Final answer:
Beams subjected to consistent loads that cause sagging will have a negative moment diagram, like a uniformly loaded simply supported beam.
Step-by-step explanation:
The type of beams where the moment diagram is always negative is when there is a consistent load that causes sagging. The negativity in the moment diagram is indicative of the fact that the moments are causing the beam to bend downwards. An example is a beam that is simply supported at two ends with a uniformly distributed load; the moment will be negative at the center of the beam where the sagging occurs.