Codes and standards tell you how to do things.
This doesn't cover every single situation, nor should it. Things like this they leave to Engineering Judgement.
In this case I would simply state your minimum clearance and say based on the worst case operating loads and deflections, there is clearance between the pipe and the grating.
Let the client come back to you and state in code XYZ it states..... Won't happen.
They might have a guidance or their own specification, but this sort of thing is not mandated by code.
Ps - don't double post - it's not allowed.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.