# Ontology-Based Representation and Reasoning in Building Construction Cost Estimation in China
## 3 The Building Cost Estimation Ontology Model
### Ontology Representation Model
#### 3.1.2 Work Item Ontology  
A work item is considered to be the smallest unit of work for cost estimation purposes [43], including all building elements in the project. In this research, project classification is based on GB 50500, and cost item indicates project coding, as described in Section 3.1.1. Taking a cast-in-place concrete beam as an example work item, the building material in a work item is composed of, e.g., cement, sand, stone, and water. The project classification may comprise a cast-in-place concrete rectangular beam (the cost item id is 010502001) or a cast-in-place concrete constructional column (the cost item id is 010502002), that is, a work item is the smallest unit of work and is indivisible in building construction cost estimation. A construction process is composed of one or more sub-processes, and sub-process is a collection of work items that produce building elements. To estimate the cost of work item, the proposed ontology needs to know the corresponding item and the quantity of the necessary resources. Therefore, the building of work item ontology can improve the utilization of the cost estimation ontology.  
The work item ontology is developed from the reuse of existing ontologies (like FreeCLassOWL ontology). FreeClassOWL ontology [44] (FC, hereafter), which was developed by the European building and construction materials market to describe construction materials and services, allows the fine-grained search of products, suppliers, and warehouses for any building-related sourcing needs. FC has over 88 million triples of real business data, 81 manufacturers/brands, 19 resellers, 56,360 product types, and 1,783,798 offerings to describe construction materials.  
In this research, entities are used to represent core concepts within the domain of cost estimation, and the four base entities in the work item ontology are actor, knowledge, resource, and schedule. A framework of the work item ontology is shown in Figure 4. Actor contains organization and personnel. Organization is made of government and company, and personnel consists of designer, constructor, operational staff, and maintainers, among others. Every process or event in the construction that involves humans produces and updates of knowledge can be seen as work plans (e.g., schedule plan), work changes (e.g., change list), designs, bills of materials, and similar items [45]. Resource includes labor, equipment, material, and specifications in the design and construction process. The 41-Materials of OmniClass is selected as the classification standard for the building material. Schedule provides the scheduling arrangement of construction, including the actual schedule and the planned schedule. The above information is required for the work item ontology, and the resulting work item ontology aims to integrate the information concerning the construction documents,developers, contractors, and facility management companies. The ontology can be built by experts' analysis and information extraction. Therefore, it is a time-consuming job and a long-term process.  
The relationships of the work item ontology use an existing relationship ontology, the GoodRelations [46] (hereafter GR) ontology, to describe the relationships of primary classes, subclasses, and individuals. The GR ontology is a standardized vocabulary for products, prices, stores, and company data that can be embedded into existing static and dynamic web pages and processed by other computers. In this research, GR is used to present a RDF (Resource Description Framework) description of the construction material and as a work item ontology extension for the terminology dictionary. The relationships in the work item ontology use parts of the GR ontology as reference, as shown in Figure 5.