double wide interior load bearing walls

[masondog]

I'm looking for a double wide that I can remodel into an office. I have seen some that have a large plywood beam at the ridge which is bolted together at the site. Are those beams meant to span the entire length of the structure with out interior supports? I want have the middle area open with a support at each end area about 12 feet in from each of the ends of the structure. In other words a 60 ft long with a 36 ft office in the middle.