Back when I was teaching 7th graders I did the default bridge building project as the culmination of a semester on forces. What I did, which was different than most build-and-break curricula is set the following:
Teams are mandatory and consist of:
1. Project Manager
2. Accountant
3. Architect
4. Carpenter
5. Transportation Chief (optional)

Each team starts out with a set amount of capital and need to purchase pretty much everything. There is a set schedule for bridge completion. The teams can know on the second day how much their entire project will cost and plan for it. Even with heavy stressing on this, it rarely happens - they're 7th graders. Team members have specific responsibilities as well as things they are not allowed to do under nearly any circumstances. For example, the architect is not allowed to build. The project manager can't write checks. The carpenter can't fill out purchase orders.

In addition to serving as a warehouse, I kept my own "books" to watch team budgets, but let them figure out if they were low. I offered an audit service (for a fee) if there were issues. Teams frequently had arguments. I offered arbitration services (for a fee). Team members sometimes did jobs they weren't supposed to or made messes that they didn't clean up. I assigned safety violations that escalated over time. 2/3 of the way through the project, I raised all my prices.

If a team went over budget, they had to appeal to the rest of the class and file for a budget override. As part of that, they had to have ready an explanation as to why they went over budget, how much money they needed and how it would be spent. If the rest of the class voted for them, they got the money and went on. If not, they went into a deficit and lost some important grades.

Final grade was on (IIRC) 10 things:
1. Completed on schedule
2. Completed on budget
3. Completed neatly
4. Matched the plans
5. Completed journal
6. Accurate bookkeeping
7. Bridge efficiency (strength/cost)
8. Safe workplace
9. Complete plans
10. Bridge met building code

And while the bridge testing is exciting, the project is all about team work. I remember overhearing things like, "if we don't settle this, it'll cost us $2500 in arbitration - we can't afford that!" I had a team with an architect who was a total control freak - the project manager had to police him to keep him out of the work site and getting the way of the carpenter. It was beautiful.

That's really cool, plinth.

The civil guys at my school used to get an assignment like this but with only two team members. The specification was a "bridge" to support the maximum load applied at the centre of a 50cm span using only 10 + 2 cm of depth. The goal is to generate the highest mass to load rating ratio.

The required building materials were Popsicle sticks and white glue.

The last year they did this assignment I was assigned to run the testing so I glued up my own for testing:
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Testing Machine:
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The tester applies a progressive load measured by a strain gauge to the centre of the bridge. Compare the maximum load to the mass of the bridge and you've got your ratio. Mine supported 163560 grams with a mass of 116.76 or a ratio of 1400.8:1 (IE: 1.4 kgs for every gram of weight).

The big problem with the required building materials is the wood is very weak where glued together. To help spread the load I cross pinned members by slicing up a stick and forcing the now toothpick like pieces of wood into holes drilled in the glued up frame.

The bridge displays at the Smithsonian American History museum are really, really fascinating. I hope they survive the ongoing renovation!