ULS vs SLS: the two questions every structural check is really asking
Almost every real design check secretly boils down to one of two very different questions — will it break, or will it feel wrong to use — and conflating the two, or checking only one, misses half of what a structure actually has to do.
Ultimate Limit State (ULS) asks a single blunt question: under the worst realistic factored combination of loads a structure might see over its life, does it have enough strength to avoid collapsing, or a member losing its ability to carry load at all? This is the check that's actually about safety in the strict sense — failing a ULS check is a genuine capacity or life-safety concern, which is exactly why ULS load combinations apply the larger, more conservative load factors and why strength calculations use the reduced design material strength covered in the previous entry, stacking margin on both the load side and the resistance side for the check that matters most.
Serviceability Limit State: a genuinely different, and genuinely real, question
Serviceability Limit State (SLS) asks something different, and easy to underrate by comparison: under the loads a structure will realistically and routinely see in everyday use — not the rare worst-case combination ULS checks — does it still function the way it's actually meant to? Does a beam deflect little enough that the ceiling below it doesn't crack, that a floor doesn't feel bouncy underfoot, that a crack in a concrete member stays narrow enough not to let in moisture and corrode the reinforcement inside it? A structure can pass every ULS strength check with real margin to spare and still be a genuine design failure at SLS — nobody wants to occupy a building that's structurally safe but visibly sags, cracks or bounces under everyday use, and fixing that kind of serviceability problem after construction is often far more disruptive than getting it right the first time.
Because ULS and SLS are asking different questions, they're checked against different load combinations — SLS uses unfactored or only lightly factored "service" loads, since the concern is everyday behavior, not a rare worst case — and often against entirely different criteria: a strength/capacity ratio for ULS, a deflection limit expressed as a fraction of span (like L/360) or a maximum allowable crack width in millimetres for SLS, rather than the same pass/fail number checked twice. Load Combination Generator explicitly builds both sets of combinations side by side, and every design calculator on the site with both a strength and a serviceability side — RC Beam, RC Slab, Steel Beam, Foundation, and the rest — checks its own governing case from each set separately, precisely because passing one doesn't tell you anything reliable about the other (see the related tools below).