This is a two story colonial style house built in 1990. From some general observation and measurement, the center of the house has settled about 1/2 to 3/4". It can easily be seen at the doorways. I've found that the cause seems to be the main beam that runs the length of the house (three sistered 2x12s) is compressing at the posts in the full basement.
I figured one way to verify the distance of settling would be to use a water level to compare the floor joist height at the sill to the height at the beam. I'd repeat this at each joist across the span to get an overall profile.
One of my major dilemmas is moreso how to correct the problem. Some initial thoughts were to use jack posts on either side of the permanent posts to relieve the weight in order to install pieces of angle iron under the beam at each post perhaps extending 8" in either direction to distribute the force rather than simply shimming.
Does anyone know if this is a common problem? Is there a way to figure out if the builder installed enough posts for the span? I'm assuming the current problem is good reason to assume NOT.