What do you mean by an arch with a large opening? An arch is a structure that goes over a large opening itself. An arch doesn't generally have a web unless you have taken a beam with a web and bent it into an arch shape. If this is the case the beam still needs to be assessed as a beam but presumably it will have a large compression component if it is acting as an arch and a smaller bending component.

Carl Bauer
www.bauerconsultbotswana.com

I think the difficulty is more than anything finding good and simple references for the design of structures with big elements. One has to join parts together from references.

Then, some practical books take practical approaches and are understandable which is excellent but fail to induce progress in the understanding of the one taking such simplified approach.

In any case you can see something about what you are seeking in

Guide to Stability Design Criteria for Metal Structures Fifth Edition
Theodore V. Galambos
Wiley

As carlbauer indicates, thinking of arches as line elements also helps...meaning: the information of similar things for straight members will be of relevance to your curved ones, and maybe directly applicable.

On the other hand, you may attack arch design by any of the classical approaches and only account of appropriate effective sections consistent with the plate slendernesses, boundary conditions and level of strength will need be entered in your model to be adequate. Your problem is centered in Von Karman-alike treatment of the plates at the zones where the arch has openings; by the way this also applies to sections with no openings! so it is a plate behaviour thing. Other than that, ordinary arches.

Now with the P-Delta tool of calculation, simple ways to ascertain the required reduced levels of bending and axial stiffness, use of short segments, and stated formulations for the limit axial strength of members with some specified level of initial in-member out of alignment imperfection one can almost reduce any arch problem or not to mere strength of materials checks of the strength of the section, and I would take such approach for such case, not without keeping an eye on what more traditional practices indicate.

But your question is amenable to find what is the effective section for different situations.  If you come to understand plates you then will know what to model, and so properly portrait the overall behaviour of your arch.