Situation

A design-and-test project in ENGR& 214 Statics: design a truss spanning 36 inches that carries 25 lbf at midspan for at least three minutes with under 2 inches of sag, built only from balsa square stock, sheet gussets, and cyanoacrylate glue. Entries were ranked on load-to-weight ratio, so the engineering problem was not strength but strength per unit mass.

The assignment specified three-person groups — one member for design, one for construction, one for documentation — and expressly disallowed solo work. Class numbers left no room for another group, so Omar completed all three roles alone.

Actions

• Evaluated two to three candidate truss configurations in truss analysis software before committing to a design.
• Selected a K truss and modified it by adding an arch to the top chord, to distribute forces more evenly and specifically to relieve the two most heavily loaded members identified in the earlier configuration. The change and its rationale were documented before construction.
• Analyzed all 49 members by hand using method of joints and method of sections, then cross-checked the results against the software output rather than trusting either alone.
• Produced a full member table: cross-section, length, axial load with tension or compression sign, capacity, safety factor, and volume for every member — 222.98 inches of stock and 13.936 in³ of material in total.
• Built the bridge, cutting and gluing the complete structure without assistance.

Outcome

The bridge passed, holding the required 25 lbf at midspan within the sag limit, and went on to carry approximately 100 lbf — roughly four times the design load — before failure. It placed second in the class on load-to-weight ratio, the competition's scoring metric.

Failure did not occur in the analyzed structure. The two side truss faces and every glued joint held; what broke were the transverse members spanning between the faces, and they failed in the wood rather than at the joints.

Lessons / safety

The structure failed where the analysis was not looking. The hand calculations and the software model were both planar analyses of the side trusses, and both proved sound — those members carried four times their design load. The transverse members connecting the two faces were never part of that analysis, and they were the ones that gave way. The lesson Omar drew is that the analysis boundary is itself a design decision: everything outside it is unverified by definition, however rigorous the work inside it.

Construction time was underestimated. The build was completed in a single session the night before the test, and construction proved the hardest part of doing a three-person project alone. Omar's stated changes, given the chance again, are to analyze the transverse members and to start construction early enough to leave room for iteration.

Material variability was discovered too late. Omar's balsa was noticeably the softest stock in the class — remarked on by classmates — but this was only apparent at the competition, after the design was fixed. The course materials warn that balsa strength varies considerably between pieces, which makes characterizing the actual stock a design input rather than an afterthought.

Skills demonstrated

• structural analysis
• method of joints
• method of sections
• load path design
• safety factor analysis
• design iteration
• failure analysis
• hand calculation verification

Evidence

Held in media/: three photographs of the completed bridge, a photograph of the load test in progress, three pages of hand-worked calculation journals, the member-table spreadsheet, the design-change memo, the final report, the software frame-analysis report, and the original assignment specification.