What is this Rm line in this biaxial interaction chart - Eurocode 2

[Pretty Girl]

This chart is located in the book "Deep Surface" by Harshana Wattage, pg 148, about biaxial N-M interaction in columns. Their chart mentions something like Rm line, but I have seen these kind of lines in other old charts and they call it a different name. Most probably it refers to the same line. The book provides a way to calculate the lines and other things. But I'm not quite sure why do we need these lines. Can't we just be below the lines and call it a safe column design? I feel like we can even calculate the asfyk/ bhfck etc without those lines. What's the actual use of those?

 

[IDS]

This chart is located in the book "Deep Surface" by Harshana Wattage, pg 148, about biaxial N-M interaction in columns. Their chart mentions something like Rm line, but I have seen these kind of lines in other old charts and they call it a different name. Most probably it refers to the same line. The book provides a way to calculate the lines and other things. But I'm not quite sure why do we need these lines. Can't we just be below the lines and call it a safe column design? I feel like we can even calculate the asfyk/ bhfck etc without those lines. What's the actual use of those?


View attachment 8749

I don't know what the Rm lines are for, other than that they seem to be related to the balance moment, which affects the capacity reduction factor calculation in some codes (but not Eurocode 2). How does the book say it is calculated?
Incidentally, if I search for Deep Surface and the authors name the only relevant link that comes up is your post, but if I enter wattagepublishers.com it goes straight to the publishers site with this book displayed!