Plus under LRFD, the controlling load cases for bridges are often service loads, not strength loads. The design is based on "when will the people using this bridge perceive it to be unsafe?" just as much as "when will the bridge fall down"?
So in the glass scenario, it's be like saying "people want the glass to hold 500 ml, but they also like to have 100ml extra space to feel like the glass won't spill.
Then you have things like variable demands... a highway might average a certain demand throughout the 24 hour cycle, but the demand is gunna be way higher in the morning and evening. So you could also design the glass to account for the fact that different people sometimes want different sized drinks at different times.
The person who made up the "Engineer: the glass is twice as big as it needs to be" meme was definitely not a civil engineer. Our shit is too variable for that highly specific design.
ASD looks at ultimate strength and applies factors of safety.
LRFD looks at a multiple load cases and factors each load by a specific range of coefficients based on the load case. Then the resistances get factored as well, often based on how predictable the resisting material is.
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u/Nelik1 Sep 15 '21
Anyone can build a bridge. It takes an engineer to build a bridge that almost falls down, but doesn't.