I have zero exposure to the IStructE beyond their website:
Link.
That at least sounds like an excellent framework. And it could be modified to fit lots of different areas. You could have a mill/industrial steel building, a steel office building, a concrete structure (could be a pump station, parking structure, office building, etc.), a wood multifamily building, and something else - maybe a masonry warehouse/office? You could even have one options that's miscellaneous structures - a retaining wall, a tank, a pier, etc. In each one, you can mix materials enough that you test a bit of everything - they all have concrete foundations, add a steel moment frame to the wood building, etc.
But it does a lot to test the 'softer' engineer skills that are so important as well as the continuity of technical concepts that are broken up by multiple choice tests or even written answers that focus on one topic.
I also really like the idea of the interview process. I think it would be beneficial to have an interview after the exam. It would serve a two purposes: further test the knowledge of the candidate and validate the written test results. For instance: if somebody made a lucky guess on something (hard to do in the long form test, but could happen - can certainly happen in the MC test), a follow up question about it could reveal the gap in their knowledge. Or if somebody is just a wretched test taker and didn't
quite pass, but they were able to speak competently on the subject to a panel the next day...it's reasonable to say that the test didn't quite measure their abilities accurately. That's not to say that the test should just be thrown out, but some measure of agency and decision making power on the part of the interviewers (with proper accountability) could be prudent.
This is how we handled qualifications in nuclear power in the Navy. Each qualification (from being allowed to talk on the phones in the power plant up to operating the nuclear reactor itself) involved a practical learning period where we had to do every task expected of that position for somebody who was already qualified and get signatures attesting to our knowledge and experience, a written test, and then an oral board with an officer of varying rank based on the qualification in question. (As a 19 year old, explaining the inner workings of a nuclear reactor for 2 hours to the CO of an aircraft carrier is...intimidating.) If you failed the written exam a certain number of times, you could request an extended oral board as a means of testing your mastery of the information. I think it's a really good and effective model - when implemented properly.