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3.3.4 IFC是什么?
这些基本实体会被组合起来用于定义AEC领域中广泛使用的构件,这些构件在IFC中叫做共享构件。它们包括建筑构件,诸如公制墙、楼板、结构构件、建筑服务元素、流程元素、管理元素以及公制特性。由于IFC是作为一种可扩展数据模型进行定义的且以构件为本,所以可通过子类型化的方式将这些实体根据需要详细定制,以便创造出更多的子实体。

从概念上讲,IFC是根据构件(如IfcObjectDefinition)及构件之间的关系(以IfcRel作为前缀的实体)进行架构的。IFC数据模型的高层次是这些构件及关系实体的特定领域扩展。他们能处理有特殊需求情况下的特定实体。所以会有结构构件以及结构分析扩展程序、建筑类、电气类、暖通类以及建筑控制构件扩展。

所有的IFC模型都能提供可通用的一般建筑空间结构用于建筑构件的布局和存取。IFC能将所有构件信息组织到“项目→场地→建筑→建筑楼层→空间”这样的层次中来。每一种较高层次的空间结构都是较低层次空间结构的集合,加上能横跨多个较低层次空间结构的构件。举例来说,楼梯通常都是能跨越所有建筑楼层的,因此说它是建筑集合的一部分。墙体总是能在一层或多层将两个或多个空间连接起来。如果在单层中进行构建,它们便是建筑楼层中典型的组成部分,如果在它们跨越了多个楼层,便是建筑集合的组成部分。由于IFC层级化构件的子类型化这种架构,用于转换的构件是嵌套在深层次的子实体定义树中的。所有的物理构件、流程元素、行为者以及其它基本构造都是以类似的方式抽象表达出来的。例如,单个墙体的轨迹如下图3-5所示。

图3-5中树的每个层次都引入了与墙体不同的特性和联系。IfcRoot是IFC中最高级别的的抽象实体,它为管理项目指定了Global ID(GUID)和其它信息,例如谁在什么时候创建了它。IfcObjectDefinition将墙安置在总建筑楼层组件内,该级别也能识别墙体中元素,例如窗、门和其它任何形式的洞口。IfcObject级别基于墙体的类型(由层级树中的较低级别定义)为墙的各种属性之间提供链接。IfcProduct可定义墙的位置及其形状。IfcElement承载着某构件与其它构件之间的关系信息,例如墙与毗邻构件的连接关系,以及由墙分隔开来的空间。它也在墙上带有各种洞口,这些洞口可选择性地用门或窗来填充。如果该墙是结构性的,那么能代表墙的结构性构件会与之关联。

墙被归类为以下形式中的一种:标准:沿着墙体的控制线以固定宽度竖向拉伸;多边形:垂直拉伸但各处剖面不同;剪切:墙体非垂向拉伸;给排水墙:带有预埋管道路径空间的墙;用户自定义:所有其它类型;未定义的。很多特性和关系都是可选择的,这使得执行者能根据他们的导出习惯剔除一部分信息。并不是所有的bim设计工具都能创建或呈现所有不同类型的墙体。

属性是承载于可选择的P-sets中的。PsetWallCommon能为定义提供平台:识别器、吸音评级、防火评级、可燃性、表面火焰蔓延、传热性、外装、延伸到主体结构(至板以上)、承重、分区(防火墙)。如有需要,也能提供更多其它更加细化的P-sets。洞口、切口和窗框槽以及伸出构件(如壁柱)可与因异型天花板造成的不规则墙一道被支持。

人们可从这个墙体案例中明白所有IFC中的建筑构件是如何完成定义的。有很多类型的组件、P-sets、特征能支持结构、机械和其它系统构件。分析模型、荷载数据以及产品性能参数也能在某些领域中得以表现。构件的几何形状也能通过IFC架构进行参数化表现,不过这项应用尚未普及。

世界各地包括美国、英国、挪威、芬兰、丹麦、德国、韩国、日本、中国、新加坡及其它国家都在努力推进IFC的应用。美国、挪威、韩国和新加坡发起了开发基于IFC的自动建筑代码检测功能和在线提交系统。成功的BIM和IFC项目数量也在逐年增加。基于IFC的项目的获奖情况见于buildingSMART 的BIM大奖网页。
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3.3.4 What Is the IFC?
The base entities are then composed to define commonly used objects in AEC, termed Shared Objects in IFC. These include building elements, such as generic walls, floors, structural elements, building service elements, process elements, management elements, and generic features. Because IFC is defined as an extensible data model and is object-oriented, the base entities can be elaborated and specialized by subtyping2 to make any number of subentities.

Conceptually, IFC is structured as objects (e.g., IfcObjectDefinition) and
their relations (entities whose names start with IfcRel). At the top level of the IFC data model are the domain-specific extensions of these object and relation entities. These deal with different specific entities needed for a particular use. Thus there are Structural Elements and Structural Analysis Extensions, Architectural, Electrical, HVAC, and Building Control Element Extensions.

All IFC models provide a common general building spatial structure for the layout and accessing of building elements. IFC organizes all object information into the hierarchy of Project → Site → Building → BuildingStorey → Space. Each higher-level spatial structure is an aggregation of lower-level ones, plus any elements that span the lower-level classes. For example, stairsusually span all building stories and thus are part of the Building Aggregation.Walls typically bound two or more spaces on one or multiple stories. They are typically part of the BuildingStorey, if structured within a single story, and part of the Building Aggregation if they span multiple stories. Because of the IFC hierarchical object subtyping structure, the objects used in exchanges are nested within a deep sub-entity definition tree. All physical objects, process objects, actors, and other basic constructs are abstractly represented similarly. For example, a simple wall entity has a trace down the tree shown in Figure 3–5.

Each level of the tree in Figure 3–5 introduces different attributes and relations to the wall entity. IfcRoot, the highest-level abstract entity in IFC, assigns a Globally Unique ID (GUID) and other information for managing the object, such as who created it and when. IfcObjectDefinition places the wall into the aggregate building story assembly. This level also identifies the components of the wall, including windows, doors, and any other openings. The IfcObject level provides links to properties of the wall, based on its type (defined lower down in the hierarchy tree). IfcProduct defines the location of the wall and its shape. IfcElement carries the relationship of this element with others, such as wall bounding relationships, and also the spaces that the wall separates. It also carries any openings within the wall and optionally their filling by doors or windows. If the wall is structural, a structural element representing the wall can be associated with it.

Walls are typed as one of the following: Standard: extruded vertically with a fixed width along its control line; Polygonal: extruded vertically but with  varying cross section; Shear: walls not extruded vertically; ElementWall: walls composed of elements such as studs and sheathing; PlumbingWall: wall with embedded routing space; Userdefined: all other types; Undefined.Many of these attributes and relations are optional, allowing implementers to exclude some of the information from their export routines. It is possible that not all BIM design tools can create or represent all of the different wall types.

Properties are carried in optional P-sets. The PSetWallCommon provides fields to define: Identifier, AcousticRating, FireRating, Combustibility, SurfaceSpreadOfFlame,  ThermalTransmittance, IsExterior, ExtendToStructure (to slab above), LoadBearing, Compartmentation (firewall). Other more detailed P-sets are also supported if needed. Openings, notches and reveals, and protruding elements, such as pilasters, are supported, along with walls clipped by irregular ceilings.

From this wall example, one gets a sense for how all building elements in IFC are defined. There are many types of assemblies, P-sets, and features that can support structural, mechanical, and other system elements. Analysis models, load data, and product performance parameters can also be represented in some areas. Objects’ geometry can also be represented parametrically using the IFC schema, but this use is not yet common.

There are significant efforts to apply the IFC in various parts of the world including the U.S., the UK, Norway, Finland, Denmark, Germany, Korea, Japan, China, Singapore, and other countries. The U.S., Norway, Korea, and Singapore have initiated efforts to develop automatic building code-checking capabilities and e-submission systems based on IFC. Also, the number of successful BIM and IFC projects increases every year. Award-winning IFC-based BIM projects can be found at the buildingSMART BIM Awards web page.

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