Spans in Bicategories

Hello all, In Benabou's 1967 paper, he gives the bicategory of spans in a category with pullbacks as an example of a bicategory. It seems to be fairly common knowledge that spans in a bicategory with weak pullbacks form a tricategory (or a bicategory if appropriate isomorphism classes are taken or fibrations required). If the bicategory has finite limits there should be a monoidal structure as well. I know people use these results fairly often, at least at the level of monoidal bicategories, but does anyone know if there exists a written account of such a theorem? Best, Alex Hoffnung [For admin and other information see: http://www.mta.ca/~cat-dist/ ]