What cap beam option are you referring to? If you want your footbridge to rest on a column with a base isolator, then draw your column to end a short distance away from the bridge.. in other words, there should be a small gap between the end of the column and the bridge. Define a link with the elastomer properties and then draw a 2 point link with your elastomer bearing to connect the column with the bridge. If the elastomer properties are nonlinear, then you'll need to run nonlinear load cases.