How can field bent rebar be modeled with IFC?
The particular case I’m thinking of is a precast, prestressed concrete bridge girder where the stirrups project vertically out of the top flange. At the bridge site, these bar are bent around the top mat of longitudinal deck reinforcement.
Can IfcReinforcingBar work?
I don’t think so, unless two different representations can be attached. One would be from the perspective of the precast component manufacture who builds the beam with the straight extension out of the top of the beam, and the other from the perspective of the finished structure with the straight bar extensions bent around the longitudinal deck steel.
But I don’t think representations work that way.
@Evandro, what @Rick_Brice is trying to convey is that the reinforcing bars have two different forms/states… a) their form when they are fabricated and leave the factory and b) their final form at installation, where they are bent into their final position/form. To make it even more complicated, the precast concrete girders also change shape between fabrication, initial installation, and “final form” due to their camber along their length… which significantly decreases over time.
Precedent BIM processes in the vertical world would say that these are two different models for two different purposes… one for fabrication, one for “final form”. Most of the work done to date has focused on “final form” and not included these fabrication models as part of the whole.
Ideally, you would have different shape representations of the same object linked or tied to specific states/steps of a defined process. Less ideally, you would have two(?) different objects, with many of the same attributes and properties, but slightly different names to indicate state and different geometry.
Very clear. I thought the question was
How can field bent rebar be modeled with IFC?
But I feel the real, more generic request is
How can I assign two representations to the same object?
Assuming that’s the case, if you check the definition of IfcProductRepresentation, you’ll see it says
IfcProductRepresentation defines a representation of a product, including its (geometric or topological) representation. A product can have zero, one or many geometric representations, and […]
Sure. Spec’ing it is one thing… getting an MVD and the vendors to implement and support are completely different ones.
I see. Since, IFC allows to describe this scenario. And IFC 4.3 MVDs, being a subset of it, allow it too.
I now understand (at my third attempt, but never too late) that question for the Users category Forums is: does anybody know if there is a tool out there with this feature?
I don’t. Sorry for hijacking the thread. 
@jwouellette summarized my question well.
I don’t think that having multiple representations accomplishes the goal. As I understand IfcProductRepresentation, and all the related classes, representations have types (SweptSolid, BRep, etc) and RepresentationIdentifiers (Box, Axis, Body, etc.). I don’t see anything where (and forgive the sloppy shorthand notation) I can model something like the following:
IfcBeam.Representation.Rep[1] = This is the rebar geometry for the precast fabricator.
IfcBeam.Representation.Rep[2] = This is the rebar geometry for the bridge builder.
I’m trying to learn what parts of IFC support conveying this information in a contract letting model. I need to convey my design intent to the builder. That intent is “when you have these beams fabricated the stirrups must extend straight out of the top flange and when you build the deck, you must bend the stirrups around the longitudinal deck bars”.
