6.5.3.2 IfcCostItem

Instances of IfcCostItem are used for cost estimates, budgets, and other forms, where a variety of identification codes are used extensively to identify the meaning of the cost. Examples include project phase codes, CSI codes, takeoff sequence numbers, and cost accounts. The model allows for all classes that are ultimately subtypes of IfcObject to inherit the ability to have one or more instances of IfcClassificationReference to be assigned. Where identification codes are required, the generic IfcRelAssociatesClassification facility should be used.

An IfcCostItem can be calculated based on quantities from objects through its relationship to IfcRelAssignsToControl.

For quantity-based costing, IfcElement, IfcTask, or IfcResource occurrence subtypes may be used. Multiple elements may be assigned of the same or different types, using IfcPhysicalQuantity entities defined at each object. Each IfcPhysicalQuantity type must be identical (for example, all values are IfcAreaQuantity) such that they can be added together.

For rate-based costing (specifically for IfcCostScheduleTypeEnum.SCHEDULEOFRATES), a single IfcTypeProduct, IfcTypeProcess, or IfcTypeResource subtype may be used to reflect rates for occurrences of such types. This enables the possibility to generate a quantity-based cost schedule for occurrences based on types with rate-based cost schedules.

IfcRelAssignsToControl is also used in the opposite direction to link the root IfcCostItem to an IfcCostSchedule where RelatingControl is the IfcCostSchedule.

Figure 6.5.3.2.A illustrates cost item assignment derived from building elements. The IfcRelAssignsToControl relationship indicates building elements for which quantities are derived. Not shown, costs may also be derived from building elements by traversing assignment relationships from the assigned IfcProduct to IfcProcess to IfcResource, where all costs ultimately originate at resources. It is also possible for cost items to have assignments from processes or resources directly.

This concept can be applied to the following resources:

• IfcProduct - For quantity-based costs based on product occurrences, spatial structures, or other physical artifacts.
• IfcProcess - For quantity-based costs based on tasks, procedures, or events.
• IfcResource - For quantity-based costs based on resource allocations.
• IfcTypeProduct - For cost rates based on product models.
• IfcTypeProcess - For cost rates based on process models of historical or projected duration.
• IfcTypeResource - For cost rates based on resource models of historical or projected productivity.

An IfcCostItem can nest other instances of IfcCostItem through its relationships to IfcRelNests. This can be used to enable the development of complex groups of costs as may be found in cost schedules through to pages, sections and complete cost schedules.

There is always a summary cost item as the root item of the tree representing the cost item nesting. Subsequent instances of IfcCostItem are assigned to the summary cost item using IfcRelNests. The summary cost item itself is assigned to IfcCostSchedule through the IfcRelAssignsToControl relationship.

Figure 6.5.3.2.B illustrates a cost item composition used for a cost schedule. Each line item has a quantity and separate unit costs where IfcCostValue.Category indicates the category of cost. The summary item has a hierarchy of costs calculated according to IfcAppliedValueRelationship.ArithmeticOperator, where IfcCostValue.Category identifies the category to be totalled. The Tax component has IfcCostValue.Category set to 'Material' which indicates it is the sum of all nested values of the 'Material' category ($3 x 3000 + $118 x 100 = $20800). The Subtotal component has IfcCostValue.Category set to an asterisk ('*') which indicates it is the sum of all nested values of all categories.

This concept can be applied to the following resources:

• IfcCostItem