I have used this form and submitted it to SJI several times. It can be be helpful, but there are limitations.
SJI will not analyze the joists. They simply review the member sizes and compare them to a table they have of known common member sizes that are specific to certain SJI joist designations, then pick the smallest joist that commonly consited of those member sizes. They will also not share the table with you, for liability reasons.
For example, if you submit a form that for a 30" deep joist erected sometime in the 1990's with a 2.5" deep seat and a top chord consisting of (2)-L1.75x1.75x3/16, you might get a response back from SJI that says something like,
"The joist you have presented resembles an 30K7 if manufactured by an SJI member company. K-Series 1986-Present."
I have spoken to them about their methodology in making these "recommendations", and they explained that they simply compare the given member sizes to common member sizes used in SJI joist designations from the time period, and pick the smallest joist with that member size. They do not do any analysis. Presumably, if the joist you submit does not resemble an SJI joist in either configuration or member sizes, you would receive a response that said something like,
"The joist you have presented does not resemble any joist manufactured by an SJI member company."
It is important to realize the limitations of their "best guess" "recommendation. In order for it to be considered acceptably reliable, and useful, you need to compare their "best guess" with results of your own analysis and the existing loads to see if everything makes sense.
