FWIW: If I can determine in answer in a relatively short time, say an hour, I usually go ahead and check it, and give the contractor an answer, and don't even bring up anything about charging the contractor.
After you do a few of these calcs, it does not take all that long to do another one once the contracotr provides the maximum wheel load. I can do most of these in 15 to 20 minutes.
The hour (or less) you spend on this will be more than paid back in a better working relation with your contractor. I find, for the most part, if you treat the contractor with repect and help him/her out when you can, they tend to help you out when they can, say, when your mistakes come to light. (And we all make mistakes on every job we do. ) A good working relation with your contractor can significantly reduce the amount of time you spend on a project in the construction phase. It also has the additional bonus of reducing your stress level significantlyy.
In addition, on a fair percentage of projects we do these days, the contractors are on board before we are. Either through deign build or through existing relationships with the owners. We get a fair portion of our workload through contractor recommendations. They appreciate the help on the little things, like this, that we do for them, and so return the favor by recommending us for projects. Don't develop a reputation as a difficult engineer to work with. This will come back to haunt you in a lot of ways over the years.