Drag the sliders — the building shows the governing combination's factored loads.
Dead load is always present. Live load might not be at peak. Wind and seismic don't strike simultaneously. Each combination below is a plausible worst-case scenario — a different crew of loads acting together, with factors that reflect how well we can estimate each one. Run all seven. The one that produces the highest demand is the one you design for.
A building is never hit by maximum wind, maximum snow, maximum live load, and maximum seismic force simultaneously. Load combinations reflect this reality. ASCE 7 §2.3 specifies which loads to combine and at what factors. The key gravity combo is 1.2D + 1.6L + 0.5(Lr or S) — dead load is almost certain (γ=1.2), live load is variable (γ=1.6), and roof loads are companion (γ=0.5, because full roof and full floor at the same time is unlikely).
The 0.9D combinations confuse every student the first time. Why would you factor dead load DOWN? Because when checking for uplift or overturning, dead load is your friend — it holds the building down. Using less dead load (0.9D) while applying full wind or seismic is the worst case for these checks. A roof connection that relies on self-weight to resist wind suction must be checked with 0.9D + 1.0W — and if the uplift exceeds 0.9 × dead load, you need mechanical anchorage.