limited dataset at a conceptual stage and RSMeans (2010) data to infer cost assemblies at different levels of detail. Functionalities Cost model import module The cost model import module has three main functionalities: generating the input dataset necessary for the cost model, performing a semantic mapping between building type specific names and name aggregation structures in PACES known as functional space areas (FSAs), and managing cost data external to the BIM model, such as location and construction duration. The module computes building data such as footprint, gross area, perimeter, external wall and roof area and building space areas. This data is generated through SMC in the form of an XML file and imported into PACES. Reporting module The reporting module uses the cost report generated by PACES to develop a comprehensive report in the form of an excel spreadsheet. This report consists of a systems-elemental estimate that shows the main systems of the building in terms of UniFormat categories, escalation, markups, overhead and conditions, a UniFormat Level III report, and a cost breakdown that provides a summary of material, equipment, labor and total costs. Building parameters are integrated in the report to provide insight on the variations among design alternatives.