Z Co-ordination
To prevent issues with Z axis coordination we add the following detail to our BEPs:
1: Define project zero not just in X and Y, but also in Z. For example, define project zero as finish floor level.
In this section example the architect has aligned his FFL with the project zero Z-level, but the structural engineer has set the project zero in his model to be the SSL. This would cause vertical coordination issues when the models are federated, as the entire structural engineer’s model will be around 50mm out, which can be a difficult discrepancy to spot.
Here the difference in each designer’s vertical origin has led to the parapet steelwork jutting out above the parapet wall. This could result in disagreements about whose model is correct, and someone may end up lowering the steelwork or raising the wall. Adjusting these components doesn’t address the core issue that models are not coordinated correctly, so this sort of issue will keep appearing.
In this example it was agreed in the BEP that FFL is project zero. The engineer has read the BEP and added a negative offset to his slab to drop it into the correct position in the Z axis.
2. Export building levels only
Only physical building storey levels should be exported (GF, 1F, 2F etc). This prevents the export of non-physical reference levels (TOS, UOS etc) that can also mess with the vertical coordination of BIM data.
BIM software that doesn’t use levels should be coordinated according to project zero as mentioned in section 1.
By reaching an agreement on these topics and adding this to the project BEP at the outset of the project will help to avoid many of the coordination issues that can arise. It also extends the BEPs usefulness slightly, adding relevant information and reference data for BIM model authors. Because of this it is important that everyone involved in the creation or management of BIM data in your practice reads the BEP.
