Another challenge. I created the coffered arch lying on the ground using walls, slabs columns with custom profiles for trims around the coffers.
After it was done and saved as a library part I created another custom profile trim for the casings. I placed it in an arched opening with trim (turned of panel). Casing was dull so added trim similar to your photo.
Granted I didn't know sizes, but this was fairly quick to do. I am sure there are many more ways to do this just using archicad.
That said Ralph will probably have a 5 minute and better solution
An advantage to figuring this out as 3D objects is all you s/e will be drawn for you.
Erika
Architect, Consultant
MacBook Pro Retina, 15-inch Yosemite 2.8 GHz Intel Core i7 16 GB 1600 MHz DDR3
Mac OSX 10.11.1
AC5-18
Onuma System
"Implementing Successful Building Information Modeling"