LRFD Load Combination (1.2D + 1.6L)

Limit-states design does not apply one blanket safety factor. It scales each load by a factor reflecting how well that load is known, and $1.2D + 1.6L$ is the workhorse combination in ASCE 7 and its cousins worldwide. Dead load gets 1.2 because you can weigh the structure; live load gets 1.6 because you cannot predict what people will put on a floor. 50 kN of dead load with 30 kN of live gives $60 + 48 = 108$ kN of required strength, an overall factor of 1.35 on the 80 kN service load.

That overall factor is not a constant, and this is the insight worth taking away. Change the mix to 70 kN dead and 10 kN live and the same combination gives 100 kN, a factor of 1.25. A live-dominated case, 10 dead and 70 live, gives 124 kN and a factor of 1.55. A dead-heavy structure — a concrete parking deck, a green roof — is inherently more predictable and the code rewards it with less margin. Allowable stress design, which simply added service loads and divided the material strength by a single factor, could not make that distinction, and it is the main reason limit-states design produces more consistent reliability across building types.

The trap is treating this one line as the check. It is one of a set: ASCE 7 gives seven basic combinations, and depending on the structure the governing case may be $1.4D$ alone, or a combination including snow, wind or earthquake, or $0.9D + 1.0W$, which uses reduced dead load precisely because self-weight helps resist uplift and overturning. That last one is the reason a light roof structure has to be checked twice, and forgetting it is how buildings lose roofs in windstorms. Named factors also drift between code editions and jurisdictions, so read the adopted edition rather than trusting a remembered number.