Question-and-Answer Resource for the Building Energy Modeling Community
 | 1 | initial version |
The standard practice is to treat this as a "thermal bridge". The standard way to model a thermal bridge is to derate the assembly according to the extra thermal transmittance this discontinuity would add.
There are different ways to input thermal bridges depending on the software used, though generally you have to sum the total thermal transmittance of the clear field assembly and subtract the additional thermal transmittance that would be caused by the thermal bridge, then enter this adjusted thermal transmittance into the model.
In the example you describe, the slab would be assigned a "psi" value (W/mK), which then can be multiplied per unit length to get a W/K transmittance. This W/K transmittance can be subtracted from an assembly in the model to derate it.
This process is described in much more detail in the references below. They should get you on the right track.
