You aren't paying attention to the phrase "expected load".
If, in the bridge scenario, it cracks under 2.95x the normal load, vs. 3.0x or 3.05x, that is not what Feynman was describing. He was explicitly talking about a partial failure under about 1.0x. Which based on other comments is not what happened here.
except 3.75g here is the expected load°, the certification requirement. the design is engineered to reach that 3.75g°, the safety margin is what lies beyond.
°with the disclaimer that the plane might be not airworthy after landing
If, in the bridge scenario, it cracks under 2.95x the normal load, vs. 3.0x or 3.05x, that is not what Feynman was describing. He was explicitly talking about a partial failure under about 1.0x. Which based on other comments is not what happened here.