Pdelta analysis of Arch using Notional Load approach

[gpg1]

I am working on the PDelta analysis of an arch rib. The FEA model I've created is a shallow arch consisting of 24 elements. I'm trying to model the PDelta effects (ignoring Pdelta effects)by the notional load approach. I've performed an elastic analysis of the arch and have my initial deflections. My question is how to calculate the notional loads (that will be applied to the nodes of the model)from the moment found by the PDelta of each element. I know there are other methods such as modified Newton-Raphson that require some mathmatical gymnastics, and I'd rather not go that route, if possible. I've already performed a moment magnification of the elastic model, and wanted to compare it to a PDelta analysis. Any help would be most appreciated.

 

[gpg1]

I forgot to say that the arch supports are fixed.