Yes, rigid means no deformation, stress or strain. Rigid bodies are able to transfer forces from one place to another. Most loads and boundary conditions including contact can be applied to a rigid body in the same way as can be applied to a flexible body, but not every single thing. In older versions, you needed to substitue a Fixed Joint for a Fixed Support. Another example is with contact between two flexible bodies, you are free to flip the Target and Contact side of the contact definition, but when one side of the pair is rigid, that side must be the Target side. The mesher will automatically create surface elements on the Target faces to pickup the contact forces to apply to the rigid body and transfer to another part of the rigid body.

I don’t know if imported pressures are supported on a rigid body. If they are, then surface nodes and elements will automatically be created. If they aren’t then you have to make that body flexible.

Given your purpose, you could start with the concrete parts such as the pier etc as rigid bodies and leave the steel parts as flexible bodies. If you need stress and strain in the concrete parts, you could change them back to flexible after you solve for the steel parts.

I will caution you that meshing thin walled structures such as the bridge girder with 3D solid elements has some requirements on the number of elements through the thickness. Where the plate experiences bending stress, there should be a minimum of 4 linear or 2 quadratic elements through the thickness to get accurate stress output. The default element order is quadratic, but you should set that manually and not leave it program controlled.

On the girder, it would be more efficient in terms of solution time to have shell elements instead of solid elements. That means going back to geometry and using the Midsurface feature to extract a surface to mesh with shell elements. The benefit of shell elements is that internally it can calculate stress at 5 or more points through the thickness so the accuracy of stress is high even with a fairly coarse mesh. This would also make the solution compute in much less time.

One compromise is to use the SOLSH190 element, which is specially formulated to be more accurate with bending stresses when there is only 1 layer of elements through the thickness. The difficulty of using that element is that you must ensure that the body can be meshed using a sweep mesh control because the element has a through-thickness direction which must be correctly oriented for it to work properly. A solid element has no such thing.

One more way to have the best of both worlds is in geometry, place a cutting plane one road-width away from the bearings along the axis of the span. Split the solid geometry using that plane. Convert to midsurface the girder between the pier where it is very easy to use midsurfacing. Leave as solid the complex geometry near the bearings. Use Bonded Contact to connect the edges of the midsurface to the cut face of the solids on the bearing side of the cutting plane.