To Dave Viking PE -- Grove and Terex have been great at supplying pontoon loads. Linkbelt has an excellent internet program for specific models. Signup and you are off and running.
JStephen is on the right track (no pun) to determine what a maximum load can be. That is, at tipping, you have a determinate load consisting of crane weight plus hook load.
The problem becomes more difficult for a lesser load when it is not tipping, and is distributed on 4 pontoons. The load is indeterminate. (Pontoons at the ends of outriggers.)
I have analyzed a number of bridges with crane loads. Generally, decks are not adequate to carry a pontoon load. For a 70 ton crane, I've seen pontoon loads in the 60 to 70 kip range. This compares to an HS20 wheel load at 16 kips.
Sometimes the maximum pontoon load is not on the lifting side. Sometimes it is on the counterweight side, and might even occur during erection. Linkbelt's program includes wind loads, additive to the heaviest loaded pontoon.
Years ago, I developed my own pontoon loads. But these are subject to some approximations, if not outright errors. Now, I go immediately to the manufacturer for pontoon loads.
When looking at thin bridge decks, the pontoons generally need to be over the girders. Small cranes will allow more margin for approximations and errors. Larger cranes, less margin.
Small and large are relative to the size of the bridge, size of the lifted load, span, spread, etc.
Michael Mills
Tulsa, Ok.
