After reading through some of the engineering websites-- and familiarizing myself with the technical language that I don’t have-- I realized they’re were talking about similar things that people have mentioned here.
http://www.eng-tips.com/viewthread.cfm?qid=436595
Here’s a good technical summary:
Basically, the engineering forums are now focusing on truss member #11, and it being strained past capacity, due to less structural redundancy. Conjecture around the failure appears to be the following:
- lack of longitudinal PT tendons attaching vertical truss-end #12 to deck
- lacked of reinforcement caused by drainpipe and other fittings through the deck and the end of truss #12 (increased chance of stress fractures)
- angle of truss #11 at approx. 30-degrees, thus increasing horizontal load strain (compare photos of thicker truss #2 with thinner truss #11)
- potential eccentric load (that is, load unevenly applied through truss-members due to asymmetry)
- retrofitted ducts in truss #11 for the use of temporary PT tendons to stabilize cantilevering due to SPMTs not being able to maneuver and support the bridge-ends
- FDOT redesign request to extend the length of bridge to make room for additional traffic lane
- north pier undersized with soft footings due to canal work and delays by Army Corps of Engineers
- emphasis on making trusses visually line up with cable-stays, despite increased instability (disregard for design safety in order to aesthetically promote future nearby gentrification projects; early conceptualization had real cable-stays dropped from design but kept for “vibration dampering”)
- no redundancy in trusswork (high failure-rate with single row of trusses and no vertical web)
- torsion strain (trusswork subject to weak cross-section twisting from wind and over-height truck collisions)
- lack of node revolution plates for flexion (joint between truss-ends and top and bottom chords was fixed and brittle)
- use of materials lacking ductility (concrete has rapid failure in tension, whereas steel fails more slowly)
- change of compression elements without accounting for mid-transport readjustments (tightening compression in non-compression members for transport, but creating tension counter-effects when deck was lowered onto piers without timely readjustment)
- push for rapid project completion in order to receive Federal funds
- loss of design focus: over-weight, over-budget, under-built, over-promised multi-purpose bridge for social gathering place (not just transit, but superfluous fittings requiring extra cable ducts for circulation fans and wifi with cafe seating over a road with heavy traffic)
And that’s just what I could recall off the top of my head! This was a bad design just waiting to fall down. Period. They’re just lucky it didn’t fall down with hundreds of people on it.
Here’s a truss simulator where you can practice what went wrong:
