Concrete tied arch

Respected imehanicians!
                      I am a tackling an important problem about the behavior of concrete tied arch. Each concrete arch rests on two steel I-beams. The steel I-beams are supported at their ends. They are connected at various locations along their length by tie rods. A picture is worth 1000 words  basic figure.pdf.

I am using Computers and Structures Inc. SAP software to model the behavior of concrete arch. I developed a 3-D model of the concrete arch and the steel beams. I used a solid element in my model. The loads on the structure consist of self-weight and distributed load on the top surface.

My question is how does the arch action develop  in this model. Do we have to treat that the concrete breaks at the center span and then the tied arch action comes into effect? Is there anyway to know whether the structure is really behaving like tie-arch or in other words, is the concrete at the mid-section not fully cracked due to the existing loads?

We see that there is a two way slab action coming into effect.So, when a non-linear load analysis was performed removing the concrete solid elements(making Ec=0) once the concrete rupture capacity was exceeded, this led to the complete cracking of concrete in the midspan region. How does the structure stand? Has the structure not experienced the loads which has delayed complete cracking of the concrete?

If you have experience modeling similar structures, please share your ideas.

Sincerely,
Mahendra.