BIM Technical Standards: Grouping & Relationship
For BIMs to be useful as more than a basic 3D model, they must contain an inherent hierarchical relationship structure. In applied terms, this means that every object within a model will have one or more relationships with other objects in the model. These relationships fall within five categories; Composition, Assignment, Connectivity, Association, and Definition.
- Composition - captures a whole-part relationship having exclusive containment such as subdividing a building into floors and rooms or a wall into studs and sheathing.
- Assignment - captures assignment relationships where one object consumes the services of another object, such as a labor resource assigned to a task, or a task assigned to a building element.
- Connectivity - indicates connectivity between objects such as a floor slab connected to a beam or a pipe connected to a sink.
- Association - indicates external references for an object such an external IFC library file where an object is defined.
- Definition - indicates an instance-of relationship such as a pipe segment being of a particular type.
At the current time, GSA has standards covering composition and connectivity relationships.
BIM objects are also sorted, or classified, into logical groups within the model. These groups go by many different names and many are automatically assigned. For example, an object placed within the confines of a room is automatically grouped or classified to be a member of that room. Other classifications are not automatic and must be managed by the data creator.
MEP Objects (Systems)
Within the BIM, MEP objects are assigned to a system. These systems are structured to represent actual or proposed, real-world systems within the building.
- System names must follow the General Naming Guidelines
- When supported by the BIM, authoring software systems are to be properly named and classified.
- All MEP objects are to be placed and maintained on the correct system.
- Systems are to be developed and assigned to all MEP objects by the project design team (Architects and Engineers) in consultation with the GSA Facility Manager.
A composition relationship captures a whole-part relationship having exclusive containment such as subdividing a building into floors and rooms or a wall into studs and sheathing.
- All objects shall be properly related (hosted) to the correct parent element.
- Where applicable, every object is to be related to it's correct floor.
- In no cases should unnecessary offsets be used as a way to position an object within the model.
- Composite objects must be constructed using the BIM authoring tool to establish correct relationships. For example, each wall object is to be made up of composite layers that are managed by the BIM authoring software.
- In the case of Revit, this standard is intended to apply to the method used to create Families. This means that a wall object should be a single instance of a wall Family that contains all parts of the wall construction. A wall should not be constructed of multiple layers of walls. For example, drywall Family, framing Family, siding Family stacked to make a single wall.
A connectivity relationship indicates connection between objects such as a floor slab connected to a beam or a pipe connected to a sink. Achieving robust connectivity relationships within the BIM is an important step in ensuring the model will be ready for downstream uses.
Connectivity relationships are often difficult to preserve when converting data between BIM applications. Care must be taken to ensure that these relationships are maintained.
- Where supported by the authoring application, objects shall be properly connected to a system.
- The network of connections is to be maintained to the furthest extents of the project's scope.
- BIMs which feature objects broken into individual elements when logical systems are available will not be accepted.
Grouping and Relationship Examples
The examples below show several different applications of both proper and improper models. The concept of grouping, categorizing, and relating objects within the BIM is complicated. Correcting these issues after the creation of the model is very time consuming. If you have any questions or concerns you should contact the GSA BIM Champion.
Composition Relationships - Wall Example
In the example shown, a wall has been improperly related to level within Revit. Although this may work properly within the Revit environment, it will not work in downstream applications such as energy analysis and property management.
Revit properties window showing a wall that is properly related (hosted) to a floor object.
Revit properties window showing a wall that is improperly related (hosted) to a floor object. Notice the large offset used to force the wall up a floor.
Example - Electrical Distribution
Within most BIM authoring tools, checking for proper connectivity is simple. Within Revit this is done by pressing the TAB key while hovering over an object.
Branch of Objects
Pressing the tab once will highlight the first level of connectivity. In this example, the branches of conduit off the circuit breaker switchboard are highlighted.
Notice the disconnects in connectivity at the marked points above.
- The feed conduit coming into the base of the panel is not properly connected.
- MDP-3 is also not connected due to a break in the connectivity at the elbow.
Network of Objects
Pressing a tab a second time will now highlight the entire connected network.
It becomes very clear at this point there are issues in the connectivity. These errors will cause sever issues when leveraging the BIM.
- No draw is being posted back to the building power distribution from this system.
- Anything connected to MDP-3 is not being calculated as a load on the switchboard.
- Facility Management software will not be able to establish what circuit is supplying MDP-3.
Example - Plumbing
Branch in plumbing network
The next level is the branch level. At this level you will have selected all objects up to the first fitting on the branch. In the image above, notice that both the supply and waste has highlighted up to the first fitting.
Branch in a network (up to equipment)
In the third level of selection you have selected all objects in the network from the initial fixture up to, but not including, other fixtures.
Example - Duct System vs. Duct Connectivity Network
This example highlights the fact that an object can be placed in the correct system while still not being connected to the network. Both are required under the GSA BIM Guidelines for Revit.
Broken duct connection
In this image you can see the same duct selected by network. Notice that a large portion of the duct system is not selected. This is caused by improper construction of the model and is not permitted under the GSA BIM Guidelines for Revit. Proper object connectivity is required for all supported objects.
Example - VAV
This example highlights the ways a single object can be assigned to multiple systems.
Multiple systems, one object
Any object can be part of multiple systems. The mechanical equipment shown in this example is part of many systems. In fact, this equipment is part of 6 different systems.