Real-Life Decision Making -- Solution
You decide to sell the contractor the joists he wants.
Both you and the contractor feel the design calls for an unnecessarily high grade of joist. It's an added expense the contractor doesn't need, and you don't want to lose the sale.
You realize that the contractor risks being sued if he fails to use the specified lumber for that particular job. However, he's not likely to get caught unless the building collapses, and you feel pretty confident that's not going to happen.
Technically, the contractor could be buying the joists for any project. So you tell him you will sell him the lower-grade joists as long as there's a clause on the invoice that specifies this lumber won't be used for that particular project. After all, you want to cover yourself from any possible liability.
The contractor agrees and you get your sale. Unfortunately, an unusually heavy snowfall that winter does cause the building to collapse. It happens late one evening when a maintenance crew is inside. While you're not legally liable, you find it difficult to look at yourself in the mirror the next day. People have been hurt, and no matter how you look at it, you feel responsible.
There were several steps you could have taken to ensure the proper lumber was used. Knowing what the contractor intended to do, you could have refused to sell the joists. You could have reported the contractor to the building inspector or the designer.
Designers and engineers specify certain criteria for good reason.