buildingSMART Forums

Span direction of a slab

When modelling floors, some software (e.g. Revit) allows you to indicate the Span Direction. However, we don’t see how to transfer this with IFC? We also don’t have a similar setting in other software.

Within Pset_PrecastSlab there are properties such as AngleToFirstAxis but we don’t really understand if this relates to the Span Direction:

The angle of rotation of the axis of the first component relative to the ‘West’ edge of the slab.

What Component? What Edge?

I am not entirely sure, but it is possible that the Span Direction in Revit might not be an actual parameter it can map to IFC to begin with. To my knowledge, Span Direction is just used to give a structural deck its direction. This is useful in detailing and sections, but not much else.


I believe Span Direction does not change the geometry of the floor, but once you section it you can see the metal deck. This is better explained in a post on revitIQ.

I then figured, even if IFC does not (yet) have a place for it, I can still export it to IFC with a UserDefined Propertyset. All I had to do is find the correct property, right? And so I snooped around the Revit properties of a floor with the Lookup plugin, but I haven’t found the actual property that hosts the Span Direction…

The way I see it, the only alternative right now is adding the property to the floors ‘manually’. You can automate most of this with Dynamo and some basic logic.

As for the AngleToFirstAxis; this specifically mentions ‘angle of rotation’, so I don’t think this is the parameter you are looking for. I don’t know which one it has to be, though…

Thank you for your comments. Span Direction is accessible via the Revit API, so could be pushed into a Shared or Project parameter if needed and as such sent to IFC. But no other software that we know of has an equivalent property and neither does IFC have this defined somewhere.

So we could, within our national industry group, come up with a custom property, but then have a problem of needing to define the right meaning and data type for it. An angle? Directionvector? I can imagine creating scripts or Add-ins to calculate this and translate into some property value.

It is something that contractors requested as input from designers when transferring models and right now this only works when they transfer native RVT files between designer and contractor. So we are not ready to make this into a “requirement” as of now.

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In the next release of our software (Bricscad) the span direction will internally be saved as a vector refered to the global coordinate system. This not only gives an angle in the XY plane but also a slope of the slab.
How to map it to IFC properties is indeed the question.

Thank you for your input, @Piet

It would be good to have a common agreement, if possible backed by buildingSMART and software developers. I guess the vector is “relative” to the site position and direction? E.g. using the project north direction and not “true north” or similar?

Would you also capture the actual Span (length) itself. For non-rectangular slabs this requires a structural analysis to properly define.